mm/hmm.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * Copyright 2013 Red Hat Inc.
   4  *
   5  * Authors: Jérôme Glisse <jglisse@redhat.com>
   6  */
   7 /*
   8  * Refer to include/linux/hmm.h for information about heterogeneous memory
   9  * management or HMM for short.
  10  */
  11 #include <linux/pagewalk.h>
  12 #include <linux/hmm.h>
  13 #include <linux/init.h>
  14 #include <linux/rmap.h>
  15 #include <linux/swap.h>
  16 #include <linux/slab.h>
  17 #include <linux/sched.h>
  18 #include <linux/mmzone.h>
  19 #include <linux/pagemap.h>
  20 #include <linux/swapops.h>
  21 #include <linux/hugetlb.h>
  22 #include <linux/memremap.h>
  23 #include <linux/sched/mm.h>
  24 #include <linux/jump_label.h>
  25 #include <linux/dma-mapping.h>
  26 #include <linux/mmu_notifier.h>
  27 #include <linux/memory_hotplug.h>
  28
  29 struct hmm_vma_walk {
  30         struct hmm_range        *range;
  31         unsigned long           last;
  32 };
  33
  34 enum {
  35         HMM_NEED_FAULT = 1 << 0,
  36         HMM_NEED_WRITE_FAULT = 1 << 1,
  37         HMM_NEED_ALL_BITS = HMM_NEED_FAULT | HMM_NEED_WRITE_FAULT,
  38 };
  39
  40 static int hmm_pfns_fill(unsigned long addr, unsigned long end,
  41                          struct hmm_range *range, unsigned long cpu_flags)
  42 {
  43         unsigned long i = (addr - range->start) >> PAGE_SHIFT;
  44
  45         for (; addr < end; addr += PAGE_SIZE, i++)
  46                 range->hmm_pfns[i] = cpu_flags;
  47         return 0;
  48 }
  49
  50 /*
  51  * hmm_vma_fault() - fault in a range lacking valid pmd or pte(s)
  52  * @addr: range virtual start address (inclusive)
  53  * @end: range virtual end address (exclusive)
  54  * @required_fault: HMM_NEED_* flags
  55  * @walk: mm_walk structure
  56  * Return: -EBUSY after page fault, or page fault error
  57  *
  58  * This function will be called whenever pmd_none() or pte_none() returns true,
  59  * or whenever there is no page directory covering the virtual address range.
  60  */
  61 static int hmm_vma_fault(unsigned long addr, unsigned long end,
  62                          unsigned int required_fault, struct mm_walk *walk)
  63 {
  64         struct hmm_vma_walk *hmm_vma_walk = walk->private;
  65         struct vm_area_struct *vma = walk->vma;
  66         unsigned int fault_flags = FAULT_FLAG_REMOTE;
  67
  68         WARN_ON_ONCE(!required_fault);
  69         hmm_vma_walk->last = addr;
  70
  71         if (required_fault & HMM_NEED_WRITE_FAULT) {
  72                 if (!(vma->vm_flags & VM_WRITE))
  73                         return -EPERM;
  74                 fault_flags |= FAULT_FLAG_WRITE;
  75         }
  76
  77         for (; addr < end; addr += PAGE_SIZE)
  78                 if (handle_mm_fault(vma, addr, fault_flags, NULL) &
  79                     VM_FAULT_ERROR)
  80                         return -EFAULT;
  81         return -EBUSY;
  82 }
  83
  84 static unsigned int hmm_pte_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
  85                                        unsigned long pfn_req_flags,
  86                                        unsigned long cpu_flags)
  87 {
  88         struct hmm_range *range = hmm_vma_walk->range;
  89
  90         /*
  91          * So we not only consider the individual per page request we also
  92          * consider the default flags requested for the range. The API can
  93          * be used 2 ways. The first one where the HMM user coalesces
  94          * multiple page faults into one request and sets flags per pfn for
  95          * those faults. The second one where the HMM user wants to pre-
  96          * fault a range with specific flags. For the latter one it is a
  97          * waste to have the user pre-fill the pfn arrays with a default
  98          * flags value.
  99          */
 100         pfn_req_flags &= range->pfn_flags_mask;
 101         pfn_req_flags |= range->default_flags;
 102
 103         /* We aren't ask to do anything ... */
 104         if (!(pfn_req_flags & HMM_PFN_REQ_FAULT))
 105                 return 0;
 106
 107         /* Need to write fault ? */
 108         if ((pfn_req_flags & HMM_PFN_REQ_WRITE) &&
 109             !(cpu_flags & HMM_PFN_WRITE))
 110                 return HMM_NEED_FAULT | HMM_NEED_WRITE_FAULT;
 111
 112         /* If CPU page table is not valid then we need to fault */
 113         if (!(cpu_flags & HMM_PFN_VALID))
 114                 return HMM_NEED_FAULT;
 115         return 0;
 116 }
 117
 118 static unsigned int
 119 hmm_range_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
 120                      const unsigned long hmm_pfns[], unsigned long npages,
 121                      unsigned long cpu_flags)
 122 {
 123         struct hmm_range *range = hmm_vma_walk->range;
 124         unsigned int required_fault = 0;
 125         unsigned long i;
 126
 127         /*
 128          * If the default flags do not request to fault pages, and the mask does
 129          * not allow for individual pages to be faulted, then
 130          * hmm_pte_need_fault() will always return 0.
 131          */
 132         if (!((range->default_flags | range->pfn_flags_mask) &
 133               HMM_PFN_REQ_FAULT))
 134                 return 0;
 135
 136         for (i = 0; i < npages; ++i) {
 137                 required_fault |= hmm_pte_need_fault(hmm_vma_walk, hmm_pfns[i],
 138                                                      cpu_flags);
 139                 if (required_fault == HMM_NEED_ALL_BITS)
 140                         return required_fault;
 141         }
 142         return required_fault;
 143 }
 144
 145 static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
 146                              __always_unused int depth, struct mm_walk *walk)
 147 {
 148         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 149         struct hmm_range *range = hmm_vma_walk->range;
 150         unsigned int required_fault;
 151         unsigned long i, npages;
 152         unsigned long *hmm_pfns;
 153
 154         i = (addr - range->start) >> PAGE_SHIFT;
 155         npages = (end - addr) >> PAGE_SHIFT;
 156         hmm_pfns = &range->hmm_pfns[i];
 157         required_fault =
 158                 hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0);
 159         if (!walk->vma) {
 160                 if (required_fault)
 161                         return -EFAULT;
 162                 return hmm_pfns_fill(addr, end, range, HMM_PFN_ERROR);
 163         }
 164         if (required_fault)
 165                 return hmm_vma_fault(addr, end, required_fault, walk);
 166         return hmm_pfns_fill(addr, end, range, 0);
 167 }
 168
 169 static inline unsigned long hmm_pfn_flags_order(unsigned long order)
 170 {
 171         return order << HMM_PFN_ORDER_SHIFT;
 172 }
 173
 174 static inline unsigned long pmd_to_hmm_pfn_flags(struct hmm_range *range,
 175                                                  pmd_t pmd)
 176 {
 177         if (pmd_protnone(pmd))
 178                 return 0;
 179         return (pmd_write(pmd) ? (HMM_PFN_VALID | HMM_PFN_WRITE) :
 180                                  HMM_PFN_VALID) |
 181                hmm_pfn_flags_order(PMD_SHIFT - PAGE_SHIFT);
 182 }
 183
 184 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 185 static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 186                               unsigned long end, unsigned long hmm_pfns[],
 187                               pmd_t pmd)
 188 {
 189         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 190         struct hmm_range *range = hmm_vma_walk->range;
 191         unsigned long pfn, npages, i;
 192         unsigned int required_fault;
 193         unsigned long cpu_flags;
 194
 195         npages = (end - addr) >> PAGE_SHIFT;
 196         cpu_flags = pmd_to_hmm_pfn_flags(range, pmd);
 197         required_fault =
 198                 hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, cpu_flags);
 199         if (required_fault)
 200                 return hmm_vma_fault(addr, end, required_fault, walk);
 201
 202         pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 203         for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++)
 204                 hmm_pfns[i] = pfn | cpu_flags;
 205         return 0;
 206 }
 207 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 208 /* stub to allow the code below to compile */
 209 int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 210                 unsigned long end, unsigned long hmm_pfns[], pmd_t pmd);
 211 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 212
 213 static inline bool hmm_is_device_private_entry(struct hmm_range *range,
 214                 swp_entry_t entry)
 215 {
 216         return is_device_private_entry(entry) &&
 217                 device_private_entry_to_page(entry)->pgmap->owner ==
 218                 range->dev_private_owner;
 219 }
 220
 221 static inline unsigned long pte_to_hmm_pfn_flags(struct hmm_range *range,
 222                                                  pte_t pte)
 223 {
 224         if (pte_none(pte) || !pte_present(pte) || pte_protnone(pte))
 225                 return 0;
 226         return pte_write(pte) ? (HMM_PFN_VALID | HMM_PFN_WRITE) : HMM_PFN_VALID;
 227 }
 228
 229 static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 230                               unsigned long end, pmd_t *pmdp, pte_t *ptep,
 231                               unsigned long *hmm_pfn)
 232 {
 233         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 234         struct hmm_range *range = hmm_vma_walk->range;
 235         unsigned int required_fault;
 236         unsigned long cpu_flags;
 237         pte_t pte = *ptep;
 238         uint64_t pfn_req_flags = *hmm_pfn;
 239
 240         if (pte_none(pte)) {
 241                 required_fault =
 242                         hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);
 243                 if (required_fault)
 244                         goto fault;
 245                 *hmm_pfn = 0;
 246                 return 0;
 247         }
 248
 249         if (!pte_present(pte)) {
 250                 swp_entry_t entry = pte_to_swp_entry(pte);
 251
 252                 /*
 253                  * Never fault in device private pages, but just report
 254                  * the PFN even if not present.
 255                  */
 256                 if (hmm_is_device_private_entry(range, entry)) {
 257                         cpu_flags = HMM_PFN_VALID;
 258                         if (is_write_device_private_entry(entry))
 259                                 cpu_flags |= HMM_PFN_WRITE;
 260                         *hmm_pfn = device_private_entry_to_pfn(entry) |
 261                                         cpu_flags;
 262                         return 0;
 263                 }
 264
 265                 required_fault =
 266                         hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);
 267                 if (!required_fault) {
 268                         *hmm_pfn = 0;
 269                         return 0;
 270                 }
 271
 272                 if (!non_swap_entry(entry))
 273                         goto fault;
 274
 275                 if (is_migration_entry(entry)) {
 276                         pte_unmap(ptep);
 277                         hmm_vma_walk->last = addr;
 278                         migration_entry_wait(walk->mm, pmdp, addr);
 279                         return -EBUSY;
 280                 }
 281
 282                 /* Report error for everything else */
 283                 pte_unmap(ptep);
 284                 return -EFAULT;
 285         }
 286
 287         cpu_flags = pte_to_hmm_pfn_flags(range, pte);
 288         required_fault =
 289                 hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
 290         if (required_fault)
 291                 goto fault;
 292
 293         /*
 294          * Since each architecture defines a struct page for the zero page, just
 295          * fall through and treat it like a normal page.
 296          */
 297         if (pte_special(pte) && !is_zero_pfn(pte_pfn(pte))) {
 298                 if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
 299                         pte_unmap(ptep);
 300                         return -EFAULT;
 301                 }
 302                 *hmm_pfn = HMM_PFN_ERROR;
 303                 return 0;
 304         }
 305
 306         *hmm_pfn = pte_pfn(pte) | cpu_flags;
 307         return 0;
 308
 309 fault:
 310         pte_unmap(ptep);
 311         /* Fault any virtual address we were asked to fault */
 312         return hmm_vma_fault(addr, end, required_fault, walk);
 313 }
 314
 315 static int hmm_vma_walk_pmd(pmd_t *pmdp,
 316                             unsigned long start,
 317                             unsigned long end,
 318                             struct mm_walk *walk)
 319 {
 320         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 321         struct hmm_range *range = hmm_vma_walk->range;
 322         unsigned long *hmm_pfns =
 323                 &range->hmm_pfns[(start - range->start) >> PAGE_SHIFT];
 324         unsigned long npages = (end - start) >> PAGE_SHIFT;
 325         unsigned long addr = start;
 326         pte_t *ptep;
 327         pmd_t pmd;
 328
 329 again:
 330         pmd = READ_ONCE(*pmdp);
 331         if (pmd_none(pmd))
 332                 return hmm_vma_walk_hole(start, end, -1, walk);
 333
 334         if (thp_migration_supported() && is_pmd_migration_entry(pmd)) {
 335                 if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0)) {
 336                         hmm_vma_walk->last = addr;
 337                         pmd_migration_entry_wait(walk->mm, pmdp);
 338                         return -EBUSY;
 339                 }
 340                 return hmm_pfns_fill(start, end, range, 0);
 341         }
 342
 343         if (!pmd_present(pmd)) {
 344                 if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 345                         return -EFAULT;
 346                 return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 347         }
 348
 349         if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
 350                 /*
 351                  * No need to take pmd_lock here, even if some other thread
 352                  * is splitting the huge pmd we will get that event through
 353                  * mmu_notifier callback.
 354                  *
 355                  * So just read pmd value and check again it's a transparent
 356                  * huge or device mapping one and compute corresponding pfn
 357                  * values.
 358                  */
 359                 pmd = pmd_read_atomic(pmdp);
 360                 barrier();
 361                 if (!pmd_devmap(pmd) && !pmd_trans_huge(pmd))
 362                         goto again;
 363
 364                 return hmm_vma_handle_pmd(walk, addr, end, hmm_pfns, pmd);
 365         }
 366
 367         /*
 368          * We have handled all the valid cases above ie either none, migration,
 369          * huge or transparent huge. At this point either it is a valid pmd
 370          * entry pointing to pte directory or it is a bad pmd that will not
 371          * recover.
 372          */
 373         if (pmd_bad(pmd)) {
 374                 if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
 375                         return -EFAULT;
 376                 return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 377         }
 378
 379         ptep = pte_offset_map(pmdp, addr);
 380         for (; addr < end; addr += PAGE_SIZE, ptep++, hmm_pfns++) {
 381                 int r;
 382
 383                 r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, hmm_pfns);
 384                 if (r) {
 385                         /* hmm_vma_handle_pte() did pte_unmap() */
 386                         return r;
 387                 }
 388         }
 389         pte_unmap(ptep - 1);
 390         return 0;
 391 }
 392
 393 #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && \
 394     defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
 395 static inline unsigned long pud_to_hmm_pfn_flags(struct hmm_range *range,
 396                                                  pud_t pud)
 397 {
 398         if (!pud_present(pud))
 399                 return 0;
 400         return (pud_write(pud) ? (HMM_PFN_VALID | HMM_PFN_WRITE) :
 401                                  HMM_PFN_VALID) |
 402                hmm_pfn_flags_order(PUD_SHIFT - PAGE_SHIFT);
 403 }
 404
 405 static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 406                 struct mm_walk *walk)
 407 {
 408         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 409         struct hmm_range *range = hmm_vma_walk->range;
 410         unsigned long addr = start;
 411         pud_t pud;
 412         int ret = 0;
 413         spinlock_t *ptl = pud_trans_huge_lock(pudp, walk->vma);
 414
 415         if (!ptl)
 416                 return 0;
 417
 418         /* Normally we don't want to split the huge page */
 419         walk->action = ACTION_CONTINUE;
 420
 421         pud = READ_ONCE(*pudp);
 422         if (pud_none(pud)) {
 423                 spin_unlock(ptl);
 424                 return hmm_vma_walk_hole(start, end, -1, walk);
 425         }
 426
 427         if (pud_huge(pud) && pud_devmap(pud)) {
 428                 unsigned long i, npages, pfn;
 429                 unsigned int required_fault;
 430                 unsigned long *hmm_pfns;
 431                 unsigned long cpu_flags;
 432
 433                 if (!pud_present(pud)) {
 434                         spin_unlock(ptl);
 435                         return hmm_vma_walk_hole(start, end, -1, walk);
 436                 }
 437
 438                 i = (addr - range->start) >> PAGE_SHIFT;
 439                 npages = (end - addr) >> PAGE_SHIFT;
 440                 hmm_pfns = &range->hmm_pfns[i];
 441
 442                 cpu_flags = pud_to_hmm_pfn_flags(range, pud);
 443                 required_fault = hmm_range_need_fault(hmm_vma_walk, hmm_pfns,
 444                                                       npages, cpu_flags);
 445                 if (required_fault) {
 446                         spin_unlock(ptl);
 447                         return hmm_vma_fault(addr, end, required_fault, walk);
 448                 }
 449
 450                 pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 451                 for (i = 0; i < npages; ++i, ++pfn)
 452                         hmm_pfns[i] = pfn | cpu_flags;
 453                 goto out_unlock;
 454         }
 455
 456         /* Ask for the PUD to be split */
 457         walk->action = ACTION_SUBTREE;
 458
 459 out_unlock:
 460         spin_unlock(ptl);
 461         return ret;
 462 }
 463 #else
 464 #define hmm_vma_walk_pud        NULL
 465 #endif
 466
 467 #ifdef CONFIG_HUGETLB_PAGE
 468 static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 469                                       unsigned long start, unsigned long end,
 470                                       struct mm_walk *walk)
 471 {
 472         unsigned long addr = start, i, pfn;
 473         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 474         struct hmm_range *range = hmm_vma_walk->range;
 475         struct vm_area_struct *vma = walk->vma;
 476         unsigned int required_fault;
 477         unsigned long pfn_req_flags;
 478         unsigned long cpu_flags;
 479         spinlock_t *ptl;
 480         pte_t entry;
 481
 482         ptl = huge_pte_lock(hstate_vma(vma), walk->mm, pte);
 483         entry = huge_ptep_get(pte);
 484
 485         i = (start - range->start) >> PAGE_SHIFT;
 486         pfn_req_flags = range->hmm_pfns[i];
 487         cpu_flags = pte_to_hmm_pfn_flags(range, entry) |
 488                     hmm_pfn_flags_order(huge_page_order(hstate_vma(vma)));
 489         required_fault =
 490                 hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
 491         if (required_fault) {
 492                 spin_unlock(ptl);
 493                 return hmm_vma_fault(addr, end, required_fault, walk);
 494         }
 495
 496         pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
 497         for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
 498                 range->hmm_pfns[i] = pfn | cpu_flags;
 499
 500         spin_unlock(ptl);
 501         return 0;
 502 }
 503 #else
 504 #define hmm_vma_walk_hugetlb_entry NULL
 505 #endif /* CONFIG_HUGETLB_PAGE */
 506
 507 static int hmm_vma_walk_test(unsigned long start, unsigned long end,
 508                              struct mm_walk *walk)
 509 {
 510         struct hmm_vma_walk *hmm_vma_walk = walk->private;
 511         struct hmm_range *range = hmm_vma_walk->range;
 512         struct vm_area_struct *vma = walk->vma;
 513
 514         if (!(vma->vm_flags & (VM_IO | VM_PFNMAP | VM_MIXEDMAP)) &&
 515             vma->vm_flags & VM_READ)
 516                 return 0;
 517
 518         /*
 519          * vma ranges that don't have struct page backing them or map I/O
 520          * devices directly cannot be handled by hmm_range_fault().
 521          *
 522          * If the vma does not allow read access, then assume that it does not
 523          * allow write access either. HMM does not support architectures that
 524          * allow write without read.
 525          *
 526          * If a fault is requested for an unsupported range then it is a hard
 527          * failure.
 528          */
 529         if (hmm_range_need_fault(hmm_vma_walk,
 530                                  range->hmm_pfns +
 531                                          ((start - range->start) >> PAGE_SHIFT),
 532                                  (end - start) >> PAGE_SHIFT, 0))
 533                 return -EFAULT;
 534
 535         hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 536
 537         /* Skip this vma and continue processing the next vma. */
 538         return 1;
 539 }
 540
 541 static const struct mm_walk_ops hmm_walk_ops = {
 542         .pud_entry      = hmm_vma_walk_pud,
 543         .pmd_entry      = hmm_vma_walk_pmd,
 544         .pte_hole       = hmm_vma_walk_hole,
 545         .hugetlb_entry  = hmm_vma_walk_hugetlb_entry,
 546         .test_walk      = hmm_vma_walk_test,
 547 };
 548
 549 /**
 550  * hmm_range_fault - try to fault some address in a virtual address range
 551  * @range:      argument structure
 552  *
 553  * Returns 0 on success or one of the following error codes:
 554  *
 555  * -EINVAL:     Invalid arguments or mm or virtual address is in an invalid vma
 556  *              (e.g., device file vma).
 557  * -ENOMEM:     Out of memory.
 558  * -EPERM:      Invalid permission (e.g., asking for write and range is read
 559  *              only).
 560  * -EBUSY:      The range has been invalidated and the caller needs to wait for
 561  *              the invalidation to finish.
 562  * -EFAULT:     A page was requested to be valid and could not be made valid
 563  *              ie it has no backing VMA or it is illegal to access
 564  *
 565  * This is similar to get_user_pages(), except that it can read the page tables
 566  * without mutating them (ie causing faults).
 567  */
 568 int hmm_range_fault(struct hmm_range *range)
 569 {
 570         struct hmm_vma_walk hmm_vma_walk = {
 571                 .range = range,
 572                 .last = range->start,
 573         };
 574         struct mm_struct *mm = range->notifier->mm;
 575         int ret;
 576
 577         mmap_assert_locked(mm);
 578
 579         do {
 580                 /* If range is no longer valid force retry. */
 581                 if (mmu_interval_check_retry(range->notifier,
 582                                              range->notifier_seq))
 583                         return -EBUSY;
 584                 ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 585                                       &hmm_walk_ops, &hmm_vma_walk);
 586                 /*
 587                  * When -EBUSY is returned the loop restarts with
 588                  * hmm_vma_walk.last set to an address that has not been stored
 589                  * in pfns. All entries < last in the pfn array are set to their
 590                  * output, and all >= are still at their input values.
 591                  */
 592         } while (ret == -EBUSY);
 593         return ret;
 594 }
 595 EXPORT_SYMBOL(hmm_range_fault);