mm: handle faults that merely update the accessed bit under the VMA lock

Move FAULT_FLAG_VMA_LOCK check out of handle_pte_fault().  This should
have a significant performance improvement for mmaped files.  Write faults
(on read-only shared pages) still take the mmap lock as we do not want to
audit all the implementations of ->pfn_mkwrite() and ->page_mkwrite().
However write-faults on private mappings are handled under the VMA lock.

[willy@infradead.org: address "suspicious RCU usage" warning]
Link: https://lkml.kernel.org/r/ZMK7jwpI4uD6tKrF@casper.infradead.org
Link: https://lkml.kernel.org/r/20230724185410.1124082-11-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Punit Agrawal <punit.agrawal@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index c122adc..f062664 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3268,6 +3268,11 @@ static vm_fault_t wp_pfn_shared(struct vm_fault *vmf)
                vm_fault_t ret;
 
                pte_unmap_unlock(vmf->pte, vmf->ptl);
+               if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+                       vma_end_read(vmf->vma);
+                       return VM_FAULT_RETRY;
+               }
+
                vmf->flags |= FAULT_FLAG_MKWRITE;
                ret = vma->vm_ops->pfn_mkwrite(vmf);
                if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
@@ -3290,6 +3295,12 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf, struct folio *folio)
                vm_fault_t tmp;
 
                pte_unmap_unlock(vmf->pte, vmf->ptl);
+               if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+                       folio_put(folio);
+                       vma_end_read(vmf->vma);
+                       return VM_FAULT_RETRY;
+               }
+
                tmp = do_page_mkwrite(vmf, folio);
                if (unlikely(!tmp || (tmp &
                                      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
@@ -3431,6 +3442,12 @@ reuse:
                return 0;
        }
 copy:
+       if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma->anon_vma) {
+               pte_unmap_unlock(vmf->pte, vmf->ptl);
+               vma_end_read(vmf->vma);
+               return VM_FAULT_RETRY;
+       }
+
        /*
         * Ok, we need to copy. Oh, well..
         */
@@ -4985,12 +5002,6 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
        if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
                return do_numa_page(vmf);
 
-       if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vmf->vma)) {
-               pte_unmap(vmf->pte);
-               vma_end_read(vmf->vma);
-               return VM_FAULT_RETRY;
-       }
-
        spin_lock(vmf->ptl);
        entry = vmf->orig_pte;
        if (unlikely(!pte_same(ptep_get(vmf->pte), entry))) {