mm/memory: slightly simplify copy_present_pte()

Let's move the pinning check into the caller, to simplify return code
logic and prepare for further changes: relocating the
page_needs_cow_for_dma() into rmap handling code.

While at it, remove the unused pte parameter and simplify the comments a
bit.

No functional change intended.

Link: https://lkml.kernel.org/r/20220428083441.37290-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Khalid Aziz <khalid.aziz@oracle.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Liang Zhang <zhangliang5@huawei.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Oded Gabbay <oded.gabbay@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index e873a61..9bdfce1 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -862,19 +862,11 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 }
 
 /*
- * Copy a present and normal page if necessary.
+ * Copy a present and normal page.
  *
- * NOTE! The usual case is that this doesn't need to do
- * anything, and can just return a positive value. That
- * will let the caller know that it can just increase
- * the page refcount and re-use the pte the traditional
- * way.
- *
- * But _if_ we need to copy it because it needs to be
- * pinned in the parent (and the child should get its own
- * copy rather than just a reference to the same page),
- * we'll do that here and return zero to let the caller
- * know we're done.
+ * NOTE! The usual case is that this isn't required;
+ * instead, the caller can just increase the page refcount
+ * and re-use the pte the traditional way.
  *
  * And if we need a pre-allocated page but don't yet have
  * one, return a negative error to let the preallocation
@@ -884,25 +876,10 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 static inline int
 copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
                  pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
-                 struct page **prealloc, pte_t pte, struct page *page)
+                 struct page **prealloc, struct page *page)
 {
        struct page *new_page;
-
-       /*
-        * What we want to do is to check whether this page may
-        * have been pinned by the parent process.  If so,
-        * instead of wrprotect the pte on both sides, we copy
-        * the page immediately so that we'll always guarantee
-        * the pinned page won't be randomly replaced in the
-        * future.
-        *
-        * The page pinning checks are just "has this mm ever
-        * seen pinning", along with the (inexact) check of
-        * the page count. That might give false positives for
-        * for pinning, but it will work correctly.
-        */
-       if (likely(!page_needs_cow_for_dma(src_vma, page)))
-               return 1;
+       pte_t pte;
 
        new_page = *prealloc;
        if (!new_page)
@@ -944,14 +921,16 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
        struct page *page;
 
        page = vm_normal_page(src_vma, addr, pte);
-       if (page) {
-               int retval;
-
-               retval = copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
-                                          addr, rss, prealloc, pte, page);
-               if (retval <= 0)
-                       return retval;
-
+       if (page && unlikely(page_needs_cow_for_dma(src_vma, page))) {
+               /*
+                * If this page may have been pinned by the parent process,
+                * copy the page immediately for the child so that we'll always
+                * guarantee the pinned page won't be randomly replaced in the
+                * future.
+                */
+               return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
+                                        addr, rss, prealloc, page);
+       } else if (page) {
                get_page(page);
                page_dup_rmap(page, false);
                rss[mm_counter(page)]++;