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BACKPORT: mm/gup: Remove enfornced COW mechanism

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With the more strict (but greatly simplified) page reuse logic in
do_wp_page(), we can safely go back to the world where cow is not
enforced with writes.

This essentially reverts commit 17839856fd58 ("gup: document and work
around 'COW can break either way' issue").  There are some context
differences due to some changes later on around it:

  2170ecfa7688 ("drm/i915: convert get_user_pages() --> pin_user_pages()", 2020-06-03)
  376a34efa4ee ("mm/gup: refactor and de-duplicate gup_fast() code", 2020-06-03)

Some lines moved back and forth with those, but this revert patch should
have striped out and covered all the enforced cow bits anyways.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

(cherry picked from commit a308c71bf1e6e19cc2e4ced31853ee0fc7cb439a)

Bug: 173684178
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ib2308e4ff3f2e9283e5c93b096047cf276edf27f
This commit is contained in:
Peter Xu 2020-08-21 19:49:57 -04:00 • committed by Michael Bestas
commit 822149ba00
1 changed files with 1 additions and 21 deletions
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22
mm/gup.c
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@ -167,17 +167,7 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
return pte_write(pte) ||
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}
/*
* A (separate) COW fault might break the page the other way and
* get_user_pages() would return the page from what is now the wrong
* VM. So we need to force a COW break at GUP time even for reads.
*/
static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
{
return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}
static struct page *follow_page_pte(struct vm_area_struct *vma,
@ -2382,10 +2372,6 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
*
* If the architecture does not support this function, simply return with no
* pages pinned.
*
* Careful, careful! COW breaking can go either way, so a non-write
* access can get ambiguous page results. If you call this function without
* 'write' set, you'd better be sure that you're ok with that ambiguity.
*/
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages)
@ -2413,12 +2399,6 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
*
* We do not adopt an rcu_read_lock(.) here as we also want to
* block IPIs that come from THPs splitting.
*
* NOTE! We allow read-only gup_fast() here, but you'd better be
* careful about possible COW pages. You'll get _a_ COW page, but
* not necessarily the one you intended to get depending on what
* COW event happens after this. COW may break the page copy in a
* random direction.
*/
if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&