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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>
This commit is contained in:
Peter Xu 2020-08-21 19:49:57 -04:00 committed by Linus Torvalds
parent 1a0cf26323
commit a308c71bf1
3 changed files with 9 additions and 46 deletions
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8
drivers/gpu/drm/i915/gem/i915_gem_userptr.c
View File

@ -599,14 +599,6 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
GFP_KERNEL |
__GFP_NORETRY |
__GFP_NOWARN);
/*
* Using __get_user_pages_fast() with a read-only
* access is questionable. A read-only page may be
* COW-broken, and then this might end up giving
* the wrong side of the COW..
*
* We may or may not care.
*/
if (pvec) {
/* defer to worker if malloc fails */
if (!i915_gem_object_is_readonly(obj))

40
mm/gup.c
View File

@ -381,22 +381,13 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
}
/*
* FOLL_FORCE or a forced COW break can write even to unwritable pte's,
* but only after we've gone through a COW cycle and they are dirty.
* FOLL_FORCE can write to even unwritable pte's, but only
* after we've gone through a COW cycle and they are dirty.
*/
static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
return pte_write(pte) || ((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 | FOLL_PIN));
return pte_write(pte) ||
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}
static struct page *follow_page_pte(struct vm_area_struct *vma,
@ -1075,11 +1066,9 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
goto out;
}
if (is_vm_hugetlb_page(vma)) {
if (should_force_cow_break(vma, foll_flags))
foll_flags |= FOLL_WRITE;
i = follow_hugetlb_page(mm, vma, pages, vmas,
&start, &nr_pages, i,
foll_flags, locked);
gup_flags, locked);
if (locked && *locked == 0) {
/*
* We've got a VM_FAULT_RETRY
@ -1093,10 +1082,6 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
continue;
}
}
if (should_force_cow_break(vma, foll_flags))
foll_flags |= FOLL_WRITE;
retry:
/*
* If we have a pending SIGKILL, don't keep faulting pages and
@ -2763,19 +2748,6 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
return -EFAULT;
/*
* The FAST_GUP case requires FOLL_WRITE even for pure reads,
* because get_user_pages() may need to cause an early COW in
* order to avoid confusing the normal COW routines. So only
* targets that are already writable are safe to do by just
* looking at the page tables.
*
* NOTE! With FOLL_FAST_ONLY we allow read-only gup_fast() here,
* because there is no slow path to fall back on. 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.
*
* Disable interrupts. The nested form is used, in order to allow
* full, general purpose use of this routine.
*
@ -2788,8 +2760,6 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
*/
if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) {
unsigned long fast_flags = gup_flags;
if (!(gup_flags & FOLL_FAST_ONLY))
fast_flags |= FOLL_WRITE;
local_irq_save(flags);
gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned);

7
mm/huge_memory.c
View File

@ -1312,12 +1312,13 @@ fallback:
}
/*
* FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
* but only after we've gone through a COW cycle and they are dirty.
* FOLL_FORCE can write to even unwritable pmd's, but only
* after we've gone through a COW cycle and they are dirty.
*/
static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
{
return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));
return pmd_write(pmd) ||
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
}
struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,