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Revert "gup: document and work around "COW can break either way" issue" 

This reverts commit 918f50807eccd63d482ef4cf778b1d2b416770a9.
the commit force COW to write model, which force COW breaking, and cause
page usage increase a lot. On upstream, commit 376a34efa ("mm/gup:
refactor and de-duplicate gup_fast() code") give another way to fix fork
secuirty issue of COW, and then revert the buggy commit by commit a308c71bf1
("mm/gup: Remove enfornced COW mechanism")

Signed-off-by: Alex Shi <alexsshi@tencent.com>
This commit is contained in:
Alex Shi 2023-02-20 23:00:30 +08:00 committed by Jianping Liu
parent 2453865ed4
commit 6bc9581ddd
3 changed files with 10 additions and 49 deletions
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8
drivers/gpu/drm/i915/gem/i915_gem_userptr.c
View File

@ -619,14 +619,6 @@ 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 */
pinned = __get_user_pages_fast(obj->userptr.ptr,
num_pages,

44
mm/gup.c
View File

@ -161,22 +161,13 @@ 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);
return pte_write(pte) ||
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}
static struct page *follow_page_pte(struct vm_area_struct *vma,
@ -832,18 +823,12 @@ 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, nonblocking);
gup_flags, nonblocking);
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
@ -2331,10 +2316,6 @@ 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)
@ -2362,12 +2343,6 @@ 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) &&
@ -2440,17 +2415,10 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
if (unlikely(!access_ok((void __user *)start, len)))
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.
*/
if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
gup_fast_permitted(start, end)) {
local_irq_disable();
gup_pgd_range(addr, end, gup_flags | FOLL_WRITE, pages, &nr);
gup_pgd_range(addr, end, gup_flags, pages, &nr);
local_irq_enable();
ret = nr;
}

7
mm/huge_memory.c
View File

@ -1457,12 +1457,13 @@ out_unlock:
}
/*
* 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,