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authorPeter Xu <peterx@redhat.com>2020-08-21 19:49:57 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2020-09-04 09:25:20 -0700
commita308c71bf1e6e19cc2e4ced31853ee0fc7cb439a (patch)
treef419a838cdc2f2616adfed4994465738a4167cac /mm
parent1a0cf26323c80e2f1c58fc04f15686de61bfab0c (diff)
mm/gup: Remove enfornced COW mechanism
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>
Diffstat (limited to 'mm')
-rw-r--r--mm/gup.c40
-rw-r--r--mm/huge_memory.c7
2 files changed, 9 insertions, 38 deletions
diff --git a/mm/gup.c b/mm/gup.c
index 6f47697f8fb0..e3bd0d0b5120 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -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);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78c84bee7e29..6f74f3d93839 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -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,