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mm/hugetlb.c: use the right pte val for compare in hugetlb_cow

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We cannot use the pte value used in set_pte_at for pte_same comparison,
because archs like ppc64, filter/add new pte flag in set_pte_at.
Instead fetch the pte value inside hugetlb_cow.  We are comparing pte
value to make sure the pte didn't change since we dropped the page table
lock.  hugetlb_cow get called with page table lock held, and we can take
a copy of the pte value before we drop the page table lock.

With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no
previous mapping (huge_pte_none entries), by forcing a cow in the fault
path.  This avoid take an addition fault to covert a read-only mapping
to read/write.  Here we were comparing a recently instantiated pte (via
set_pte_at) to the pte values from linux page table.  As explained above
on ppc64 such pte_same check returned wrong result, resulting in us
taking an additional fault on ppc64.

Fixes: 6a119eae94 ("powerpc/mm: Add a _PAGE_PTE bit")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Aneesh Kumar K.V <[email protected]>
Reported-by: Jan Stancek <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Scott Wood <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Aneesh Kumar K.V 2016-12-12 16:41:56 -08:00 • committed by Linus Torvalds
commit 3999f52e31
1 changed files with 7 additions and 5 deletions
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12
mm/hugetlb.c
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@ -3450,15 +3450,17 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
* Keep the pte_same checks anyway to make transition from the mutex easier.
*/
static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long address, pte_t *ptep, pte_t pte,
struct page *pagecache_page, spinlock_t *ptl)
unsigned long address, pte_t *ptep,
struct page *pagecache_page, spinlock_t *ptl)
{
pte_t pte;
struct hstate *h = hstate_vma(vma);
struct page *old_page, *new_page;
int ret = 0, outside_reserve = 0;
unsigned long mmun_start; /* For mmu_notifiers */
unsigned long mmun_end; /* For mmu_notifiers */
pte = huge_ptep_get(ptep);
old_page = pte_page(pte);
retry_avoidcopy:
@ -3733,7 +3735,7 @@ retry:
hugetlb_count_add(pages_per_huge_page(h), mm);
if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
/* Optimization, do the COW without a second fault */
ret = hugetlb_cow(mm, vma, address, ptep, new_pte, page, ptl);
ret = hugetlb_cow(mm, vma, address, ptep, page, ptl);
}
spin_unlock(ptl);
@ -3888,8 +3890,8 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
if (flags & FAULT_FLAG_WRITE) {
if (!huge_pte_write(entry)) {
ret = hugetlb_cow(mm, vma, address, ptep, entry,
pagecache_page, ptl);
ret = hugetlb_cow(mm, vma, address, ptep,
pagecache_page, ptl);
goto out_put_page;
}
entry = huge_pte_mkdirty(entry);