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mm: introduce __page_add_new_anon_rmap()

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When dealing with speculative page fault handler, we may race with VMA
being split or merged. In this case the vma->vm_start and vm->vm_end
fields may not match the address the page fault is occurring.

This can only happens when the VMA is split but in that case, the
anon_vma pointer of the new VMA will be the same as the original one,
because in __split_vma the new->anon_vma is set to src->anon_vma when
*new = *vma.

So even if the VMA boundaries are not correct, the anon_vma pointer is
still valid.

If the VMA has been merged, then the VMA in which it has been merged
must have the same anon_vma pointer otherwise the merge can't be done.

So in all the case we know that the anon_vma is valid, since we have
checked before starting the speculative page fault that the anon_vma
pointer is valid for this VMA and since there is an anon_vma this
means that at one time a page has been backed and that before the VMA
is cleaned, the page table lock would have to be grab to clean the
PTE, and the anon_vma field is checked once the PTE is locked.

This patch introduce a new __page_add_new_anon_rmap() service which
doesn't check for the VMA boundaries, and create a new inline one
which do the check.

When called from a page fault handler, if this is not a speculative one,
there is a guarantee that vm_start and vm_end match the faulting address,
so this check is useless. In the context of the speculative page fault
handler, this check may be wrong but anon_vma is still valid as explained
above.

Change-Id: I72c47830181579f8c9618df879077d321653b5f1
Signed-off-by: Laurent Dufour <[email protected]>
Patch-mainline: linux-mm @ Tue, 17 Apr 2018 16:33:22
[[email protected]: trivial merge conflict fixes +
checkpatch fixes]
Signed-off-by: Vinayak Menon <[email protected]>
This commit is contained in:
Laurent Dufour 2018-04-17 16:33:22 +02:00 • committed by Vinayak Menon
commit d56126780c
3 changed files with 16 additions and 9 deletions
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5
mm/rmap.c
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@ -1138,7 +1138,7 @@ void do_page_add_anon_rmap(struct page *page,
}
/**
* page_add_new_anon_rmap - add pte mapping to a new anonymous page
* __page_add_new_anon_rmap - add pte mapping to a new anonymous page
* @page: the page to add the mapping to
* @vma: the vm area in which the mapping is added
* @address: the user virtual address mapped
@ -1148,12 +1148,11 @@ void do_page_add_anon_rmap(struct page *page,
* This means the inc-and-test can be bypassed.
* Page does not have to be locked.
*/
void page_add_new_anon_rmap(struct page *page,
void __page_add_new_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address, bool compound)
{
int nr = compound ? hpage_nr_pages(page) : 1;
VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
__SetPageSwapBacked(page);
if (compound) {
VM_BUG_ON_PAGE(!PageTransHuge(page), page);