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 <ldufour@linux.vnet.ibm.com>
Link: https://lore.kernel.org/lkml/1523975611-15978-17-git-send-email-ldufour@linux.vnet.ibm.com/
Bug: 161210518
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
a1dbf20e8e