[PATCH v9 16/24] mm: Introduce __page_add_new_anon_rmap()

[David Rientjes]

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> 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.
> 
> Signed-off-by: Laurent Dufour <ldufour at linux.vnet.ibm.com>

I'm indifferent on this: it could be argued both sides that the new 
function and its variant for a simple VM_BUG_ON() isn't worth it and it 
would should rather be done in the callers of page_add_new_anon_rmap().  
It feels like it would be better left to the caller and add a comment to 
page_add_anon_rmap() itself in mm/rmap.c.