Re: THP, rmap and page_referenced_one()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> Hi,
> 
> I have been wondering about the following:
> 
> Before the THP work, the if (vma->vm_flags & VM_LOCKED) test in
> page_referenced_one() was placed after the page_check_address() call,
> but now it is placed above it. Could this be a problem ?
> 
> My understanding is that the page_check_address() check may return
> false positives - for example, if an anon page was created before a
> process forked, rmap will indicate that the page could be mapped in
> both of the processes, even though one of them might have since broken
> COW. What would happen if the child process mlocks the corresponding
> VMA ? my understanding is that this would break COW, but not cause
> rmap to be updated, so the parent's page would still be marked in rmap
> as being possibly mapped in the children's VM_LOCKED vma. With the
> VM_LOCKED check now placed above the page_check_address() call, this
> would cause vmscan to see both the parent's and the child's pages as
> being unevictable.
> 
> Am I missing something there ? In particular, I am not sure if marking
> the children's VMA as mlocked would somehow cause rmap to realize it
> can't share pages with the parent anymore (but I don't think that's
> the case, and it could cause other issues if it was...)

Hi

I think you are right. 
page_check_address() should be called before VM_LOCKED check.


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