Re: [PATCH] mm: fix possible cause of a page_mapped BUG

[Michel Lespinasse <walken@google.com>]

On Wed, May 4, 2011 at 5:38 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>> A more conservative alternative could
>> be to enable the guard page special case under an new GUP flag, but
>> this loses much of the elegance of your original proposal...
>
> How about only doing that only for FOLL_MLOCK?

Sounds reasonable.

> Also, looking at mm/mlock.c, why _do_ we call get_user_pages() even if
> the vma isn't mlocked? That looks bogus. Since we have dropped the
> mm_semaphore, an unlock may have happened, and afaik we should *not*
> try to bring those pages back in at all. There's this whole comment
> about that in the caller ("__mlock_vma_pages_range() double checks the
> vma flags, so that it won't mlock pages if the vma was already
> munlocked."), but despite that it would actually call
> __get_user_pages() even if the VM_LOCKED bit had been cleared (it just
> wouldn't call it with the FOLL_MLOCK flag).

There are two reasons VM_LOCKED might be cleared in
__mlock_vma_pages_range(). It could be that one of the VM_SPECIAL
flags were set on the VMA, in which case mlock() won't set VM_LOCKED
but it still must make the pages present. Or, there is an munlock()
executing concurrently with mlock() - in that case, the conservative
thing to do is to give the same results as if the mlock() had
completed before the munlock(). That is, the mlock() would have broken
COW / made the pages present and the munlock() would have cleared the
VM_LOCKED and PageMlocked flags.

> UNTESTED! And maybe there was some really subtle reason to still call
> __get_user_pages() without that FOLL_MLOCK thing that I'm missing.

I think we want the mm/memory.c part of this proposal without the
mm/mlock.c part.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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