linux-kernel - Re: [PATCH] mm: fix possible cause of a page

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Date:	Wed, 4 May 2011 17:38:40 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Robert Święcki <robert@...ecki.net>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] mm: fix possible cause of a page_mapped BUG

On Wed, May 4, 2011 at 5:09 PM, Michel Lespinasse <walken@...gle.com> wrote:
>
> FYI, the attached code causes an infinite loop in kernels that have
> the 95042f9eb7 commit:

Mmm.

Yes. The atomic fault will never work, and the get_user_pages() thing
won't either, so things will just loop forever.

> Linus, I am not sure as to what would be the preferred way to fix
> this. One option could be to modify fault_in_user_writeable so that it
> passes a non-NULL page pointer, and just does a put_page on it
> afterwards. While this would work, this is kinda ugly and would slow
> down futex operations somewhat.

No, that's just ugly as hell.

> 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?

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).

So maybe something like the attached?

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

                           Linus

View attachment "patch.diff" of type "text/x-patch" (1442 bytes)