From: Andrea Arcangeli <aarcange@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, bugzilla-daemon@bugzilla.kernel.org,
bugme-daemon@bugzilla.kernel.org, bugs@casparzhang.com
Subject: Re: [Bugme-new] [Bug 33682] New: mprotect got stuck when THP is "always" enabled
Date: Tue, 19 Apr 2011 13:25:06 +0200 [thread overview]
Message-ID: <20110419112506.GB5641@random.random> (raw)
In-Reply-To: <20110418230651.54da5b82.akpm@linux-foundation.org>
On Mon, Apr 18, 2011 at 11:06:51PM -0700, Andrew Morton wrote:
>
> (switched to email. Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
Ok.
/dev/zero mapped with MAP_PRIVATE is special as it doesn't set vm_ops
but is vm_file backed. The page fault code only checks the vm_ops
being NULL to decide if to use THP or not (it doesn't check
vm_file). But then the huge_memory.c code also checks vm_file (like in
vma_adjust_trans_huge). So I think what's going on in the special case
of /dev/zero MAP_PRIVATE is that we could have THP enabled on a
filebacked vma (but no vm_ops backed) that doesn't fully enable all
operations necessary to be safe like vma_adjust_trans_huge because
vm_file being set. To let /dev/zero to use THP safely I guess we can
remove the vm_file checks and only use the vm_ops checks across
huge_memory.c/huge_mm.h or to add add "!vma->vm_ops && !vma->vm_file"
to the page fault so THP isn't used. The former is more risky than
adding the vm_file check in the page fault path but it'd match more
closely what do_anonymous_page does, so I'm thinking what are the
implications of removing vm_file across the huge_memory.c/huge_mm.h. I
didn't manage to reboot into a fixed kernel to test yet... I'll let
you know the results after some testing.
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next prev parent reply other threads:[~2011-04-19 11:26 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <bug-33682-10286@https.bugzilla.kernel.org/>
2011-04-19 6:06 ` [Bugme-new] [Bug 33682] New: mprotect got stuck when THP is "always" enabled Andrew Morton
2011-04-19 11:25 ` Andrea Arcangeli [this message]
2011-04-19 13:51 ` Andrea Arcangeli
2011-04-19 18:59 ` Rik van Riel
2011-04-20 10:01 ` Mel Gorman
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