linux-kernel - Re: [PATCH] mm: thp: Acquire the anon

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1357388687.9001.3.camel@kernel.cn.ibm.com>
Date:	Sat, 05 Jan 2013 06:24:47 -0600
From:	Simon Jeons <simon.jeons@...il.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Mel Gorman <mgorman@...e.de>, Zhouping Liu <zliu@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>, hughd@...gle.com,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] mm: thp: Acquire the anon_vma rwsem for lock during
 split

On Fri, 2013-01-04 at 17:32 -0800, Michel Lespinasse wrote:
> On Fri, Jan 4, 2013 at 6:08 AM, Mel Gorman <mgorman@...e.de> wrote:
> > Despite the reason for these commits, NUMA balancing is not the direct
> > source of the problem. split_huge_page() expected the anon_vma lock to be
> > exclusive to serialise the whole split operation. Ordinarily it is expected
> > that the anon_vma lock would only be required when updating the avcs but
> > THP also uses it. The locking requirements for THP are complex and there
> > is some overlap but broadly speaking they include the following
> >
> > 1. mmap_sem for read or write prevents THPs being created underneath
> > 2. anon_vma is taken for write if collapsing a huge page
> > 3. mm->page_table_lock should be taken when checking if pmd_trans_huge as
> >    split_huge_page can run in parallel
> > 4. wait_split_huge_page uses anon_vma taken for write mode to serialise
> >    against other THP operations
> > 5. compound_lock is used to serialise between
> >    __split_huge_page_refcount() and gup
> >
> > split_huge_page takes anon_vma for read but that does not serialise against
> > parallel split_huge_page operations on the same page (rule 2). One process
> > could be modifying the ref counts while the other modifies the page tables
> > leading to counters not being reliable. This patch takes the anon_vma
> > lock for write to serialise against parallel split_huge_page and parallel
> > collapse operations as it is the most fine-grained lock available that
> > protects against both.
> 
> Your comment about this being the most fine-grained lock made me
> think, couldn't we use lock_page() on the THP page here ?
> 
> Now I don't necessarily want to push you that direction, because I
> haven't fully thought it trough and because what you propose brings us
> closer to what happened before anon_vma became an rwlock, which is
> more obviously safe. But I felt I should still mention it, since we're
> really only trying to protect from concurrent operations on the same
> THP page, so locking at just that granularity would seem desirable.

Why you said that anon_vma lock who will protect page associated to a
list of vmas is fine-grained then page lock who just protect one page?

> 


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/