linux-kernel - Re: implicit AOP_FLAG_NOFS for grab_cache_page_write

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]

Date:   Fri, 17 Apr 2020 00:29:31 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>
Subject: Re: implicit AOP_FLAG_NOFS for grab_cache_page_write_begin

On Wed, Apr 15, 2020 at 09:02:28AM +0200, Michal Hocko wrote:
> Hi,
> I have just received a bug report about memcg OOM [1]. The underlying
> issue is memcg specific but the stack trace made me look at the write(2)
> patch and I have noticed that iomap_write_begin enforces AOP_FLAG_NOFS
> which means that all the page cache that has to be allocated is
> GFP_NOFS. What is the reason for this? Do all filesystems really need
> the reclaim protection? I was hoping that those filesystems which really
> need NOFS context would be using the scope API
> (memalloc_nofs_{save,restore}.

This comes from the historic XFS code, and this commit from Dave
in particular:

commit aea1b9532143218f8599ecedbbd6bfbf812385e1
Author: Dave Chinner <dchinner@...hat.com>
Date:   Tue Jul 20 17:54:12 2010 +1000

    xfs: use GFP_NOFS for page cache allocation

    Avoid a lockdep warning by preventing page cache allocation from
    recursing back into the filesystem during memory reclaim.