Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 17 Apr 2020 00:29:31 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     "Darrick J. Wong" <darrick.wong@oracle.com>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Dave Chinner <david@fromorbit.com>
Subject: Re: implicit AOP_FLAG_NOFS for grab_cache_page_write_begin
Message-ID: <20200417072931.GA20822@infradead.org>
References: <20200415070228.GW4629@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200415070228.GW4629@dhcp22.suse.cz>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@redhat.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.