linux-kernel - implicit AOP_FLAG_NOFS for grab_cache_page_write

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Message-ID: <20200415070228.GW4629@dhcp22.suse.cz>
Date:   Wed, 15 Apr 2020 09:02:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: implicit AOP_FLAG_NOFS for grab_cache_page_write_begin

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}.

Could you clarify please?

[1] http://lkml.kernel.org/r/20200414212558.58eaab4de2ecf864eaa87e5d@linux-foundation.org
-- 
Michal Hocko
SUSE Labs