linux-kernel - Re: [PATCH] ext4: Fix deadlock on page reclaim

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Message-ID: <20190726225508.GA13729@mit.edu>
Date:   Fri, 26 Jul 2019 18:55:08 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Dave Chinner <david@...morbit.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Damien Le Moal <damien.lemoal@....com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Naohiro Aota <naohiro.aota@....com>,
        Masato Suzuki <masato.suzuki@....com>
Subject: Re: [PATCH] ext4: Fix deadlock on page reclaim

On Sat, Jul 27, 2019 at 08:44:23AM +1000, Dave Chinner wrote:
> > 
> > This looks like something that could hit every file systems, so
> > shouldn't we fix this in common code?  We could also look into
> > just using memalloc_nofs_save for the page cache allocation path
> > instead of the per-mapping gfp_mask.
> 
> I think it has to be the entire IO path - any allocation from the
> underlying filesystem could recurse into the top level filesystem
> and then deadlock if the memory reclaim submits IO or blocks on
> IO completion from the upper filesystem. That's a bloody big hammer
> for something that is only necessary when there are stacked
> filesystems like this....

Yeah.... that's why using memalloc_nofs_save() probably makes the most
sense, and dm_zoned should use that before it calls into ext4.

       	   	    	       	    	   - Ted