linux-ext4 - Re: [PATCH v4 0/2] ext4: fix DAX dma vs truncate/hole-punch

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Message-ID: <20180806154943.GA17666@lst.de>
Date:   Mon, 6 Aug 2018 17:49:43 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dave Chinner <david@...morbit.com>
Cc:     Ross Zwisler <zwisler@...nel.org>, Jan Kara <jack@...e.cz>,
        linux-nvdimm@...ts.01.org, darrick.wong@...cle.com,
        lczerner@...hat.com, linux-ext4 <linux-ext4@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v4 0/2] ext4: fix DAX dma vs truncate/hole-punch

> > > This allows the direct I/O path to do I/O and raise & lower page->_refcount
> > > while we're executing a truncate/hole punch.  This leads to us trying to free
> > > a page with an elevated refcount.
> 
> I don't see how this is possible in XFS - maybe I'm missing
> something, but "direct IO submission during truncate" is not
> something that should ever be happening in XFS, DAX or not.

The pages involved in a direct I/O are not that of the file that
the direct I/O read/write syscalls are called on, but those of the
memory regions the direct I/O read/write syscalls operate on.
Those pages could be file backed and undergo a truncate at the
same time.