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Date:   Wed, 24 May 2023 14:33:13 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Dave Chinner <david@fromorbit.com>
Cc:     Hannes Reinecke <hare@suse.de>, Christoph Hellwig <hch@lst.de>,
        Jens Axboe <axboe@kernel.dk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Howells <dhowells@redhat.com>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
        linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 16/17] block: use iomap for writes to block devices
Message-ID: <ZG4SGYOogQtEZrll@casper.infradead.org>
References: <20230424054926.26927-1-hch@lst.de>
 <20230424054926.26927-17-hch@lst.de>
 <b96b397e-2f5e-7910-3bb3-7405d0e293a7@suse.de>
 <ZG09wR4WOI8zDxJK@dread.disaster.area>
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On Wed, May 24, 2023 at 08:27:13AM +1000, Dave Chinner wrote:
> On Fri, May 19, 2023 at 04:22:01PM +0200, Hannes Reinecke wrote:
> > I'm hitting this during booting:
> > [    5.016324]  <TASK>
> > [    5.030256]  iomap_iter+0x11a/0x350
> > [    5.030264]  iomap_readahead+0x1eb/0x2c0
> > [    5.030272]  read_pages+0x5d/0x220
> > [    5.030279]  page_cache_ra_unbounded+0x131/0x180
> > [    5.030284]  filemap_get_pages+0xff/0x5a0
> 
> Why is filemap_get_pages() using unbounded readahead? Surely
> readahead should be limited to reading within EOF....

It isn't using unbounded readahead; that's an artifact of this
incomplete stack trace.  Actual call stack:

page_cache_ra_unbounded
do_page_cache_ra
ondemand_readahead
page_cache_sync_ra
page_cache_sync_readahead
filemap_get_pages

As you can see, do_page_cache_ra() does limit readahead to i_size.
Is ractl->mapping->host the correct way to find the inode?  I always
get confused.

> I think Christoph's code is correct. IMO, any attempt to read beyond
> the end of the device should throw out a warning and return an
> error, not silently return zeros.
> 
> If readahead is trying to read beyond the end of the device, then it
> really seems to me like the problem here is readahead, not the iomap
> code detecting the OOB read request....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com