linux-ext4 - Re: [PATCH 16/17] block: use iomap for writes to block devices

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Date:   Wed, 24 May 2023 14:33:13 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>,
        Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] block: use iomap for writes to block devices

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@...morbit.com