linux-kernel - Re: [RFC 3/9] selftests: block_seek_hole: add loop block driver tests

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Message-ID: <20240403135142.GC2524049@fedora>
Date: Wed, 3 Apr 2024 09:51:42 -0400
From: Stefan Hajnoczi <stefanha@...hat.com>
To: Eric Blake <eblake@...hat.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alasdair Kergon <agk@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>, dm-devel@...ts.linux.dev,
	David Teigland <teigland@...hat.com>,
	Mike Snitzer <snitzer@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@....de>, Joe Thornber <ejt@...hat.com>
Subject: Re: [RFC 3/9] selftests: block_seek_hole: add loop block driver tests

On Fri, Mar 29, 2024 at 07:38:17AM -0500, Eric Blake wrote:
> On Thu, Mar 28, 2024 at 04:39:04PM -0400, Stefan Hajnoczi wrote:
> > Run the tests with:
> > 
> >   $ make TARGETS=block_seek_hole -C tools/selftests run_tests
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@...hat.com>
> > ---
> >  tools/testing/selftests/Makefile              |   1 +
> >  .../selftests/block_seek_hole/Makefile        |  17 +++
> >  .../testing/selftests/block_seek_hole/config  |   1 +
> >  .../selftests/block_seek_hole/map_holes.py    |  37 +++++++
> >  .../testing/selftests/block_seek_hole/test.py | 103 ++++++++++++++++++
> >  5 files changed, 159 insertions(+)
> >  create mode 100644 tools/testing/selftests/block_seek_hole/Makefile
> >  create mode 100644 tools/testing/selftests/block_seek_hole/config
> >  create mode 100755 tools/testing/selftests/block_seek_hole/map_holes.py
> >  create mode 100755 tools/testing/selftests/block_seek_hole/test.py
> > 
> 
> > +
> > +def map_holes(fd):
> > +    end = os.lseek(fd, 0, os.SEEK_END)
> > +    offset = 0
> > +
> > +    print('TYPE START END SIZE')
> > +
> > +    while offset < end:
> > +        contents = 'DATA'
> > +        new_offset = os.lseek(fd, offset, os.SEEK_HOLE)
> > +        if new_offset == offset:
> > +            contents = 'HOLE'
> > +            try:
> > +              new_offset = os.lseek(fd, offset, os.SEEK_DATA)
> > +            except OSError as err:
> > +                if err.errno == errno.ENXIO:
> > +                    new_offset = end
> > +                else:
> > +                    raise err
> > +            assert new_offset != offset
> > +        print(f'{contents} {offset} {new_offset} {new_offset - offset}')
> > +        offset = new_offset
> 
> Over the years, I've seen various SEEK_HOLE implementation bugs where
> things work great on the initial boundary, but fail when requested on
> an offset not aligned to the start of the extent boundary.  It would
> probably be worth enhancing the test to prove that:
> 
> if lseek(fd, offset, SEEK_HOLE) == offset:
>   new_offset = lseek(fd, offset, SEEK_DATA)
>   assert new_offset > offset
>   assert lseek(fd, new_offset - 1, SEEK_HOLE) == new_offset - 1
> else:
>   assert lseek(fd, offset, SEEK_DATA) == offset
>   new_offset = lseek(fd, offset, SEEK_HOLE)
>   assert new_offset > offset
>   assert lseek(fd, new_offset - 1, SEEK_DATA) == new_offset - 1
> 
> Among other things, this would prove that even though block devices
> generally operate on a minimum granularity of a sector, lseek() still
> gives byte-accurate results for a random offset that falls in the
> middle of a sector, and doesn't accidentally round down reporting an
> offset less than the value passed in to the request.

Sure. I'll add a test for that.

Stefan

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