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From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-block@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 16/34] iomap: add initial support for writes without buffer heads
Date: Wed, 23 May 2018 08:38:06 +1000	[thread overview]
Message-ID: <20180522223806.GX23861@dastard> (raw)
In-Reply-To: <20180522082454.GB9801@lst.de>

On Tue, May 22, 2018 at 10:24:54AM +0200, Christoph Hellwig wrote:
> On Tue, May 22, 2018 at 10:07:45AM +1000, Dave Chinner wrote:
> > > Something doesn't smell right here.  The only pages we need to read in
> > > are the first and last pages in the write_begin range, and only if they
> > > aren't page aligned and the underlying extent is IOMAP_MAPPED, right?
> > 
> > And not beyond EOF, too.
> > 
> > The bufferhead code handles this via the buffer_new() flag - it
> > triggers the skipping of read IO and the states in which it is
> > set are clearly indicated in iomap_to_bh(). That same logic needs to
> > apply here.
> 
> The buffer_new logic itself isn't really something to copy directly
> as it has all kinds of warts..

Sure, my point was that it documents all the cases where we can
avoid reading from disk, not that we should copy the logic.

> > > I also noticed that speculative preallocation kicks in by the second 80M
> > > write() call and writeback for the second call can successfully allocate
> > > the entire preallocation, which means that the third (or nth) write call
> > > can have a real extent already mapped in, and then we end up reading it.
> > 
> > Yeah, that's because there's no check against EOF here. These writes
> > are all beyond EOF, so there shouldn't be any read at all...
> 
> The EOF case is already handled in iomap_block_needs_zeroing.

Ok, I missed that detail as it's in a different patch. It looks like
if (pos > EOF) it will zeroed. But in this case I think that pos ==
EOF and so it was reading instead. That smells like off-by-one bug
to me.

> We just
> need to skip the read for ranges entirely covered by the write.

Yup.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply	other threads:[~2018-05-22 22:38 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 16:47 buffered I/O without buffer heads in xfs and iomap v2 Christoph Hellwig
2018-05-18 16:47 ` [PATCH 01/34] block: add a lower-level bio_add_page interface Christoph Hellwig
2018-05-18 16:56   ` Jens Axboe
2018-05-18 16:47 ` [PATCH 02/34] fs: factor out a __generic_write_end helper Christoph Hellwig
2018-05-18 16:47 ` [PATCH 03/34] fs: move page_cache_seek_hole_data to iomap.c Christoph Hellwig
2018-05-18 16:48 ` [PATCH 04/34] fs: remove the buffer_unwritten check in page_seek_hole_data Christoph Hellwig
2018-05-18 16:48 ` [PATCH 05/34] fs: use ->is_partially_uptodate in page_cache_seek_hole_data Christoph Hellwig
2018-05-21 19:53   ` Darrick J. Wong
2018-05-22  7:56     ` Christoph Hellwig
2018-05-18 16:48 ` [PATCH 06/34] mm: give the 'ret' variable a better name __do_page_cache_readahead Christoph Hellwig
2018-05-18 16:48 ` [PATCH 07/34] mm: return an unsigned int from __do_page_cache_readahead Christoph Hellwig
2018-05-18 16:48 ` [PATCH 08/34] mm: split ->readpages calls to avoid non-contiguous pages lists Christoph Hellwig
2018-05-22 14:52   ` Brian Foster
2018-05-18 16:48 ` [PATCH 09/34] iomap: inline data should be an iomap type, not a flag Christoph Hellwig
2018-05-18 16:48 ` [PATCH 10/34] iomap: fix the comment describing IOMAP_NOWAIT Christoph Hellwig
2018-05-18 16:48 ` [PATCH 11/34] iomap: move IOMAP_F_BOUNDARY to gfs2 Christoph Hellwig
2018-05-18 16:48 ` [PATCH 12/34] iomap: use __bio_add_page in iomap_dio_zero Christoph Hellwig
2018-05-18 16:48 ` [PATCH 13/34] iomap: add a iomap_sector helper Christoph Hellwig
2018-05-18 16:48 ` [PATCH 14/34] iomap: add an iomap-based bmap implementation Christoph Hellwig
2018-05-18 16:48 ` [PATCH 15/34] iomap: add an iomap-based readpage and readpages implementation Christoph Hellwig
2018-05-18 16:48 ` [PATCH 16/34] iomap: add initial support for writes without buffer heads Christoph Hellwig
2018-05-21 23:27   ` Darrick J. Wong
2018-05-22  0:07     ` Dave Chinner
2018-05-22  8:24       ` Christoph Hellwig
2018-05-22 22:38         ` Dave Chinner [this message]
2018-05-23  6:03           ` Christoph Hellwig
2018-05-22  8:20     ` Christoph Hellwig
2018-05-22  8:31     ` Christoph Hellwig
2018-05-22  9:39       ` Christoph Hellwig
2018-05-18 16:48 ` [PATCH 17/34] xfs: use iomap_bmap Christoph Hellwig
2018-05-18 16:48 ` [PATCH 18/34] xfs: use iomap for blocksize == PAGE_SIZE readpage and readpages Christoph Hellwig
2018-05-18 16:48 ` [PATCH 19/34] xfs: simplify xfs_bmap_punch_delalloc_range Christoph Hellwig
2018-05-18 16:48 ` [PATCH 20/34] xfs: simplify xfs_aops_discard_page Christoph Hellwig
2018-05-18 16:48 ` [PATCH 21/34] xfs: move locking into xfs_bmap_punch_delalloc_range Christoph Hellwig
2018-05-18 16:48 ` [PATCH 22/34] xfs: make xfs_writepage_map extent map centric Christoph Hellwig
2018-05-18 16:48 ` [PATCH 23/34] xfs: remove the now unused XFS_BMAPI_IGSTATE flag Christoph Hellwig
2018-05-18 16:48 ` [PATCH 24/34] xfs: remove xfs_reflink_find_cow_mapping Christoph Hellwig
2018-05-18 16:48 ` [PATCH 25/34] xfs: remove xfs_reflink_trim_irec_to_next_cow Christoph Hellwig
2018-05-18 16:48 ` [PATCH 26/34] xfs: simplify xfs_map_blocks by using xfs_iext_lookup_extent directly Christoph Hellwig
2018-05-18 16:48 ` [PATCH 27/34] xfs: don't clear imap_valid for a non-uptodate buffers Christoph Hellwig
2018-05-18 16:48 ` [PATCH 28/34] xfs: remove the imap_valid flag Christoph Hellwig
2018-05-18 16:48 ` [PATCH 29/34] xfs: don't look at buffer heads in xfs_add_to_ioend Christoph Hellwig
2018-05-18 16:48 ` [PATCH 30/34] xfs: move all writeback buffer_head manipulation into xfs_map_at_offset Christoph Hellwig
2018-05-18 16:48 ` [PATCH 31/34] xfs: remove xfs_start_page_writeback Christoph Hellwig
2018-05-18 16:48 ` [PATCH 32/34] xfs: refactor the tail of xfs_writepage_map Christoph Hellwig
2018-05-18 16:48 ` [PATCH 33/34] xfs: do not set the page uptodate in xfs_writepage_map Christoph Hellwig
2018-05-18 16:48 ` [PATCH 34/34] xfs: allow writeback on pages without buffer heads Christoph Hellwig
2018-05-21 20:46 ` buffered I/O without buffer heads in xfs and iomap v2 Darrick J. Wong
2018-05-22 20:33   ` Christoph Hellwig
2018-05-23 14:43 buffered I/O without buffer heads in xfs and iomap v3 Christoph Hellwig
2018-05-23 14:43 ` [PATCH 16/34] iomap: add initial support for writes without buffer heads Christoph Hellwig
2018-05-30  6:21   ` Darrick J. Wong

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