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From: Dave Chinner <david@fromorbit.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>,
	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: Tue, 22 May 2018 10:07:45 +1000	[thread overview]
Message-ID: <20180522000745.GU23861@dastard> (raw)
In-Reply-To: <20180521232700.GB14384@magnolia>

On Mon, May 21, 2018 at 04:27:00PM -0700, Darrick J. Wong wrote:
> On Fri, May 18, 2018 at 06:48:12PM +0200, Christoph Hellwig wrote:
> > For now just limited to blocksize == PAGE_SIZE, where we can simply read
> > in the full page in write begin, and just set the whole page dirty after
> > copying data into it.  This code is enabled by default and XFS will now
> > be feed pages without buffer heads in ->writepage and ->writepages.
> > 
> > If a file system sets the IOMAP_F_BUFFER_HEAD flag on the iomap the old
> > path will still be used, this both helps the transition in XFS and
> > prepares for the gfs2 migration to the iomap infrastructure.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/iomap.c            | 132 ++++++++++++++++++++++++++++++++++++++----
> >  fs/xfs/xfs_iomap.c    |   6 +-
> >  include/linux/iomap.h |   2 +
> >  3 files changed, 127 insertions(+), 13 deletions(-)
> > 
> > diff --git a/fs/iomap.c b/fs/iomap.c
> > index 821671af2618..cd4c563db80a 100644
> > --- a/fs/iomap.c
> > +++ b/fs/iomap.c
> > @@ -314,6 +314,58 @@ iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
> >  		truncate_pagecache_range(inode, max(pos, i_size), pos + len);
> >  }
> >  
> > +static int
> > +iomap_read_page_sync(struct inode *inode, loff_t block_start, struct page *page,
> > +		unsigned poff, unsigned plen, struct iomap *iomap)
> > +{
> > +	struct bio_vec bvec;
> > +	struct bio bio;
> > +	int ret;
> > +
> > +	bio_init(&bio, &bvec, 1);
> > +	bio.bi_opf = REQ_OP_READ;
> > +	bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
> > +	bio_set_dev(&bio, iomap->bdev);
> > +	__bio_add_page(&bio, page, plen, poff);
> > +	ret = submit_bio_wait(&bio);
> > +	if (ret < 0 && iomap_block_needs_zeroing(inode, block_start, iomap))
> > +		zero_user(page, poff, plen);
> > +	return ret;
> > +}
> > +
> > +static int
> > +__iomap_write_begin(struct inode *inode, loff_t pos, unsigned len,
> > +		struct page *page, struct iomap *iomap)
> > +{
> > +	loff_t block_size = i_blocksize(inode);
> > +	loff_t block_start = pos & ~(block_size - 1);
> > +	loff_t block_end = (pos + len + block_size - 1) & ~(block_size - 1);
> > +	unsigned poff = block_start & (PAGE_SIZE - 1);
> > +	unsigned plen = min_t(loff_t, PAGE_SIZE - poff, block_end - block_start);
> > +	int status;
> > +
> > +	WARN_ON_ONCE(i_blocksize(inode) < PAGE_SIZE);
> > +
> > +	if (PageUptodate(page))
> > +		return 0;
> > +
> > +	if (iomap_block_needs_zeroing(inode, block_start, iomap)) {
> > +		unsigned from = pos & (PAGE_SIZE - 1), to = from + len;
> > +		unsigned pend = poff + plen;
> > +
> > +		if (poff < from || pend > to)
> > +			zero_user_segments(page, poff, from, to, pend);
> > +	} else {
> > +		status = iomap_read_page_sync(inode, block_start, page,
> > +				poff, plen, iomap);
> 
> 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.

> 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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply	other threads:[~2018-05-22  0:07 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 [this message]
2018-05-22  8:24       ` Christoph Hellwig
2018-05-22 22:38         ` Dave Chinner
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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