From: Jens Axboe <axboe@kernel.dk>
To: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Cc: hch@infradead.org, akpm@linux-foundation.org
Subject: [PATCHSET 0/3] Improve IOCB_NOWAIT O_DIRECT
Date: Mon, 8 Feb 2021 15:18:26 -0700 [thread overview]
Message-ID: <20210208221829.17247-1-axboe@kernel.dk> (raw)
Hi,
Ran into an issue with IOCB_NOWAIT and O_DIRECT, which causes a rather
serious performance issue. If IOCB_NOWAIT is set, the generic/iomap
iterators check for page cache presence in the given range, and return
-EAGAIN if any is there. This is rather simplistic and looks like
something that was never really finished. For !IOCB_NOWAIT, we simply
call filemap_write_and_wait_range() to issue (if any) and wait on the
range. The fact that we have page cache entries for this range does
not mean that we cannot safely do O_DIRECT IO to/from it.
This series adds filemap_range_needs_writeback(), which checks if
we have pages in the range that do require us to call
filemap_write_and_wait_range(). If we don't, then we can proceed just
fine with IOCB_NOWAIT.
The problem manifested itself in a production environment, where someone
is doing O_DIRECT on a raw block device. Due to other circumstances,
blkid was triggered on this device periodically, and blkid very helpfully
does a number of page cache reads on the device. Now the mapping has
page cache entries, and performance falls to pieces because we can no
longer reliably do IOCB_NOWAIT O_DIRECT.
Patch 1 adds the helper, patch 2 uses it for the generic iterators, and
patch 3 applies the same to the iomap direct-io code.
fs/iomap/direct-io.c | 10 ++++-----
include/linux/fs.h | 2 ++
mm/filemap.c | 52 +++++++++++++++++++++++++++++++++++++++++---
3 files changed, 56 insertions(+), 8 deletions(-)
--
Jens Axboe
next reply other threads:[~2021-02-08 22:19 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-08 22:18 Jens Axboe [this message]
2021-02-08 22:18 ` [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper Jens Axboe
2021-02-08 23:02 ` Matthew Wilcox
2021-02-08 23:21 ` Jens Axboe
2021-02-08 22:18 ` [PATCH 2/3] mm: use filemap_range_needs_writeback() for O_DIRECT IO Jens Axboe
2021-02-08 22:18 ` [PATCH 3/3] iomap: " Jens Axboe
2021-02-08 23:28 ` [PATCHSET 0/3] Improve IOCB_NOWAIT O_DIRECT Dave Chinner
2021-02-08 23:37 ` Jens Axboe
2021-02-09 0:14 ` Dave Chinner
2021-02-09 2:15 ` Jens Axboe
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