From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>,
Johannes Weiner <hannes@cmpxchg.org>,
linux-fsdevel@vger.kernel.org, Jan Kara <jack@suse.cz>
Subject: Re: [PATCH 1/2] mm/filemap: don't initiate writeback if mapping has no dirty pages
Date: Tue, 23 Jul 2019 11:16:51 +0300 [thread overview]
Message-ID: <bdc6c53d-a7bb-dcc4-20ba-6c7fa5c57dbd@yandex-team.ru> (raw)
In-Reply-To: <20190722175230.d357d52c3e86dc87efbd4243@linux-foundation.org>
On 23.07.2019 3:52, Andrew Morton wrote:
>
> (cc linux-fsdevel and Jan)
>
> On Mon, 22 Jul 2019 12:36:08 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:
>
>> Functions like filemap_write_and_wait_range() should do nothing if inode
>> has no dirty pages or pages currently under writeback. But they anyway
>> construct struct writeback_control and this does some atomic operations
>> if CONFIG_CGROUP_WRITEBACK=y - on fast path it locks inode->i_lock and
>> updates state of writeback ownership, on slow path might be more work.
>> Current this path is safely avoided only when inode mapping has no pages.
>>
>> For example generic_file_read_iter() calls filemap_write_and_wait_range()
>> at each O_DIRECT read - pretty hot path.
>>
>> This patch skips starting new writeback if mapping has no dirty tags set.
>> If writeback is already in progress filemap_write_and_wait_range() will
>> wait for it.
>>
>> ...
>>
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -408,7 +408,8 @@ int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start,
>> .range_end = end,
>> };
>>
>> - if (!mapping_cap_writeback_dirty(mapping))
>> + if (!mapping_cap_writeback_dirty(mapping) ||
>> + !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
>> return 0;
>>
>> wbc_attach_fdatawrite_inode(&wbc, mapping->host);
>
> How does this play with tagged_writepages? We assume that no tagging
> has been performed by any __filemap_fdatawrite_range() caller?
>
Checking also PAGECACHE_TAG_TOWRITE is cheap but seems redundant.
To-write tags are supposed to be a subset of dirty tags:
to-write is set only when dirty is set and cleared after starting writeback.
Special case set_page_writeback_keepwrite() which does not clear to-write
should be for dirty page thus dirty tag is not going to be cleared either.
Ext4 calls it after redirty_page_for_writepage()
XFS even without clear_page_dirty_for_io()
Anyway to-write tag without dirty tag or at clear page is confusing.
next prev parent reply other threads:[~2019-07-23 8:16 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-22 9:36 [PATCH 1/2] mm/filemap: don't initiate writeback if mapping has no dirty pages Konstantin Khlebnikov
2019-07-22 9:36 ` [PATCH 2/2] mm/filemap: rewrite mapping_needs_writeback in less fancy manner Konstantin Khlebnikov
2019-07-23 0:52 ` [PATCH 1/2] mm/filemap: don't initiate writeback if mapping has no dirty pages Andrew Morton
2019-07-23 8:16 ` Konstantin Khlebnikov [this message]
2019-07-30 14:14 ` Jan Kara
2019-07-30 14:57 ` Konstantin Khlebnikov
2019-07-30 15:48 ` Jan Kara
2019-07-30 18:15 ` Konstantin Khlebnikov
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