linux-kernel - Re: [PATCH] Optimize wait_sb

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Date:	Thu, 27 Jun 2013 16:22:04 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Dave Chinner <david@...morbit.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tux3@...3.org
Subject: Re: [PATCH] Optimize wait_sb_inodes()

Dave Chinner <david@...morbit.com> writes:

>> Otherwise, vfs can't know the data is whether after sync point or before
>> sync point, and have to wait or not. FS is using the behavior like
>> data=journal has tracking of those already, and can reuse it.
>
> The VFS writeback code already differentiates between data written
> during a sync operation and that dirtied after a sync operation.
> Perhaps you should look at the tagging for WB_SYNC_ALL writeback
> that write_cache_pages does....
>
> But, anyway, we don't have to do that on the waiting side of things.
> All we need to do is add the inode to a "under IO" list on the bdi
> when the mapping is initially tagged with pages under writeback,
> and remove it from that list during IO completion when the mapping
> is no longer tagged as having pages under writeback.
>
> wait_sb_inodes() just needs to walk that list and wait on each inode
> to complete IO. It doesn't require *any awareness* of the underlying
> filesystem implementation or how the IO is actually issued - if
> there's IO in progress at the time wait_sb_inodes() is called, then
> it waits for it.
>
>> > Fix the root cause of the problem - the sub-optimal VFS code.
>> > Hacking around it specifically for out-of-tree code is not the way
>> > things get done around here...
>> 
>> I'm thinking the root cause is vfs can't have knowledge of FS internal,
>> e.g. FS is handling data transactional way, or not.
>
> If the filesystem has transactional data/metadata that the VFS is
> not tracking, then that is what the ->sync_fs call is for. i.e. so
> the filesystem can then do what ever extra writeback/waiting it
> needs to do that the VFS is unaware of.
>
> We already cater for what Tux3 needs in the VFS - all you've done is
> found an inefficient algorithm that needs fixing.

write_cache_pages() is library function to be called from per-FS. So, it
is not under vfs control can be assume already. And it doesn't do right
things via write_cache_pages() for data=journal, because it handles for
each inodes, not at once. So, new dirty data can be inserted while
marking.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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