linux-kernel - Re: Sync writeback still broken

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Message-ID: <AANLkTikEpV7GWG9fd3=zfrhOZtbPvxJWfXAO30iN8aae@mail.gmail.com>
Date:	Fri, 29 Oct 2010 18:16:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>, stable@...nel.org,
	Greg KH <gregkh@...e.de>, Jens Axboe <jaxboe@...ionio.com>
Subject: Re: Sync writeback still broken

On Fri, Oct 29, 2010 at 5:57 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> Guys, what is the status of this?
>
> The original patch in that email thread still makes no sense and the
> commit log for it cannot be the real issue. But the _problem_ seems to
> be real, and the code is apparently a total mess, still.

Btw, is the problem just that insane WB_SYNC_ALL thing?

The problem with WB_SYNC_ALL seems to be that it synchrnously writes
out one inode at a time. And it's not just the data, it's the inode
itself.

So rather than write out all pages for _all_ inodes, and then wait for
them, and write out _all_ metadata, and then wait for that, it seems
like the WB_SYNC_ALL code does the truly stupid thing, which is to
"write out some data for one inode, then _synchronously_ wait for
that, then write out the metadata for that single inode, then
_synchronously_ wait for THAT" and then rinse and repeat for each
inode.

The sane thing for "WB_SYNC_ALL" would be to:
 - for_each_inode: write out all data (no waiting)
 - for_each_inode: wait for the data for that inode, write out the inode
 - for_each_inode: wait for the inode

so that you avoid the whole synchronous wait thing, and can do all
inodes in one go.

I dunno. Who even uses WB_SYNC_ALL? It's just "sync()" itself, isn't
it? And "umount()", I guess. I didn't actually look at the code.

                  Linus
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