linux-kernel - Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal

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Date:	Mon, 5 Oct 2015 22:55:56 +0100
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Peter Anvin <hpa@...or.com>, "Theodore Ts'o" <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal

On Mon, Oct 5, 2015 at 10:18 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Your ext4 patch may well fix the issue, and be the right thing to do
> (_regardless_ of the revert, in fact - while it might make the revert
> unnecessary, it might also be a good idea even if we do revert).

Thinking a bit more about your patch, I actually am getting more and
more convinced that it's the wrong thing to do.

Why?

Because the whole "Setting copied=0 will tell the upper layers to
repeat the write" just seems a nasty layering violation, where the
low-level filesystem uses a magic return code to introduce a special
case at the upper layers. But the upper layers are actually already
*aware* of the special case, and in fact have a comment about it.

So I think that the whole "setting copied to 0" would actually make a
lot more sense in the *caller*. Just do it in generic_perform_write()
instead. Then all the special cases and the restarting is all
together.

What do you guys think? This basically simplifies the low-level
filesystem rules, and says:

 - the filesystem will only ever see a partial "->write_end()" for the
case where the page was up-to-date, so that there is no issue with
"oops, we now have part of the page that may not have been written at
all"

 - if the page wasn't up-to-date before, ->write_end() will either be
everything we said we'd do in ->write_begin(), or it will be nothing
at all.

Hmm? This would seem to keep the special cases at the right layer, and
actually allow low-level filesystems to simplify things (ie the
special "copied = 0" special case in ext4 goes away.

The ext4 side still worries me, though. You made that
"page_zero_new_buffers()" conditional on "copied" being non-zero, but
I'm not convinced it can be conditional. Even if we retry, that retry
may end up failing (for example, because the source isn't mapped, so
we return -EFAULT rather than re-doing the write), but we have those
new buffers that got allocated in write_begin(), and now nobody has
ever written any data to them at all, so they have random stale
contents.

So I do think this needs more thought. Or at least somebody should
explain to me better why it's all ok.

I'm attaching the "copied = 0" special case thing at the
generic_perform_write() level as a patch for comments. But for now I
still think that reverting would seem to be the safer thing (which
still possibly leaves things buggy with a racy unmap, but at least
it's the old bug that we've never hit in practice).

Dave? Ted? Comments?

               Linus

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