linux-ext4 - Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal

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Date:	Tue, 6 Oct 2015 10:01:04 +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 Tue, Oct 6, 2015 at 12:33 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
>
> Did you mean that as a cleanup, or to fix the regression?

Purely as a cleanup to try to avoid the (already existing) special
case in at least ext4 - and possibly others.

So that patch was meant just for discussion - it's not really fixing
any existing bugs, and I didn't actually keep it live in my tree.

I'm planning on just doing the revert for now, but I'll wait a bit to
see how this thread pans out first.

> Since the page isn't faulted in yet, iov_iter_copy_from_user_atomic()
> had already set copied=0

Not necessarily. For your case that only does one-byte writes, yes.
But in the generic case you may well have a page-crossing source area
in user space, and get a partial success from the user copy. It's that
partial success case (when the rest of the missing data isn't
necessarily already up-to-date) that I'd like to have low-level
filesystems not have to worry about.

           Linus
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