linux-kernel - Re: [PATCH] iomap: move prefaulting out of hot write path

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Message-ID: <486185f6-7da7-4fdc-9206-8f1eebd341cf@intel.com>
Date: Thu, 9 Oct 2025 08:15:00 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Darrick J. Wong" <djwong@...nel.org>, alexjlzheng@...il.com,
 dave.hansen@...ux.intel.com
Cc: brauner@...nel.org, linux-xfs@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jinliang Zheng <alexjlzheng@...cent.com>
Subject: Re: [PATCH] iomap: move prefaulting out of hot write path

On 10/9/25 08:01, Darrick J. Wong wrote:
> On Thu, Oct 09, 2025 at 05:08:51PM +0800, alexjlzheng@...il.com wrote:
>> From: Jinliang Zheng <alexjlzheng@...cent.com>
>>
>> Prefaulting the write source buffer incurs an extra userspace access
>> in the common fast path. Make iomap_write_iter() consistent with
>> generic_perform_write(): only touch userspace an extra time when
>> copy_folio_from_iter_atomic() has failed to make progress.
>>
>> This patch is inspired by commit 665575cff098 ("filemap: move
>> prefaulting out of hot write path").
> Seems fine to me, but I wonder if dhansen has any thoughts about this
> patch ... which exactly mirrors one he sent eight months ago?

I don't _really_ care all that much. But, yeah, I would have expected
a little shout-out or something when someone copies the changelog and
code verbatim from another patch:

	https://lore.kernel.org/lkml/20250129181753.3927F212@davehans-spike.ostc.intel.com/

and then copies a comment from a second patch I did.

But I guess I was cc'd at least. Also, if my name isn't on this one,
then I don't have to fix any of the bugs it causes. Right? ;)

Just one warning: be on the lookout for bugs in the area. The
prefaulting definitely does a good job of hiding bugs in other bits
of the code. The generic_perform_write() gunk seems to have uncovered
a bug or two.

Also, didn't Christoph ask you to make the comments wider the last
time Alex posted this? I don't think that got changed.

	https://lore.kernel.org/lkml/aIt8BYa6Ti6SRh8C@infradead.org/

Overall, the change still seems as valid to me as it did when I wrote the
patch in the first place. Although it feels funny to ack my own
patch.