linux-kernel - Re: [REGRESSION] 9pfs issues on 6.12-rc1

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Message-ID: <aF49vp50BkfjJOTG@codewreck.org>
Date: Fri, 27 Jun 2025 15:44:14 +0900
From: Dominique Martinet <asmadeus@...ewreck.org>
To: Christian Theune <ct@...ingcircus.io>,
	David Howells <dhowells@...hat.com>
Cc: Ryan Lahfa <ryan@...fa.xyz>, Antony Antony <antony.antony@...unet.com>,
	Antony Antony <antony@...nome.org>,
	Christian Brauner <brauner@...nel.org>,
	Eric Van Hensbergen <ericvh@...nel.org>,
	Latchesar Ionkov <lucho@...kov.net>,
	Christian Schoenebeck <linux_oss@...debyte.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Maximilian Bosch <maximilian@...sch.me>,
	regressions@...ts.linux.dev, v9fs@...ts.linux.dev,
	netfs@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION] 9pfs issues on 6.12-rc1

Hi all,

sorry for the slow reply; I wasn't in Cc of most of the mails back in
October so this is a pain to navigate... Let me recap a bit:
- stuff started failing in 6.12-rc1
- David first posted "9p: Don't revert the I/O iterator after
reading"[1], which fixed the bug, but then found a "better" fix as
"iov_iter: Fix iov_iter_get_pages*() for folio_queue" [2] which was
merged instead (so the first patch was not merged)

But it turns out the second patch is not enough (or causes another
issue?), and the reverting it + applying first one works, is that
correct?
What happens if you keep [2] and just apply [1], does that still bug?

(I've tried reading through the thread now and I don't even see what was
the "bad" patch in the first place, although I assume it's ee4cdf7ba857
("netfs: Speed up buffered reading") -- was that confirmed?)

David, as you worked on this at the time it'd be great if you could have
another look, I have no idea what made you try [1] in the first place
but unless you think 9p is doing something wrong like double-reverting
on error or something like that I'd like to understand a bit more what
happens... Although given 6.12 is getting used more now it could make
sense to just apply [1] first until we understand, and have a proper fix
come second -- if someone can confirm we don't need to revert [2].

[1] https://lore.kernel.org/all/3327438.1729678025@warthog.procyon.org.uk/T/#mc97a248b0f673dff6dc8613b508ca4fd45c4fefe
[2] https://lore.kernel.org/all/3327438.1729678025@warthog.procyon.org.uk/T/#m89597a1144806db4ae89992953031cdffa0b0bf9

Thanks,
-- 
Dominique