Re: [PATCH v2] fs/9p: Don't open remote file with APPEND mode when writeback cache is used

[Dominique Martinet <asmadeus@codewreck.org>]

Thanks for the v2

Tingmao Wang wrote on Sun, Nov 02, 2025 at 11:56:30PM +0000:
> I haven't done a bisect to figure out if this regression was introduced by
> a change or was this behaviour always present - 6.14 has the same problem
> and 6.13 did not compile for me due to some problem with bool / true /
> false being a keyword in c23, and it could not compile with c17 either.

Ok so you got me at the "it happens in cache=loose too", so I went ahead
with bisect... I had the same problem with newer gcc being c23 and older
kernels being silly, you'd think `KCFLAGS=-std=gnu11` is enough but some
arch makefiles ignore that so I went in heavy patching
arch/x86/boot/Makefile and arch/x86/boot/compressed/Makefile . . .
... Anyway, it certainly didn't break recently, but it's not ages ago
either: the repro you made (thank you!!) starts failing in 6.4 cache
rework: the first bad commit is actually 21e26d5e54ab ("fs/9p: Fix bit
operation logic error"), but that's just a fixup of the previous commit
so 4eb3117888a9 ("fs/9p: Rework cache modes and add new options to
Documentation") is the cluprit... and... I'm not quite sure I get why
without digging deeper...
(But at least that made me realize the commit just before, 1543b4c5071c
("fs/9p: remove writeback fid and fix per-file modes"), removed the
writeback fid I was talking about... But that commit was working (with a
sane ~P9_OWRITE mask), so it's not directly that removal that broke
things, but something else in the rework)


Anyway, I think this commit is a strict improvement over the current
situation, I'll just slap a Fixes tag and push to -next for now, and if
time allows I'll try to have a closer look...

Thanks,
-- 
Dominique