On Sun, May 25, 2025 at 01:32:33PM -0700, Linus Torvalds wrote:
> But yeah, maybe the drop-behind case never triggers in practice, and I
> should just revert commit 974c5e6139db ("xfs: flag as supporting
> FOP_DONTCACHE") for now.
>
> That's kind of sad too, but at least that's new to 6.15 and we
> wouldn't have a kernel release that triggers this issue.
>
> I realize that Vlastimil had a suggested possible fix, but doing
> _that_ kind of surgery at this point in the release isn't an option,
> I'm afraid. And delaying 6.15 for this also seems a bit excessive - if
> it turns out to be easy to fix, we can always just backport the fix
> and undo the revert.
>
> Sounds like a plan?
>
> I'm somewhat surprised that this was only noticed now if it triggers
> so easily for Al with xfstests on xfs. But better late than never, I
> guess..
I wonder if we shouldn't do ...
+++ b/include/linux/fs.h
@@ -3725,6 +3725,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags,
return -EOPNOTSUPP;
}
if (flags & RWF_DONTCACHE) {
+ /* Houston, we have a problem */
+ return -EOPNOTSUPP;
/* file system must support it */
if (!(ki->ki_filp->f_op->fop_flags & FOP_DONTCACHE))
return -EOPNOTSUPP;
in case some other filesystem adds support for it? I don't see anything
in -next right now, but I see Darrick playing with it here for FUSE:
https://lore.kernel.org/all/174787195629.1483178.7917092102987513364.stgit@frogsfrogsfrogs/
Jeff playing with it for nfsd here:
https://lore.kernel.org/all/370dd4ae06d44f852342b7ee2b969fc544bd1213.camel@xxxxxxxxxx/
Trond implementing it for NFS client here:
https://lore.kernel.org/all/cover.1745381692.git.trond.myklebust@xxxxxxxxxxxxxxx/
I thought I saw someone implement it for ext4, but perhaps I'm confused
with something else. Anyway, some kind of not-xfs-specific patch is
appropriate here, I think?
Oh, and we're only just seeing it, I think, because you need to recompile
xfstests to test this functionality ... and I certainly don't re-pull
and re-compile xfstests on a regular basis; I just use the one I pulled
and compiled, um, months ago.
