Re: pagecache locking (was: bcachefs status update) merged)

Jan Kara
2019-06-19T10:38:46Z

On Tue 18-06-19 07:21:56, Amir Goldstein wrote:
> > > Right, but regardless of the spec we have to consider that the
> > > behaviour of XFS comes from it's Irix heritage (actually from EFS,
> > > the predecessor of XFS from the late 1980s)
> >
> > Sure. And as I mentioned, I think it's technically the nicer guarantee.
> >
> > That said, it's a pretty *expensive* guarantee. It's one that you
> > yourself are not willing to give for O_DIRECT IO.
> >
> > And it's not a guarantee that Linux has ever had. In fact, it's not
> > even something I've ever seen anybody ever depend on.
> >
> > I agree that it's possible that some app out there might depend on
> > that kind of guarantee, but I also suspect it's much much more likely
> > that it's the other way around: XFS is being unnecessarily strict,
> > because everybody is testing against filesystems that don't actually
> > give the total atomicity guarantees.
> >
> > Nobody develops for other unixes any more (and nobody really ever did
> > it by reading standards papers - even if they had been very explicit).
> >
> > And honestly, the only people who really do threaded accesses to the same file
> >
> > (a) don't want that guarantee in the first place
> >
> > (b) are likely to use direct-io that apparently doesn't give that
> > atomicity guarantee even on xfs
> >
> > so I do think it's moot.
> >
> > End result: if we had a really cheap range lock, I think it would be a
> > good idea to use it (for the whole QoI implementation), but for
> > practical reasons it's likely better to just stick to the current lack
> > of serialization because it performs better and nobody really seems to
> > want anything else anyway.
> >
>
> This is the point in the conversation where somebody usually steps in
> and says "let the user/distro decide". Distro maintainers are in a much
> better position to take the risk of breaking hypothetical applications.
>
> I should point out that even if "strict atomic rw" behavior is desired, then
> page cache warmup [1] significantly improves performance.
> Having mentioned that, the discussion can now return to what is the
> preferred way to solve the punch hole vs. page cache add race.
>
> XFS may end up with special tailored range locks, which beings some
> other benefits to XFS, but all filesystems need the solution for the punch
> hole vs. page cache add race.
> Jan recently took a stab at it for ext4 [2], but that didn't work out.

Yes, but I have idea how to fix it. I just need to push acquiring ext4's
i_mmap_sem down a bit further so that only page cache filling is protected
by it but not copying of data out to userspace. But I didn't get to coding
it last week due to other stuff.

> So I wonder what everyone thinks about Kent's page add lock as the
> solution to the problem.
> Allegedly, all filesystems (XFS included) are potentially exposed to
> stale data exposure/data corruption.

When we first realized that hole-punching vs page faults races are an issue I
was looking into a generic solution but at that time there was a sentiment
against adding another rwsem to struct address_space (or inode) so we ended
up with a private lock in ext4 (i_mmap_rwsem), XFS (I_MMAPLOCK), and other
filesystems these days. If people think that growing struct inode for
everybody is OK, we can think about lifting private filesystem solutions
into a generic one. I'm fine with that.

That being said as Dave said we use those fs-private locks also for
serializing against equivalent issues arising for DAX. So the problem is
not only about page cache but generally about doing IO and caching
block mapping information for a file range. So the solution should not be
too tied to page cache.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR