Re: [PATCH v2 1/3] fs: add infrastructure for multigrain inode i_m/ctime

[Jeff Layton]

On Thu, 2023-04-27 at 11:51 +0200, Christian Brauner wrote:
> On Wed, Apr 26, 2023 at 05:48:38AM -0400, Jeff Layton wrote:
> > On Wed, 2023-04-26 at 09:07 +0200, Christian Brauner wrote:
> > > On Mon, Apr 24, 2023 at 11:11:02AM -0400, Jeff Layton wrote:
> > > > The VFS always uses coarse-grained timestamp updates for filling out the
> > > > ctime and mtime after a change. This has the benefit of allowing
> > > > filesystems to optimize away a lot metaupdates, to around once per
> > > > jiffy, even when a file is under heavy writes.
> > > > 
> > > > Unfortunately, this has always been an issue when we're exporting via
> > > > NFSv3, which relies on timestamps to validate caches. Even with NFSv4, a
> > > > lot of exported filesystems don't properly support a change attribute
> > > > and are subject to the same problems with timestamp granularity. Other
> > > > applications have similar issues (e.g backup applications).
> > > > 
> > > > Switching to always using fine-grained timestamps would improve the
> > > > situation for NFS, but that becomes rather expensive, as the underlying
> > > > filesystem will have to log a lot more metadata updates.
> > > > 
> > > > What we need is a way to only use fine-grained timestamps when they are
> > > > being actively queried:
> > > > 
> > > > Whenever the mtime changes, the ctime must also change since we're
> > > > changing the metadata. When a superblock has a s_time_gran >1, we can
> > > > use the lowest-order bit of the inode->i_ctime as a flag to indicate
> > > > that the value has been queried. Then on the next write, we'll fetch a
> > > > fine-grained timestamp instead of the usual coarse-grained one.
> > > > 
> > > > We could enable this for any filesystem that has a s_time_gran >1, but
> > > > for now, this patch adds a new SB_MULTIGRAIN_TS flag to allow filesystems
> > > > to opt-in to this behavior.
> > > 
> > > Hm, the patch raises the flag in s_flags. Please at least move this to
> > > s_iflags as SB_I_MULTIGRAIN and treat this as an internal flag. There's
> > > no need to give the impression that this will become a mount option.
> > > 
> > > Also, this looks like it's a filesystem property not a superblock
> > > property as the granularity isn't changeable. So shouldn't this be an
> > > FS_* flag instead?
> > 
> > It could be a per-sb thing if there was some filesystem that wanted to
> > do that, but I'm hoping that most will not want to do that.
> 
> Yeah, I'd really hope this isn't an sb thing.
> 
> > 
> > My initial patches for this actually did use a FS_* flag, but I figured
> 
> Oh, I might've just missed that.
> 

Sorry, I didn't actually post that set. But I did go with a FS_* flag
before I made it a SB_* flag.

> > that was one more pointer to chase when you wanted to check the flag.
> 
> Hm, unless you have reasons to think that it would be noticable in terms
> of perf I'd rather do the correct thing and have it be an FS_* flag.

Sure. I'll make the switch before the next posting.

Thanks for the review!
-- 
Jeff Layton <jlayton@xxxxxxxxxx>