Hi,
On 25/11/2019 17:05, Linus Torvalds wrote:
On Mon, Nov 25, 2019 at 2:53 AM Steven Whitehouse <swhiteho@xxxxxxxxxx> wrote:
Linus, is that roughly what you were thinking of?
So the concept looks ok, but I don't really like the new flags as they
seem to be gfs2-specific rather than generic.
That said, I don't _gate_ them either, since they aren't in any
critical code sequence, and it's not like they do anything really odd.
I still think the whole gfs2 approach is broken. You're magically ok
with using stale data from the cache just because it's cached, even if
another client might have truncated the file or something.
If another node tries to truncate the file, that will require an
exclusive glock, and in turn that means the all the other nodes will
have to drop their glock(s) shared or exclusive. That process
invalidates the page cache on those nodes, such that any further
requests on those nodes will find the cache empty and have to call into
the filesystem.
If a page is truncated on another node, then when the local node gives
up its glock, after any copying (e.g. for read) has completed then the
truncate will take place. The local node will then have to reread any
data relating to new pages or return an error in case the next page to
be read has vanished due to the truncate. It is a pretty small window,
and the advantage is that in cases where the page is in cache, we can
directly use the cached page without having to call into the filesystem
at all. So it is page atomic in that sense.
The overall aim here is to avoid taking (potentially slow) cluster locks
when at all possible, yet at the same time deliver close to local fs
semantics whenever we can. You can think of GFS2's glock concept (at
least as far as the inodes we are discussing here) as providing a
combination of (page) cache control and cluster (dlm) locking.
So you're ok with saying "the file used to be X bytes in size, so
we'll just give you this data because we trust that the X is correct".
But you're not ok to say "oh, the file used to be X bytes in size, but
we don't want to give you a short read because it might not be correct
any more".
See the disconnect? You trust the size in one situation, but not in another one.
Well we are not trusting the size at all... the original algorithm
worked entirely off "is this page in cache and uptodate?" and for
exactly the reason that we know the size in the inode might be out of
date, if we are not currently holding a glock in either shared or
exclusive mode. We also know that if there is a page in cache and
uptodate then we must be holding the glock too.
I also don't really see that you *need* the new flag at all. Since
you're doing to do a speculative read and then a real read anyway, and
since the only thing that you seem to care about is the file size
(because the *data* you will trust if it is cached), then why don't
you just use the *existing* generic read, and *IFF* you get a
truncated return value, then you go and double-check that the file
hasn't changed in size?
See what I'm saying? I think gfs2 is being very inconsistent in when
it trusts the file size, and I don't see that you even need the new
behavior that patch gives, because you might as well just use the
existing code (just move the i_size check earlier, and then teach gfs2
to double-check the "I didn't get as much as I expected" case).
Linus
I'll leave the finer details to Andreas here, since it is his patch, and
hopefully we can figure out a good path forward. We are perhaps also a
bit reluctant to go off and (nearly) duplicate code that is already in
the core vfs library functions, since that often leads to things getting
out of sync (our implementation of ->writepages is one case where that
happened in the past) and missing important bug fixes/features in some
cases. Hopefully though we can iterate on this a bit and come up with
something which will resolve all the issues,
Steve.
