Re: [PATCH 4/6] nfsd: filecache: introduce NFSD_FILE_RECENT

Jeff Layton
2025-02-13T11:27:24Z

On Thu, 2025-02-13 at 09:39 +1100, NeilBrown wrote:
> On Tue, 11 Feb 2025, Jeff Layton wrote:
> > On Mon, 2025-02-10 at 13:31 +1100, NeilBrown wrote:
> > > On Mon, 10 Feb 2025, Chuck Lever wrote:
> > > > On 2/9/25 6:23 PM, NeilBrown wrote:
> > > > > On Sat, 08 Feb 2025, Chuck Lever wrote:
> > > > > > On 2/7/25 12:15 AM, NeilBrown wrote:
> > > > > > > The filecache lru is walked in 2 circumstances for 2 different reasons.
> > > > > > >
> > > > > > > 1/ When called from the shrinker we want to discard the first few
> > > > > > > entries on the list, ignoring any with NFSD_FILE_REFERENCED set
> > > > > > > because they should really be at the end of the LRU as they have been
> > > > > > > referenced recently. So those ones are ROTATED.
> > > > > > >
> > > > > > > 2/ When called from the nfsd_file_gc() timer function we want to discard
> > > > > > > anything that hasn't been used since before the previous call, and
> > > > > > > mark everything else as unused at this point in time.
> > > > > > >
> > > > > > > Using the same flag for both of these can result in some unexpected
> > > > > > > outcomes. If the shrinker callback clears NFSD_FILE_REFERENCED then the
> > > > > > > nfsd_file_gc() will think the file hasn't been used in a while, while
> > > > > > > really it has.
> > > > > > >
> > > > > > > I think it is easier to reason about the behaviour if we instead have
> > > > > > > two flags.
> > > > > > >
> > > > > > > NFSD_FILE_REFERENCED means "this should be at the end of the LRU, please
> > > > > > > put it there when convenient"
> > > > > > > NFSD_FILE_RECENT means "this has been used recently - since the last
> > > > > > > run of nfsd_file_gc()
> > > > > > >
> > > > > > > When either caller finds an NFSD_FILE_REFERENCED entry, that entry
> > > > > > > should be moved to the end of the LRU and the flag cleared. This can
> > > > > > > safely happen at any time. The actual order on the lru might not be
> > > > > > > strictly least-recently-used, but that is normal for linux lrus.
> > > > > > >
> > > > > > > The shrinker callback can ignore the "recent" flag. If it ends up
> > > > > > > freeing something that is "recent" that simply means that memory
> > > > > > > pressure is sufficient to limit the acceptable cache age to less than
> > > > > > > the nfsd_file_gc frequency.
> > > > > > >
> > > > > > > The gc caller should primarily focus on NFSD_FILE_RECENT. It should
> > > > > > > free everything that doesn't have this flag set, and should clear the
> > > > > > > flag on everything else. When it clears the flag it is convenient to
> > > > > > > clear the "REFERENCED" flag and move to the end of the LRU too.
> > > > > > >
> > > > > > > With this, calls from the shrinker do not prematurely age files. It
> > > > > > > will focus only on freeing those that are least recently used.
> > > > > > >
> > > > > > > Signed-off-by: NeilBrown <neilb@suse.de>
> > > > > > > ---
> > > > > > > fs/nfsd/filecache.c | 21 +++++++++++++++++++--
> > > > > > > fs/nfsd/filecache.h | 1 +
> > > > > > > fs/nfsd/trace.h | 3 +++
> > > > > > > 3 files changed, 23 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > > > > > index 04588c03bdfe..9faf469354a5 100644
> > > > > > > --- a/fs/nfsd/filecache.c
> > > > > > > +++ b/fs/nfsd/filecache.c
> > > > > > > @@ -318,10 +318,10 @@ nfsd_file_check_writeback(struct nfsd_file *nf)
> > > > > > > mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK);
> > > > > > > }
> > > > > > >
> > > > > > > -
> > > > > > > static bool nfsd_file_lru_add(struct nfsd_file *nf)
> > > > > > > {
> > > > > > > set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
> > > > > > > + set_bit(NFSD_FILE_RECENT, &nf->nf_flags);
> > > > > > > if (list_lru_add_obj(&nfsd_file_lru, &nf->nf_lru)) {
> > > > > > > trace_nfsd_file_lru_add(nf);
> > > > > > > return true;
> > > > > > > @@ -528,6 +528,23 @@ nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru,
> > > > > > > return LRU_REMOVED;
> > > > > > > }
> > > > > > >
> > > > > > > +static enum lru_status
> > > > > > > +nfsd_file_gc_cb(struct list_head *item, struct list_lru_one *lru,
> > > > > > > + void *arg)
> > > > > > > +{
> > > > > > > + struct nfsd_file *nf = list_entry(item, struct nfsd_file, nf_lru);
> > > > > > > +
> > > > > > > + if (test_and_clear_bit(NFSD_FILE_RECENT, &nf->nf_flags)) {
> > > > > > > + /* "REFERENCED" really means "should be at the end of the LRU.
> > > > > > > + * As we are putting it there we can clear the flag
> > > > > > > + */
> > > > > > > + clear_bit(NFSD_FILE_REFERENCED, &nf->nf_flags);
> > > > > > > + trace_nfsd_file_gc_aged(nf);
> > > > > > > + return LRU_ROTATE;
> > > > > > > + }
> > > > > > > + return nfsd_file_lru_cb(item, lru, arg);
> > > > > > > +}
> > > > > > > +
> > > > > > > static void
> > > > > > > nfsd_file_gc(void)
> > > > > > > {
> > > > > > > @@ -537,7 +554,7 @@ nfsd_file_gc(void)
> > > > > > >
> > > > > > > for_each_node_state(nid, N_NORMAL_MEMORY) {
> > > > > > > unsigned long nr = list_lru_count_node(&nfsd_file_lru, nid);
> > > > > > > - ret += list_lru_walk_node(&nfsd_file_lru, nid, nfsd_file_lru_cb,
> > > > > > > + ret += list_lru_walk_node(&nfsd_file_lru, nid, nfsd_file_gc_cb,
> > > > > > > &dispose, &nr);
> > > > > > > }
> > > > > > > trace_nfsd_file_gc_removed(ret, list_lru_count(&nfsd_file_lru));
> > > > > > > diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
> > > > > > > index d5db6b34ba30..de5b8aa7fcb0 100644
> > > > > > > --- a/fs/nfsd/filecache.h
> > > > > > > +++ b/fs/nfsd/filecache.h
> > > > > > > @@ -38,6 +38,7 @@ struct nfsd_file {
> > > > > > > #define NFSD_FILE_PENDING (1)
> > > > > > > #define NFSD_FILE_REFERENCED (2)
> > > > > > > #define NFSD_FILE_GC (3)
> > > > > > > +#define NFSD_FILE_RECENT (4)
> > > > > > > unsigned long nf_flags;
> > > > > > > refcount_t nf_ref;
> > > > > > > unsigned char nf_may;
> > > > > > > diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
> > > > > > > index ad2c0c432d08..9af723eeb2b0 100644
> > > > > > > --- a/fs/nfsd/trace.h
> > > > > > > +++ b/fs/nfsd/trace.h
> > > > > > > @@ -1039,6 +1039,7 @@ DEFINE_CLID_EVENT(confirmed_r);
> > > > > > > { 1 << NFSD_FILE_HASHED, "HASHED" }, \
> > > > > > > { 1 << NFSD_FILE_PENDING, "PENDING" }, \
> > > > > > > { 1 << NFSD_FILE_REFERENCED, "REFERENCED" }, \
> > > > > > > + { 1 << NFSD_FILE_RECENT, "RECENT" }, \
> > > > > > > { 1 << NFSD_FILE_GC, "GC" })
> > > > > > >
> > > > > > > DECLARE_EVENT_CLASS(nfsd_file_class,
> > > > > > > @@ -1317,6 +1318,7 @@ DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_lru_del_disposed);
> > > > > > > DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_in_use);
> > > > > > > DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_writeback);
> > > > > > > DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_referenced);
> > > > > > > +DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_aged);
> > > > > > > DEFINE_NFSD_FILE_GC_EVENT(nfsd_file_gc_disposed);
> > > > > > >
> > > > > > > DECLARE_EVENT_CLASS(nfsd_file_lruwalk_class,
> > > > > > > @@ -1346,6 +1348,7 @@ DEFINE_EVENT(nfsd_file_lruwalk_class, name, \
> > > > > > > TP_ARGS(removed, remaining))
> > > > > > >
> > > > > > > DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_gc_removed);
> > > > > > > +DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_gc_recent);
> > > > > > > DEFINE_NFSD_FILE_LRUWALK_EVENT(nfsd_file_shrinker_removed);
> > > > > > >
> > > > > > > TRACE_EVENT(nfsd_file_close,
> > > > > >
> > > > > > The other patches in this series look like solid improvements. This one
> > > > > > could be as well, but it will take me some time to understand it.
> > > > > >
> > > > > > I am generally in favor of replacing the logic that removes and adds
> > > > > > these items with a single atomic bitop, and I'm happy to see NFSD stick
> > > > > > with the use of an existing LRU facility while documenting its unique
> > > > > > requirements ("nfsd_file_gc_aged" and so on).
> > > > > >
> > > > > > I would still prefer the backport to be lighter -- looks like the key
> > > > > > changes are 3/6 and 6/6. Is there any chance the series can be
> > > > > > reorganized to facilitate backporting? I have to ask, and the answer
> > > > > > might be "no", I realize.
> > > > >
> > > > > I'm going with "no".
> > > > > To be honest, I was hoping that the complexity displayed here needed
> > > > > to work around the assumptions of list_lru what don't match our needs
> > > > > would be sufficient to convince you that list_lru isn't worth pursuing.
> > > > > I see that didn't work.
> > > >
> > > > Fair enough.
> > > >
> > > >
> > > > > So I am no longer invested in this patch set. You are welcome to use it
> > > > > if you wish and to make any changes that you think are suitable, but I
> > > > > don't think it is a good direction to go and will not be offering any
> > > > > more code changes to support the use of list_lru here.
> > > >
> > > > If I may observe, you haven't offered a compelling explanation of why an
> > > > imperfect fit between list_lru and the filecache adds more technical
> > > > debt than does the introduction of a bespoke LRU mechanism.
> > > >
> > > > I'm open to that argument, but I need stronger rationale (or performance
> > > > data) to back it up. So far I can agree that the defect rate in this
> > > > area is somewhat abnormal, but that seems to be because we don't
> > > > understand how to use the list_lru API to its best advantage.
> > > >
> > >
> > > I would characterise the cause of the defect rate differently.
> > > I would say it is because we are treating this as an lru-style problem
> > > when it isn't an lru-style problem. list_lru is great for lrus. That
> > > isn't what we have.
> > >
> > > What we have is a desire to keep files open between consecutive IO
> > > requests without any clear indication of when we have seen the last in a
> > > series of IO requests. So we make a policy decision "keep files open
> > > until there have been no IOs for 2 seconds - then close them".
> > > This is a good and sensible policy that nothing to do with the "LRU"
> > > concept.
> > >
> > > We implement this policy by keeping all unused files on a list, set a
> > > flag every time the file is used, clearing the flag on a timer tick
> > > (every 2 seconds) and closing anything which still has the flag cleared
> > > 2 seconds later.
> > >
> > > Still nothing in this description that is at all related to LRU
> > > concepts.
> > >
> > > Now we decide that it would be good the add a shinker - fair enough as
> > > we don't *need* these to remain. How should the shrinker choose files
> > > to close? It probably doesn't matter beyond avoiding files that still
> > > have the not-timed-out flag set.
> > >
> > > But we try to also impose an LRU disciple over the list, and we use
> > > list_lru.
> > > The interfaces for list_lru() are well documented but the intent is
> > > not. Most users of list_lru (gfs2/quota might be an exception) only
> > > explicitly delete things from the lru when it is time to discard them
> > > completely. They rely on the shrinker to detect things that are in use
> > > again, and to remove them. And possibly to detect things that have been
> > > referenced and to rotate them. But if the shrinker doesn't run because
> > > there isn't much memory pressure they are just left alone.
> > >
> > > This is what list_lru is optimised for - for shrinker driven scanning
> > > which skips or removes or rotates things that can't or shouldn't
> > > be freed, and frees others. You would expect to normally only scan a
> > > small fraction of the list, because realistically you want to keep most
> > > of them.
> > >
> > > For filecache we don't want to keep them very long. So I think it
> > > matters a lot less what we choose for shrinking. I'm tempted to suggest
> > > we don't bother with the shrinker. Old files will be discarded soon
> > > anyway if they aren't used, and slowness in memory allocation (due to
> > > any memory pressure) will naturally slow down the addition of new files
> > > to the cache. So the cache will shrink naturally.
> > >
> > > I'm not 100% certain of that, but I do think that the needs of the
> > > shrinker should not dominate the design as they currently do.
> > >
> > > Note that maybe we *don't* need to close files so quickly. Maybe we
> > > could discard the whole timer thing, and then it would make sense to use
> > > list_lru(). What is the cost of keeping them open?
> > >
> > > All I can think of is that it affects unlink. An unlinked file won't be
> > > removed while there is a reference to the inode. Maybe we should
> > > address that by taking a lease on the file while it is in the
> > > filecache?? When the lease is broken, we discard the file from the
> > > cache.
> >
> >
> > It may also affect other applications trying to take out leases. The
> > filecache has the nfsd_file_lease_notifier that tells it when someone
> > is trying to take out a lease on a file. That happens then it will try
> > to close the file first.
>
> It wouldn't have to be an FL_LEASE lease. We could invent a new thing:
> FL_CACHED which gets added to the locks list and triggers a notification
> whenever anyone tries to break leases.
>
> The nfsd_file_lease_notifier is interesting... I wasn't aware of that.
>
>
> >
> > >
> > > If that could work (which might involve creating a new internal lease
> > > type that is only broken on unlink), then we could remove the timeout
> > > and leave files in the cache indefinitely. Then it would make perfect
> > > sense to use list_lru() because the problem would start to look exactly
> > > like an LRU problem. But I don't think that is what we have today.
> > >
> >
> > The filecache already sets a fsnotify_mark on the inode to watch for
> > its i_nlink to go to 0, and then removes it from the cache when that
> > happens. I think we could keep these files open for quite a bit longer
> > if we chose to do so.
>
> Chuck mentioned that holding he v3 files open longer could interfere
> with v4 DENY_READ etc opens. But I think they only every test for other
> v4 opens. A v3 (or local) open never blocks a v4 DENY open. Does that
> match your understanding?
>

Yes, that's correct. nfsd's share/deny locking doesn't extend past
other NFSv4 files, so that shouldn't be a problem.

> Do you know of any other reason that we currently time out files after
> 2-4 seconds?
>

No. The basic idea was to keep files open for "a little while" so that
when we're doing v3 READ/WRITE operations we can avoid having to do the
entire open/close. ~2-4s was considered long enough for that "little
while". We could easily keep them open longer, particularly since we
have some mechanisms to close them when needed for competing access
(leases and unlink activity, mostly).

> >
> > One thing that Chuck has brought up a few times is that maybe we should
> > consider making v4 not use the filecache at all. If that simplifies
> > things then that might be a good thing to consider as well.
>
> As v4 stores the file with the open state it shouldn't need the cache.
> Maybe the filecache makes life a bit easier for localio?
>
> I don't know that it would make the garbage-collection/shrinking any
> simpler but it does superficially seem like an unnecessary indirection.
> Do you know of any specific benefit it brings v4?
>

None that I can think of.

Originally, I had thought that we would keep both v2/3 and v4 files in
the cache and that they would be shared. It hasn't really worked out
that way, so we could drop v4 files from the filecache altogether.

The only real "problem" is that struct nfsd_file is referenced in a lot
of nfsv4 code, so we'd need to do a big conversion back to it using
regular struct file pointers instead.
--
Jeff Layton <jlayton@kernel.org>