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

[Chuck Lever <chuck.lever@oracle.com>]

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.


-- 
Chuck Lever