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fscache: Implement simple cookie state machine

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Implement a very simple cookie state machine to handle lookup,
invalidation, withdrawal, relinquishment and, to be added later, commit on
LRU discard.

Three cache methods are provided: ->lookup_cookie() to look up and, if
necessary, create a data storage object; ->withdraw_cookie() to free the
resources associated with that object and potentially delete it; and
->prepare_to_write(), to do prepare for changes to the cached data to be
modified locally.

Changes
=======
ver #3:
 - Fix a race between LRU discard and relinquishment whereby the former
   would override the latter and thus the latter would never happen[1].

ver #2:
 - Don't hold n_accesses elevated whilst cache is bound to a cookie, but
   rather add a flag that prevents the state machine from being queued when
   n_accesses reaches 0.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/599331.1639410068@warthog.procyon.org.uk/ [1]
Link: https://lore.kernel.org/r/163819599657.215744.15799615296912341745.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906903925.143852.1805855338154353867.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967105456.1823006.14730395299835841776.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021510706.640689.7961423370243272583.stgit@warthog.procyon.org.uk/ # v4
This commit is contained in:
David Howells 2021-10-20 15:53:34 +01:00
parent 29f18e79fe
commit 5d00e426f9
3 changed files with 303 additions and 47 deletions
Download patch file Download diff file Expand all files Collapse all files

27
include/linux/fscache-cache.h
View file

@ -57,6 +57,15 @@ struct fscache_cache_ops {
/* Free the cache's data attached to a volume */
void (*free_volume)(struct fscache_volume *volume);
/* Look up a cookie in the cache */
bool (*lookup_cookie)(struct fscache_cookie *cookie);
/* Withdraw an object without any cookie access counts held */
void (*withdraw_cookie)(struct fscache_cookie *cookie);
/* Prepare to write to a live cache object */
void (*prepare_to_write)(struct fscache_cookie *cookie);
};
extern struct workqueue_struct *fscache_wq;
@ -72,6 +81,7 @@ extern int fscache_add_cache(struct fscache_cache *cache,
void *cache_priv);
extern void fscache_withdraw_cache(struct fscache_cache *cache);
extern void fscache_withdraw_volume(struct fscache_volume *volume);
extern void fscache_withdraw_cookie(struct fscache_cookie *cookie);
extern void fscache_io_error(struct fscache_cache *cache);
@ -85,8 +95,21 @@ extern void fscache_put_cookie(struct fscache_cookie *cookie,
enum fscache_cookie_trace where);
extern void fscache_end_cookie_access(struct fscache_cookie *cookie,
enum fscache_access_trace why);
extern void fscache_set_cookie_state(struct fscache_cookie *cookie,
enum fscache_cookie_state state);
extern void fscache_cookie_lookup_negative(struct fscache_cookie *cookie);
extern void fscache_caching_failed(struct fscache_cookie *cookie);
/**
* fscache_cookie_state - Read the state of a cookie
* @cookie: The cookie to query
*
* Get the state of a cookie, imposing an ordering between the cookie contents
* and the state value. Paired with fscache_set_cookie_state().
*/
static inline
enum fscache_cookie_state fscache_cookie_state(struct fscache_cookie *cookie)
{
return smp_load_acquire(&cookie->state);
}
/**
* fscache_get_key - Get a pointer to the cookie key