Re: [PATCH 0/4] nfsd: filecache: change garbage collect lists

Chuck Lever
2025-01-23T14:29:20Z

On 1/22/25 5:16 PM, NeilBrown wrote:
> On Thu, 23 Jan 2025, Chuck Lever wrote:
>> On 1/21/25 10:54 PM, NeilBrown wrote:
>>>
>>> The nfsd filecache currently uses list_lru for tracking files recently
>>> used in NFSv3 requests which need to be "garbage collected" when they
>>> have becoming idle - unused for 2-4 seconds.
>>>
>>> I do not believe list_lru is a good tool for this. It does no allow the
>>> timeout which filecache requires so we have to add a timeout mechanism
>>> which holds the list_lru for while the whole list is scanned looking for
>>> entries that haven't been recently accessed. When the list is largish
>>> (even a few hundred) this can block new requests which need the lock to
>>> remove a file to access it.
>>>
>>> This patch removes the list_lru and instead uses 2 simple linked lists.
>>> When a file is accessed it is removed from whichever list it is one,
>>> then added to the tail of the first list. Every 2 seconds the second
>>> list is moved to the "freeme" list and the first list is moved to the
>>> second list. This avoids any need to walk a list to find old entries.
>>>
>>> These lists are per-netns rather than global as the freeme list is
>>> per-netns as the actual freeing is done in nfsd threads which are
>>> per-netns.
>>>
>>> This should not be applied until we resolve how to handle the
>>> race-detection code in nfsd_file_put(). However I'm posting it now to
>>> get any feedback so that a final version can be posted as soon as that
>>> issue is resolved.
>>>
>>> Thanks,
>>> NeilBrown
>>>
>>>
>>> [PATCH 1/4] nfsd: filecache: use nfsd_file_dispose_list() in
>>> [PATCH 2/4] nfsd: filecache: move globals nfsd_file_lru and
>>> [PATCH 3/4] nfsd: filecache: change garbage collection list
>>> [PATCH 4/4] nfsd: filecache: change garbage collection to a timer.
>>
>> Hi Neil -
>>
>> I would like Dave Chinner to chime in on this approach. When you
>> resend, please Cc: him. Thanks!
>
> Sure, I can do that. But why Dave in particular?
> I would like to add a comment to the cover letter explaining to Dave
> what he was added to see and I don't know what to say.

Dave helped me with edead3a55804 ("NFSD: Fix the filecache LRU
shrinker") and a few other related fixes, and he is the original
author of the list_lru facility (see a38e40824844 ("list: add a new LRU
list type"). He might have additional insight on whether the filecache's
use of list_lru is salvageable or how a replacement of that mechanism
should work.

--
Chuck Lever