Re: [PATCH v5 01/20] list_lru: add list_lru_rotate

[Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>]

On Mon, Oct 05, 2015 at 07:02:23AM -0400, Jeff Layton wrote:
> Add a function that can move an entry to the MRU end of the list.
> 
> Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
> Cc: linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org
> Reviewed-by: Vladimir Davydov <vdavydov-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
> Signed-off-by: Jeff Layton <jeff.layton-7I+n7zu2hftEKMMhf/gKZA@public.gmane.org>

Having read through patch 10 (nfsd: add a new struct file caching
facility to nfsd) that uses this function, I think it is unnecessary
as it's usage is incorrect from the perspective of the list_lru
shrinker management.

What you are attempting to do is rotate the object to the tail of
the LRU when the last reference is dropped, so that it gets a full
trip through the LRU before being reclaimed by the shrinker. And to
ensure this "works", the scan from the shrinker checks for reference
counts and skip the item being isolated (i.e. return LRU_SKIP) and
so leave it in it's place in the LRU.

i.e. you're attempting to manage LRU-ness of the list yourself when,
in fact, the list_lru infrastructure does this and doesn't have the
subtle bugs your version has. By trying to manage it yourself, the
list_lru lists are no longer sorted into memory pressure driven
LRU order.

e.g. your manual rotation technique means if there are nr_to_walk
referenced items at the head of the list, the shrinker will skip
them all and do nothing, even though there are reclaimable objects
further down the list. i.e. it can't do any reclaim because it
doesn't sort the list into LRU order any more.

This comes from using LRU_SKIP improperly. LRU_SKIP is there for
objects that we can't lock in the isolate callback due to lock
inversion issues (e.g. see dentry_lru_isolate()), and so we need to
look at it again on the next scan pass. hence it gets left in place.

However, if we can lock the item and peer at it's reference counts
safely and we decide that we cannot reclaim it because it is
referenced, the isolate callback should be returning LRU_ROTATE
to move the referenced item to the tail of the list. (Again, see
dentry_lru_isolate() for an example.) The means that
the next nr_to_walk scan of the list will not rescan that item and
skip it again (unless the list is very short), but will instead scan
items that it hasn't yet reached.

This avoids the "shrinker does nothing due to skipped items at the
head of the list" problem, and makes the LRU function as an actual
LRU. i.e.  referenced items all cluster towards the tail of the LRU
under memory pressure and the head of the LRU contains the
reclaimable objects.

So I think the correct solution is to use LRU_ROTATE correctly
rather than try to manage the LRU list order externally like this.

Cheers,

Dave.
-- 
Dave Chinner
david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org
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