Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed

[Michal Hocko <mhocko@kernel.org>]

On Wed 29-08-18 15:58:52, Waiman Long wrote:
> On 08/29/2018 03:51 AM, Michal Hocko wrote:
> > On Tue 28-08-18 13:19:40, Waiman Long wrote:
> >> For negative dentries that are accessed once and never used again, they
> >> should be removed first before other dentries when shrinker is running.
> >> This is done by putting negative dentries at the head of the LRU list
> >> instead at the tail.
> >>
> >> A new DCACHE_NEW_NEGATIVE flag is now added to a negative dentry when it
> >> is initially created. When such a dentry is added to the LRU, it will be
> >> added to the head so that it will be the first to go when a shrinker is
> >> running if it is never accessed again (DCACHE_REFERENCED bit not set).
> >> The flag is cleared after the LRU list addition.
> > Placing object to the head of the LRU list can be really tricky as Dave
> > pointed out. I am not familiar with the dentry cache reclaim so my
> > comparison below might not apply. Let me try anyway.
> >
> > Negative dentries sound very similar to MADV_FREE pages from the reclaim
> > POV. They are primary candidate for reclaim, yet you want to preserve
> > aging to other easily reclaimable objects (including other MADV_FREE
> > pages). What we do for those pages is to move them from the anonymous
> > LRU list to the inactive file LRU list. Now you obviously do not have
> > anon/file LRUs but something similar to active/inactive LRU lists might
> > be a reasonably good match. Have easily reclaimable dentries on the
> > inactive list including negative dentries. If negative entries are
> > heavily used then they can promote to the active list because there is
> > no reason to reclaim them soon.
> >
> > Just my 2c
> 
> As mentioned in my reply to Dave, I did considered using a 2 LRU list
> solution. However, that will add more complexity to the dcache LRU
> management code than my current approach and probably more potential for
> slowdown.

I completely agree with Dave here. This is not easy but trying to sneak
in something that works for an _artificial_ workload is simply a no go.
So if it takes to come with a more complex solution to cover more
general workloads then be it. Someone has to bite a bullet and explore
that direction. It won't be a simple project but well, if negative
dentries really matter then it is worth making the reclaim design robust
and comprehensible rather than adhoc and unpredictable.
-- 
Michal Hocko
SUSE Labs