Re: [PATCH RFC] mm: mglru: provide a separate list for lazyfree anon folios

[Barry Song <21cnbao@gmail.com>]

On Wed, Sep 18, 2024 at 12:02 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 14.09.24 08:37, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > This follows up on the discussion regarding Gaoxu's work[1]. It's
> > unclear if there's still interest in implementing a separate LRU
> > list for lazyfree folios, but I decided to explore it out of
> > curiosity.
> >
> > According to Lokesh, MADV_FREE'd anon folios are expected to be
> > released earlier than file folios. One option, as implemented
> > by Gao Xu, is to place lazyfree anon folios at the tail of the
> > file's `min_seq` generation. However, this approach results in
> > lazyfree folios being released in a LIFO manner, which conflicts
> > with LRU behavior, as noted by Michal.
> >
> > To address this, this patch proposes maintaining a separate list
> > for lazyfree anon folios while keeping them classified under the
> > "file" LRU type to minimize code changes. These lazyfree anon
> > folios will still be counted as file folios and share the same
> > generation with regular files. In the eviction path, the lazyfree
> > list will be prioritized for scanning before the actual file
> > LRU list.
> >
>
> What's the downside of another LRU list? Do we have any experience on that?

Essentially, the goal is to address the downsides of using a single LRU list for
files and lazyfree anonymous pages - seriously more files re-faults.

I'm not entirely clear on the downsides of having an additional LRU
list. While it
does increase complexity, it doesn't seem to be significant.

Let's wait for Gaoxu's test results before deciding on the next steps.
I was just
curious about how difficult it would be to add a separate list, so I
took two hours
to explore it :-)

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry