Re: [PATCH v2] mm: add lazyfree folio to lru tail

[Lokesh Gidra]

Thanks Suren for looping in

On Fri, Aug 23, 2024 at 4:39 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Wed, Aug 21, 2024 at 2:47 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> >
> > On Wed, Aug 21, 2024 at 8:46 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Fri 16-08-24 07:48:01, gaoxu wrote:
> > > > Replace lruvec_add_folio with lruvec_add_folio_tail in the lru_lazyfree_fn:
> > > > 1. The lazy-free folio is added to the LRU_INACTIVE_FILE list. If it's
> > > >    moved to the LRU tail, it allows for faster release lazy-free folio and
> > > >    reduces the impact on file refault.
> > >
> > > This has been discussed when MADV_FREE was introduced. The question was
> > > whether this memory has a lower priority than other inactive memory that
> > > has been marked that way longer ago. Also consider several MADV_FREE
> > > users should they be LIFO from the reclaim POV?

Thinking from the user's perspective, it seems to me that FIFO within
MADV_FREE'ed pages makes more sense. As a user I expect the longer a
MADV_FREE'ed page hasn't been touched, the chances are higher that it
may not be around anymore.
> >
> > The priority of this memory compared to other inactive memory that has been
> > marked for a longer time likely depends on the user's expectations - How soon
> > do users expect MADV_FREE to be reclaimed compared with old file folios.
> >
> > art guys moved to MADV_FREE from MADV_DONTNEED without any
> > useful performance data and reason in the changelog:
> > https://android-review.googlesource.com/c/platform/art/+/2633132
> >
> > Since art is the Android Java heap, it can be quite large. This increases the
> > likelihood of packing the file LRU and reduces the chances of reclaiming
> > anonymous memory, which could result in more file re-faults while helping
> > anonymous folio persist longer in memory.

Individual heaps of android apps are not big, and even in there we
don't call MADV_FREE on the entire heap.
> >
> > I am really curious why art guys have moved to MADV_FREE if we have
> > an approach to reach them.

Honestly, it makes little sense as a user that calling MADV_FREE on an
anonymous mapping will impact file LRU. That was never the intention
with our ART change.

>From our perspective, once a set of pages are MADV_FREE'ed, they are
like a page-cache. It gives an opportunity, without hurting memory
use, to avoid overhead of page-faults, which happen frequently after
GC is done on running apps.

IMHO, within LRU_INACTIVE_FILE, MADV_FREE'ed pages should be
prioritized for reclamation over file ones.
>
> Adding Lokesh.
> Lokesh, could you please comment on the reasoning behind the above
> mentioned change?

Adding Nicolas as well, in case he wants to add something.
>
> >
> > >
> > > --
> > > Michal Hocko
> > > SUSE Labs
> > >
> >
> > Thanks
> > Barry