Message-ID: <CAGsJ_4zYRCotNTL2kKO202Rk2o78w5pZs25Y_iq=7OfS-KoX9w@mail.gmail.com>
Date: Fri, 18 Oct 2024 18:12:01 +1300
From: Barry Song <21cnbao@...il.com>
To: Minchan Kim <minchan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, yuzhao@...gle.com, linux-mm@...ck.org, 
	david@...hat.com, fengbaopeng@...or.com, gaoxu2@...or.com, 
	hailong.liu@...o.com, kaleshsingh@...gle.com, linux-kernel@...r.kernel.org, 
	lokeshgidra@...gle.com, mhocko@...e.com, ngeoffray@...gle.com, shli@...com, 
	surenb@...gle.com, v-songbaohua@...o.com, yipengxiang@...or.com, 
	Gao Xu <gaoxu2@...onor.com>
Subject: Re: [PATCH v2] mm: mglru: provide a separate list for lazyfree anon folios

On Fri, Oct 18, 2024 at 6:58 AM Minchan Kim <minchan@...nel.org> wrote:
>
> On Thu, Oct 17, 2024 at 06:59:09PM +1300, Barry Song wrote:
> > On Thu, Oct 17, 2024 at 11:58 AM Andrew Morton
> > <akpm@...ux-foundation.org> wrote:
> > >
> > > On Wed, 16 Oct 2024 16:30:30 +1300 Barry Song <21cnbao@...il.com> wrote:
> > >
> > > > 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.
> > >
> > > Thanks.  I'll await input from other MGLRU developers before adding
> > > this for testing.
> >
> > Thanks!
> >
> > Hi Minchan, Yu,
> >
> > Any comments? I understand that Minchan may have a broader plan
> > to "enable the system to maintain a quickly reclaimable memory
> > pool and provide a knob for admins to control its size." While I
> > have no objection to that plan, I believe improving MADV_FREE
> > performance is a more urgent priority and a low-hanging fruit at this
> > stage.
>
> Hi Barry,
>
> I have no idea why my email didn't send well before. I sent following
> reply on Sep 24. Hope it works this time.

Hi Minchan,

I guess not. Your *this* email ended up in my spam folder of gmail, and
my oppo.com account still hasn’t received it. Any idea why?

>
> ====== &< ======
>
> My proposal involves the following:
>
> 1. Introduce an "easily reclaimable" LRU list. This list would hold pages
>    that can be quickly freed without significant overhead.

I assume you plan to keep both lazyfree anon pages and 'reclaimed'
file folios (reclaimed in the normal LRU lists but still in the easily-
reclaimable list) in this 'easily reclaimable' LRU list. However, I'm
not sure this will work, as this patch aims to help reclaim lazyfree
anon pages before file folios to reduce both file and anon refaults.
If we place 'reclaimed' file folios and lazyfree anon folios in the
same list, we may need to revisit how to reclaim lazyfree anon folios
before reclaiming the 'reclaimed' file folios.

>
> 2. Implement a parameter to control the size of this list. This allows for
>    system tuning based on available memory and performance requirements.

If we include only 'reclaimed' file folios in this 'easily
reclaimable' LRU list, the
parameter makes sense. However, if we also add lazyfree folios to the list, the
parameter becomes less meaningful since we can't predict how many
lazyfree anon folios user space might have. I still feel lazyfree anon folios
are different with "reclaimed" file folios (I mean reclaimed from normal
lists but still in 'easily-reclaimable' list).

>
> 3. Modify kswapd behavior to utilize this list. When kswapd is awakened due
>    to memory pressure, it should attempt to drop those pages first to refill
>    free pages up to the high watermark by first reclaiming.
>
> 4. Before kswapd goes to sleep, it should scan the tail of the LRU list and
>    move cold pages to the easily reclaimable list, unmapping them from the
>    page table.
>
> 5. Whenever page cache hit, move the page into evictable LRU.
>
> This approach allows the system to maintain a pool of readily available
> memory, mitigating the "aging" problem. The trade-off is the potential for
> minor page faults and LRU movement ovehreads if these pages in ez_reclaimable
> LRU are accessed again.

I believe you're aware of an implementation from Samsung that uses
cleancache. Although it was dropped from the mainline kernel, it still
exists in the Android kernel. Samsung's rbincache, based on cleancache,
maintains a reserved memory region for holding reclaimed file folios.
Instead of LRU movement, rbincache uses memcpy to transfer data between
the pool and the page cache.

>
> Furthermore, we could put some asynchrnous writeback pages(e.g., swap
> out or writeback the fs pages) into the list, too.
> Currently, what we are doing is rotate those pages back to head of LRU
> and once writeback is done, move the page to the tail of LRU again.
> We can simply put the page into ez_reclaimable LRU without rotating
> back and forth.

If this is about establishing a pool of easily reclaimable file folios, I
fully support the idea and am eager to try it, especially for Android,
where there are certainly strong use cases. However, I suspect it may
be controversial and could take months to gain acceptance. Therefore,
I’d prefer we first focus on landing a smaller change to address the
madv_free performance issue and treat that idea as a separate
incremental patch set.

My current patch specifically targets the issue of reclaiming lazyfree
anon folios before reclaiming file folios. It appears your proposal is
independent (though related) work, and I don't believe it should delay
resolving the madv_free issue. Additionally, that pool doesn’t effectively
address the reclamation priority between files and lazyfree anon folios.

In conclusion:

1. I agree that the pool is valuable, and I’d like to develop it as an
incremental patch set. However, this is a significant step that will
require considerable time.
2. It could be quite tricky to include both lazyfree anon folios and
reclaimed file folios (which are reclaimed in normal lists but not in
the 'easily-reclaimable' list) in the same LRU list. I’d prefer to
start by replacing Samsung's rbincache to reduce file folio I/O if we
decide to implement the pool.
3. I believe we should first focus on landing this fix patch for the
madv_free performance issue.

What are your thoughts? I spoke with Yu, and he would like to hear
your opinion.

Thanks
Barry

Powered by blists - more mailing lists