Received-SPF: pass (google.com: domain of linux-kernel+bounces-23485-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Precedence: bulk
MIME-Version: 1.0
References: <20231024142706.195517-1-hezhongkun.hzk@bytedance.com>
 <CAKEwX=OiNB+pPhb-3Tf7O=F7psKE3EOpwmbPSeLSOyuHpj3i+Q@mail.gmail.com>
 <CACSyD1P6HmH9tSvONnNxYv8P+am_hH2dK3UJQd9_+o6EWkPsXA@mail.gmail.com>
 <CAKEwX=PC3C-PrWAH3XiYGyR4ujqBJQBBX6uRa2jXKCy9VMyRCQ@mail.gmail.com>
 <CACSyD1O7t0+BXUujJ81RAdEys3MUnmpu0sRADLazoyvayx5DLA@mail.gmail.com>
 <CAKEwX=P5AC+ubnunnZr5vMiC6fFU+E_E7jg_FZztWwZRYSxTWQ@mail.gmail.com>
 <CACSyD1Nnc_w3epbt6+EMt7a-4pAzgW1hbE=G5Fy5Tc5R5+uxKw@mail.gmail.com>
 <CAKEwX=NuXR9Ot1eRFsp9n-3Tq9yhjD9up+jyvXeOzQ4xK9kEPA@mail.gmail.com>
 <CAKEwX=Oj2dR6a4-DeccvcVdJ-J7b=83uCWQAf5u7U0sySudnkw@mail.gmail.com>
 <CAJD7tkb2oda=4f0s8w8xn+t_TM1b2Q_otbb86VPQ9R1m2uqDTA@mail.gmail.com>
 <CACSyD1ODCikYLDzO4LkQeDzB4sqDWCULwCdehw9inP-qyw3_Jg@mail.gmail.com>
 <CAJD7tkY=zmGiPoWNjVaVeU+NPxV2t48J5-CxEP9=nBK8nAh0XA@mail.gmail.com>
 <CAKEwX=Na3dg+KZwvtQi-Nj79Am-1tttDw50_qStkobmYGUC6NA@mail.gmail.com> <CACSyD1Pp8gkxwTXZuinm6wiZs0e5U2B5oND4rj29dzmRApFjhQ@mail.gmail.com>
In-Reply-To: <CACSyD1Pp8gkxwTXZuinm6wiZs0e5U2B5oND4rj29dzmRApFjhQ@mail.gmail.com>
From: Yosry Ahmed <yosryahmed@google.com>
Date: Thu, 11 Jan 2024 03:27:44 -0800
Message-ID: <CAJD7tkYmOCCR7iipEFF69woXQxOr7byj+GuafGQVBL=UCHUK5A@mail.gmail.com>
Subject: Re: [External] Re: [PATCH] mm: zswap: fix the lack of page lru flag
 in zswap_writeback_entry
To: Zhongkun He <hezhongkun.hzk@bytedance.com>
Cc: Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org, hannes@cmpxchg.org, 
	sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	Chris Li <chrisl@kernel.org>, Matthew Wilcox <willy@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Wed, Jan 10, 2024 at 7:49=E2=80=AFPM Zhongkun He
<hezhongkun.hzk@bytedance.com> wrote:
>
> >
> > This sounds dangerous. This is going to introduce a rather large
> > unexpected side effect - we're changing the readahead behavior in a
> > seemingly small zswap optimization. In fact, I'd argue that if we do
> > this, the readahead behavior change will be the "main effect", and the
> > zswap-side change would be a "happy consequence". We should run a lot
> > of benchmarking and document the change extensively if we pursue this
> > route.
> >
>
> I agree with the unexpected side effect,  and here I need
> to clarify the original intention of this patch.Please see the memory
> offloading steps below.
>
>
> memory      zswap(reclaim)          memory+swap (writeback)
> 1G                 0.5G                        1G(tmp memory) + 1G=EF=BC=
=88swap=EF=BC=89
>
> If the decompressed memory cannot be released in time,
> zswap's writeback has great side effects(mostly clod pages).
> On the one hand, the memory space has not been reduced,
> but has increased (from 0.5G->1G).
> At the same time, it is not put the pages to the tail of the lru.
> When the memory is insufficient, other pages will be squeezed out
> and released early.
> With this patch=EF=BC=8C we can put the tmp pages to the tail and reclaim=
 it
> in time when the memory is insufficient or actively reclaimed.
> So I think this patch makes sense and hope it can be fixed with a
> suitable approaches.
>
> >
> > Unless some page flag/readahead expert can confirm that the first
> > option is safe, my vote is on this option. I mean, it's fairly minimal
> > codewise, no? Just a bunch of plumbing. We can also keep the other
> > call sites intact if we just rename the old versions - something along
> > the line of:
> >
> > __read_swap_cache_async_head(..., bool add_to_lru_head)
> > {
> > ...
> > if (add_to_lru_head)
> >   folio_add_lru(folio)
> > else
> >   folio_add_lru_tail(folio);
> > }
> >
> > __read_swap_cache_async(...)
> > {
> >    return __read_swap_cache_async_tail(..., true);
> > }
> >
> > A bit boilerplate? Sure. But this seems safer, and I doubt it's *that*
> > much more work.
> >
>
> Yes=EF=BC=8C agree. I will try it again.

I agree with Nhat here. Unless someone with enough readahead/page
flags knowledge says putting PG_readahead pages at the tail of the LRU
is okay (doubtful), I think we should opt for introducing a
folio_add_lru_tail() as I initially suggested.