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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 10 Aug 2023 20:21:19 -0700
Message-ID: <CAJD7tkZoE5is2_pBHs5w7-4Am8HuWdbkBn89OYoHfbRv5rSKsQ@mail.gmail.com>
Subject: Re: [PATCH] zswap: don't warn if none swapcache folio is passed to zswap_load
To:     Yu Zhao <yuzhao@google.com>
Cc:     "Yin, Fengwei" <fengwei.yin@intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, willy@infradead.org, hannes@cmpxchg.org
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On Thu, Aug 10, 2023 at 8:12=E2=80=AFPM Yu Zhao <yuzhao@google.com> wrote:
>
> On Thu, Aug 10, 2023 at 5:46=E2=80=AFPM Yosry Ahmed <yosryahmed@google.co=
m> wrote:
> >
> > On Thu, Aug 10, 2023 at 4:44=E2=80=AFPM Yu Zhao <yuzhao@google.com> wro=
te:
> > >
> > > On Thu, Aug 10, 2023 at 5:31=E2=80=AFPM Yin, Fengwei <fengwei.yin@int=
el.com> wrote:
> > > >
> > > >
> > > >
> > > > On 8/11/2023 7:15 AM, Yosry Ahmed wrote:
> > > > > On Thu, Aug 10, 2023 at 4:09=E2=80=AFPM Yin, Fengwei <fengwei.yin=
@intel.com> wrote:
> > > > >>
> > > > >>
> > > > >>
> > > > >> On 8/11/2023 2:44 AM, Yu Zhao wrote:
> > > > >>> On Thu, Aug 10, 2023 at 3:58=E2=80=AFAM Yin Fengwei <fengwei.yi=
n@intel.com> wrote:
> > > > >>>>
> > > > >>>> With mm-unstable branch, if trigger swap activity and it's pos=
sible
> > > > >>>> see following warning:
> > > > >>>> [  178.093511][  T651] WARNING: CPU: 2 PID: 651 at mm/zswap.c:=
1387 zswap_load+0x67/0x570
> > > > >>>> [  178.095155][  T651] Modules linked in:
> > > > >>>> [  178.096103][  T651] CPU: 2 PID: 651 Comm: gmain Not tainted=
 6.5.0-rc4-00492-gad3232df3e41 #148
> > > > >>>> [  178.098372][  T651] Hardware name: QEMU Standard PC (i440FX=
 + PIIX,1996), BIOS 1.14.0-2 04/01/2014
> > > > >>>> [  178.101114][  T651] RIP: 0010:zswap_load+0x67/0x570
> > > > >>>> [  178.102359][  T651] Code: a0 78 4b 85 e8 ea db ff ff 48 8b =
00 a8 01 0f 84 84 04 00 00 48 89 df e8 d7 db ff ff 48 8b 00 a9 00 00 08 00 =
0f 85 c4
> > > > >>>> [  178.106376][  T651] RSP: 0018:ffffc900011b3760 EFLAGS: 0001=
0246
> > > > >>>> [  178.107675][  T651] RAX: 0017ffffc0080001 RBX: ffffea0004a9=
91c0 RCX:ffffc900011b37dc
> > > > >>>> [  178.109242][  T651] RDX: 0000000000000000 RSI: 000000000000=
0001 RDI:ffffea0004a991c0
> > > > >>>> [  178.110916][  T651] RBP: ffffea0004a991c0 R08: 000000000000=
0243 R09:00000000c9a1aafc
> > > > >>>> [  178.112377][  T651] R10: 00000000c9657db3 R11: 000000003c96=
57db R12:0000000000014b9c
> > > > >>>> [  178.113698][  T651] R13: ffff88813501e710 R14: ffff88810d59=
1000 R15:0000000000000000
> > > > >>>> [  178.115008][  T651] FS:  00007fb21a9ff700(0000) GS:ffff8881=
3bc80000(0000) knlGS:0000000000000000
> > > > >>>> [  178.116423][  T651] CS:  0010 DS: 0000 ES: 0000 CR0: 000000=
0080050033
> > > > >>>> [  178.117421][  T651] CR2: 00005632cbfc81f6 CR3: 000000013145=
0002 CR4:0000000000370ee0
> > > > >>>> [  178.118683][  T651] DR0: 0000000000000000 DR1: 000000000000=
0000 DR2:0000000000000000
> > > > >>>> [  178.119894][  T651] DR3: 0000000000000000 DR6: 00000000fffe=
0ff0 DR7:0000000000000400
> > > > >>>> [  178.121087][  T651] Call Trace:
> > > > >>>> [  178.121654][  T651]  <TASK>
> > > > >>>> [  178.122109][  T651]  ? zswap_load+0x67/0x570
> > > > >>>> [  178.122658][  T651]  ? __warn+0x81/0x170
> > > > >>>> [  178.123119][  T651]  ? zswap_load+0x67/0x570
> > > > >>>> [  178.123608][  T651]  ? report_bug+0x167/0x190
> > > > >>>> [  178.124150][  T651]  ? handle_bug+0x3c/0x70
> > > > >>>> [  178.124615][  T651]  ? exc_invalid_op+0x13/0x60
> > > > >>>> [  178.125192][  T651]  ? asm_exc_invalid_op+0x16/0x20
> > > > >>>> [  178.125753][  T651]  ? zswap_load+0x67/0x570
> > > > >>>> [  178.126231][  T651]  ? lock_acquire+0xbb/0x290
> > > > >>>> [  178.126745][  T651]  ? folio_add_lru+0x40/0x1c0
> > > > >>>> [  178.127261][  T651]  ? find_held_lock+0x2b/0x80
> > > > >>>> [  178.127776][  T651]  swap_readpage+0xc7/0x5c0
> > > > >>>> [  178.128273][  T651]  do_swap_page+0x86d/0xf50
> > > > >>>> [  178.128770][  T651]  ? __pte_offset_map+0x3e/0x290
> > > > >>>> [  178.129321][  T651]  ? __pte_offset_map+0x1c4/0x290
> > > > >>>> [  178.129883][  T651]  __handle_mm_fault+0x6ad/0xca0
> > > > >>>> [  178.130419][  T651]  handle_mm_fault+0x18b/0x410
> > > > >>>> [  178.130992][  T651]  do_user_addr_fault+0x1f1/0x820
> > > > >>>> [  178.132076][  T651]  exc_page_fault+0x63/0x1a0
> > > > >>>> [  178.132599][  T651]  asm_exc_page_fault+0x22/0x30
> > > > >>>>
> > > > >>>> It's possible that swap_readpage() is called with none swapcac=
he folio
> > > > >>>> in do_swap_page() and trigger this warning. So we shouldn't as=
sume
> > > > >>>> zswap_load() always takes swapcache folio.
> > > > >>>
> > > > >>> Did you use a bdev with QUEUE_FLAG_SYNCHRONOUS? Otherwise it so=
unds
> > > > >>> like a bug to me.
> > > > >> I hit this warning with zram which has QUEUE_FLAG_SYNCHRONOUS se=
t. Thanks.
> > > > >
> > > > > Does it make sense to keep the warning and instead change it to c=
heck
> > > > > SWP_SYNCHRONOUS_IO as well? Something like:
> > > > >
> > > > > VM_WARN_ON_ONCE(!folio_test_swapcache(folio) &&
> > > > > !swap_type_to_swap_info(type)->flags && SWP_SYNCHRONOUS_IO);
> > > > >
> > > > > Of course this is too ugly, so perhaps we want a helper to check =
if a
> > > > > swapfile is synchronous.
> > > > My understanding was that the WARN here is zswap_load() doesn't exp=
ect
> > > > a folio not in swapcache. With zram, swap_readpage() must accept th=
e
> > > > folio not in swapcache. So this warn should not be there.
> > > >
> > > > But your comment make more sense to me. I will update the patch not
> > > > to remove this WARN. Thanks.
> > >
> > > That can cause another warning.
> > >
> > > Please don't overegineer.
> >
> > How so?
> >
> > Using zswap with zram is a weird combination
>
> Not at all -- it can achieve tiering between different compressors:
> fast but low compression ratio for zswap but the opposite for zram.

That's definitely an interesting use case, thanks for pointing this out.

I would prefer creating a helper and using it in both do_swap_fault()
and zswap_load() in the WARN_ON (with data_race()), but I am not
against just removing the WARN_ON either. I will leave it up to you
and Yin :)

>
> > if anything I would
> > prefer leaving the warning as-is than removing it to be honest.