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From:   Yu Zhao <yuzhao@google.com>
Date:   Thu, 10 Aug 2023 21:02:55 -0600
Message-ID: <CAOUHufYpnQaf1qRaO7AR78EVHkcbq4aPd9BXutXyyVEbt_Cthw@mail.gmail.com>
Subject: Re: [PATCH] zswap: don't warn if none swapcache folio is passed to zswap_load
To:     "Yin, Fengwei" <fengwei.yin@intel.com>
Cc:     Yosry Ahmed <yosryahmed@google.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 6:37=E2=80=AFPM Yin, Fengwei <fengwei.yin@intel.com=
> wrote:
>
>
>
> On 8/11/2023 7:43 AM, Yu Zhao wrote:
> > On Thu, Aug 10, 2023 at 5:31=E2=80=AFPM Yin, Fengwei <fengwei.yin@intel=
.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@int=
el.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.yin@in=
tel.com> wrote:
> >>>>>>
> >>>>>> With mm-unstable branch, if trigger swap activity and it's possibl=
e
> >>>>>> 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 + P=
IIX,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 a=
8 01 0f 84 84 04 00 00 48 89 df e8 d7 db ff ff 48 8b 00 a9 00 00 08 00 0f 8=
5 c4
> >>>>>> [  178.106376][  T651] RSP: 0018:ffffc900011b3760 EFLAGS: 00010246
> >>>>>> [  178.107675][  T651] RAX: 0017ffffc0080001 RBX: ffffea0004a991c0=
 RCX:ffffc900011b37dc
> >>>>>> [  178.109242][  T651] RDX: 0000000000000000 RSI: 0000000000000001=
 RDI:ffffea0004a991c0
> >>>>>> [  178.110916][  T651] RBP: ffffea0004a991c0 R08: 0000000000000243=
 R09:00000000c9a1aafc
> >>>>>> [  178.112377][  T651] R10: 00000000c9657db3 R11: 000000003c9657db=
 R12:0000000000014b9c
> >>>>>> [  178.113698][  T651] R13: ffff88813501e710 R14: ffff88810d591000=
 R15:0000000000000000
> >>>>>> [  178.115008][  T651] FS:  00007fb21a9ff700(0000) GS:ffff88813bc8=
0000(0000) knlGS:0000000000000000
> >>>>>> [  178.116423][  T651] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080=
050033
> >>>>>> [  178.117421][  T651] CR2: 00005632cbfc81f6 CR3: 0000000131450002=
 CR4:0000000000370ee0
> >>>>>> [  178.118683][  T651] DR0: 0000000000000000 DR1: 0000000000000000=
 DR2:0000000000000000
> >>>>>> [  178.119894][  T651] DR3: 0000000000000000 DR6: 00000000fffe0ff0=
 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 swapcache f=
olio
> >>>>>> in do_swap_page() and trigger this warning. So we shouldn't assume
> >>>>>> zswap_load() always takes swapcache folio.
> >>>>>
> >>>>> Did you use a bdev with QUEUE_FLAG_SYNCHRONOUS? Otherwise it sounds
> >>>>> like a bug to me.
> >>>> I hit this warning with zram which has QUEUE_FLAG_SYNCHRONOUS set. T=
hanks.
> >>>
> >>> Does it make sense to keep the warning and instead change it to check
> >>> 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 expect
> >> a folio not in swapcache. With zram, swap_readpage() must accept the
> >> 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.
> My understanding is that WARN may be wanted by zswap code.
>
> >
> > Please don't overegineer.

The original patch looks good to me. What Yosry suggested seems not
only overengineered but also can cause a new KCSAN warning.