Message-ID: <CAMgjq7D9tfW626dki+cC_SjHK-Z=KnPvXmWmmSQO6xsyxokrfQ@mail.gmail.com>
Date: Tue, 20 Aug 2024 17:29:40 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>, 
	syzbot <syzbot+ce6029250d7fd4d0476d@...kaller.appspotmail.com>, 
	akpm@...ux-foundation.org, chengming.zhou@...ux.dev, hannes@...xchg.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, nphamcs@...il.com, 
	syzkaller-bugs@...glegroups.com, Chris Li <chrisl@...nel.org>, 
	Ying <ying.huang@...el.com>, Ryan Roberts <ryan.roberts@....com>
Subject: Re: [syzbot] [mm?] WARNING in zswap_swapoff

On Tue, Aug 20, 2024 at 5:22 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Tue, Aug 20, 2024 at 8:47 PM Kairui Song <ryncsn@...il.com> wrote:
> >
> > On Tue, Aug 20, 2024 at 4:13 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > > On Fri, Aug 16, 2024 at 12:52 PM syzbot
> > > <syzbot+ce6029250d7fd4d0476d@...kaller.appspotmail.com> wrote:
> > > >
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    367b5c3d53e5 Add linux-next specific files for 20240816
> >
> > I can't find this commit, seems this commit is not in linux-next any more?
> >
> > > > git tree:       linux-next
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=12489105980000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=61ba6f3b22ee5467
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=ce6029250d7fd4d0476d
> > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > > >
> > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > >
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/0b1b4e3cad3c/disk-367b5c3d.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/5bb090f7813c/vmlinux-367b5c3d.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/6674cb0709b1/bzImage-367b5c3d.xz
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+ce6029250d7fd4d0476d@...kaller.appspotmail.com
> > > >
> > > > ------------[ cut here ]------------
> > > > WARNING: CPU: 0 PID: 11298 at mm/zswap.c:1700 zswap_swapoff+0x11b/0x2b0 mm/zswap.c:1700
> > > > Modules linked in:
> > > > CPU: 0 UID: 0 PID: 11298 Comm: swapoff Not tainted 6.11.0-rc3-next-20240816-syzkaller #0
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/27/2024
> > > > RIP: 0010:zswap_swapoff+0x11b/0x2b0 mm/zswap.c:1700
> > > > Code: 74 05 e8 78 73 07 00 4b 83 7c 35 00 00 75 15 e8 1b bd 9e ff 48 ff c5 49 83 c6 50 83 7c 24 0c 17 76 9b eb 24 e8 06 bd 9e ff 90 <0f> 0b 90 eb e5 48 8b 0c 24 80 e1 07 80 c1 03 38 c1 7c 90 48 8b 3c
> > > > RSP: 0018:ffffc9000302fa38 EFLAGS: 00010293
> > > > RAX: ffffffff81f4d66a RBX: dffffc0000000000 RCX: ffff88802c19bc00
> > > > RDX: 0000000000000000 RSI: 0000000000000002 RDI: ffff888015986248
> > > > RBP: 0000000000000000 R08: ffffffff81f4d620 R09: 1ffffffff1d476ac
> > > > R10: dffffc0000000000 R11: fffffbfff1d476ad R12: dffffc0000000000
> > > > R13: ffff888015986200 R14: 0000000000000048 R15: 0000000000000002
> > > > FS:  00007f9e628a5380(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 0000001b30f15ff8 CR3: 000000006c5f0000 CR4: 00000000003506f0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > >  <TASK>
> > > >  __do_sys_swapoff mm/swapfile.c:2837 [inline]
> > > >  __se_sys_swapoff+0x4653/0x4cf0 mm/swapfile.c:2706
> > > >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> > > >  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
> > > >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > > > RIP: 0033:0x7f9e629feb37
> > > > Code: 73 01 c3 48 8b 0d f1 52 0d 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 a8 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c1 52 0d 00 f7 d8 64 89 01 48
> > > > RSP: 002b:00007fff17734f68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a8
> > > > RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f9e629feb37
> > > > RDX: 00007f9e62a9e7e8 RSI: 00007f9e62b9beed RDI: 0000563090942a20
> > > > RBP: 0000563090942a20 R08: 0000000000000000 R09: 77872e07ed164f94
> > > > R10: 000000000000001f R11: 0000000000000246 R12: 00007fff17735188
> > > > R13: 00005630909422a0 R14: 0000563073724169 R15: 00007f9e62bdda80
> > > >  </TASK>
> > >
> > > I am hoping syzbot would find a reproducer and bisect this for us.
> > > Meanwhile, from a high-level it looks to me like we are missing a
> > > zswap_invalidate() call in some paths.
> > >
> > > If I have to guess, I would say it's related to the latest mTHP swap
> > > changes, but I am not following closely. Perhaps one of the following
> > > things happened:
> > >
> > > (1) We are not calling zswap_invalidate() in some invalidation paths.
> > > It used to not be called for the cluster freeing path, so maybe we end
> > > up with some order-0 swap entries in a cluster? or maybe there is an
> > > entirely new invalidation path that does not go through
> > > free_swap_slot() for order-0 entries?
> > >
> > > (2) Some higher order swap entries (i.e. a cluster) end up in zswap
> > > somehow. zswap_store() has a warning to cover that though. Maybe
> > > somehow some swap entries are allocated as a cluster, but then pages
> > > are swapped out one-by-one as order-0 (which can go to zswap), but
> > > then we still free the swap entries as a cluster?
> >
> > Hi Yosry, thanks for the report.
> >
> > There are many mTHP related optimizations recently, for this problem I
> > can reproduce this locally. Can confirm the problem is gone for me
> > after reverting:
> >
> > "mm: attempt to batch free swap entries for zap_pte_range()"
> >
> > Hi Barry,
> >
> > If a set of continuous slots are having the same value, they are
> > considered a mTHP and freed, bypassing the slot cache, and causing
> > zswap leak.
> > This didn't happen in put_swap_folio because that function is
> > expecting an actual mTHP folio behind the slots but
> > free_swap_and_cache_nr is simply walking the slots.
>
> Hi Kairui,
>
> I don't understand, if anyone has a folio backend, the code will
> go fallback to __try_to_reclaim_swap(),  it won't call
> swap_entry_range_free().
>
>      ci = lock_cluster_or_swap_info(si, offset);
>         if (!swap_is_last_map(si, offset, nr, &has_cache)) {
>                 unlock_cluster_or_swap_info(si, ci);
>                 goto fallback;
>         }
>         for (i = 0; i < nr; i++)
>                 WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
>         unlock_cluster_or_swap_info(si, ci);
>
>         if (!has_cache) {
>                 spin_lock(&si->lock);
>                 swap_entry_range_free(si, entry, nr);
>                 spin_unlock(&si->lock);
>         }
>         return has_cache;
>
> Am i missing something?

Hi Barry,

Per my understanding, ZSWAP invalidation could happen after the folio
is gone from the swap cache, especially in free_swap_and_cache_nr, it
will iterate and zap the swap slots without swapping them in.
So a slot doesn't have a folio backed doesn't mean it doesn't have ZSWAP data.

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