Message-ID: <CAMgjq7BqyjON0PgfYePDETfWbs99TO1f3zsFM8DwnQG-JqoOyQ@mail.gmail.com>
Date: Mon, 9 Jun 2025 16:29:13 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Chris Li <chrisl@...nel.org>, 
	Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>, Usama Arif <usamaarif642@...il.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/shmem, swap: fix softlockup with mTHP swapin

On Mon, Jun 9, 2025 at 12:30 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Mon, Jun 9, 2025 at 2:32 PM Kairui Song <ryncsn@...il.com> wrote:
> >
> > On Mon, Jun 9, 2025 at 7:58 AM Barry Song <21cnbao@...il.com> wrote:
> > >
> > > On Mon, Jun 9, 2025 at 7:27 AM Kairui Song <ryncsn@...il.com> wrote:
> > > >
> > > > From: Kairui Song <kasong@...cent.com>
> > > >
> > > > Following softlockup can be easily reproduced on my test machine with:
> > > >
> > > > echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
> > > > swapon /dev/zram0 # zram0 is a 48G swap device
> > > > mkdir -p /sys/fs/cgroup/memory/test
> > > > echo 1G > /sys/fs/cgroup/test/memory.max
> > > > echo $BASHPID > /sys/fs/cgroup/test/cgroup.procs
> > > > while true; do
> > > >     dd if=/dev/zero of=/tmp/test.img bs=1M count=5120
> > > >     cat /tmp/test.img > /dev/null
> > > >     rm /tmp/test.img
> > > > done
> > > >
> > > > Then after a while:
> > > > watchdog: BUG: soft lockup - CPU#0 stuck for 763s! [cat:5787]
> > > > Modules linked in: zram virtiofs
> > > > CPU: 0 UID: 0 PID: 5787 Comm: cat Kdump: loaded Tainted: G             L      6.15.0.orig-gf3021d9246bc-dirty #118 PREEMPT(voluntary)·
> > > > Tainted: [L]=SOFTLOCKUP
> > > > Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
> > > > RIP: 0010:mpol_shared_policy_lookup+0xd/0x70
> > > > Code: e9 b8 b4 ff ff 31 c0 c3 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 0f 1f 44 00 00 41 54 55 53 <48> 8b 1f 48 85 db 74 41 4c 8d 67 08 48 89 fb 48 89 f5 4c 89 e7 e8
> > > > RSP: 0018:ffffc90002b1fc28 EFLAGS: 00000202
> > > > RAX: 00000000001c20ca RBX: 0000000000724e1e RCX: 0000000000000001
> > > > RDX: ffff888118e214c8 RSI: 0000000000057d42 RDI: ffff888118e21518
> > > > RBP: 000000000002bec8 R08: 0000000000000001 R09: 0000000000000000
> > > > R10: 0000000000000bf4 R11: 0000000000000000 R12: 0000000000000001
> > > > R13: 00000000001c20ca R14: 00000000001c20ca R15: 0000000000000000
> > > > FS:  00007f03f995c740(0000) GS:ffff88a07ad9a000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 00007f03f98f1000 CR3: 0000000144626004 CR4: 0000000000770eb0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > PKRU: 55555554
> > > > Call Trace:
> > > >  <TASK>
> > > >  shmem_alloc_folio+0x31/0xc0
> > > >  shmem_swapin_folio+0x309/0xcf0
> > > >  ? filemap_get_entry+0x117/0x1e0
> > > >  ? xas_load+0xd/0xb0
> > > >  ? filemap_get_entry+0x101/0x1e0
> > > >  shmem_get_folio_gfp+0x2ed/0x5b0
> > > >  shmem_file_read_iter+0x7f/0x2e0
> > > >  vfs_read+0x252/0x330
> > > >  ksys_read+0x68/0xf0
> > > >  do_syscall_64+0x4c/0x1c0
> > > >  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > > > RIP: 0033:0x7f03f9a46991
> > > > Code: 00 48 8b 15 81 14 10 00 f7 d8 64 89 02 b8 ff ff ff ff eb bd e8 20 ad 01 00 f3 0f 1e fa 80 3d 35 97 10 00 00 74 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 4f c3 66 0f 1f 44 00 00 55 48 89 e5 48 83 ec
> > > > RSP: 002b:00007fff3c52bd28 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > > > RAX: ffffffffffffffda RBX: 0000000000040000 RCX: 00007f03f9a46991
> > > > RDX: 0000000000040000 RSI: 00007f03f98ba000 RDI: 0000000000000003
> > > > RBP: 00007fff3c52bd50 R08: 0000000000000000 R09: 00007f03f9b9a380
> > > > R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000040000
> > > > R13: 00007f03f98ba000 R14: 0000000000000003 R15: 0000000000000000
> > > >  </TASK>
> > > >
> > > > The reason is simple, readahead brought some order 0 folio in swap
> > > > cache, and the swapin mTHP folio being allocated is in confict with it,
> > > > so swapcache_prepare fails and causes shmem_swap_alloc_folio to return
> > > > -EEXIST, and shmem simply retries again and again causing this loop.
> > > >
> > > > Fix it by applying a similar fix for anon mTHP swapin.
> > > >
> > > > The performance change is very slight, time of swapin 10g zero folios
> > > > (test for 12 times):
> > > > Before:  2.49s
> > > > After:   2.52s
> > > >
> > > > Fixes: 1dd44c0af4fa1 ("mm: shmem: skip swapcache for swapin of synchronous swap device")
> > > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > > >
> > > > ---
> > > >
> > > > I found this issue while doing a performance comparing of mm-new with
> > > > swap table series [1] on top of mm-new. This issue no longer exists
> > > > if the swap table series is applied, because it elimated both
> > > > SWAP_HAS_CACHE and SWP_SYNCHRONOUS_IO swapin completely while improving
> > > > the performance and simplify the code, and the race swapin is solved
> > > > differently by then.
> > > >
> > > > (The zero map fix might still need to stay for a while, but could be
> > > > optimized too later with swap table).
> > > >
> > > > It will be good if the swap table series could get reviewed and merged
> > > > to avoid more fixes like this. SWAP_HAS_CACHE and SWP_SYNCHRONOUS_IO has
> > > > a history of causing many issues. I'll do a swap table rebase on top of
> > > > this fix, if this one is accepted.
> > > >
> > > > And for a comparision, swap in 10G into shmem:
> > > >
> > > > Before this patch:  2.49s
> > > > After this patch:   2.52s
> > > > After swap table:   2.37s (Removing SWAP_HAS_CACHE and SWP_SYNCHRONOUS_IO,
> > > >                            still not in the best shape but looking good)
> > > >
> > > > Link: https://lore.kernel.org/linux-mm/20250514201729.48420-1-ryncsn@gmail.com/ [1]
> > > >
> > > >  mm/memory.c | 20 --------------------
> > > >  mm/shmem.c  | 12 +++++++++++-
> > > >  mm/swap.h   | 19 +++++++++++++++++++
> > > >  3 files changed, 30 insertions(+), 21 deletions(-)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 9ead7ab07e8e..3845ed068d74 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -4313,26 +4313,6 @@ static struct folio *__alloc_swap_folio(struct vm_fault *vmf)
> > > >  }
> > > >
> > > >  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > > -static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
> > > > -{
> > > > -       struct swap_info_struct *si = swp_swap_info(entry);
> > > > -       pgoff_t offset = swp_offset(entry);
> > > > -       int i;
> > > > -
> > > > -       /*
> > > > -        * While allocating a large folio and doing swap_read_folio, which is
> > > > -        * the case the being faulted pte doesn't have swapcache. We need to
> > > > -        * ensure all PTEs have no cache as well, otherwise, we might go to
> > > > -        * swap devices while the content is in swapcache.
> > > > -        */
> > > > -       for (i = 0; i < max_nr; i++) {
> > > > -               if ((si->swap_map[offset + i] & SWAP_HAS_CACHE))
> > > > -                       return i;
> > > > -       }
> > > > -
> > > > -       return i;
> > > > -}
> > > > -
> > > >  /*
> > > >   * Check if the PTEs within a range are contiguous swap entries
> > > >   * and have consistent swapcache, zeromap.
> > > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > > index 73182e904f9c..484cd3043a78 100644
> > > > --- a/mm/shmem.c
> > > > +++ b/mm/shmem.c
> > > > @@ -1995,6 +1995,14 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
> > > >          */
> > > >         if (swapcache_prepare(entry, nr_pages)) {
> > > >                 folio_put(new);
> > > > +
> > > > +               /*
> > > > +                * A smaller folio is in the swap cache, mTHP swapin will always fail
> > > > +                * until it's gone. Return -EINVAL to fallback to order 0.
> > > > +                */
> > > > +               if (non_swapcache_batch(entry, nr_pages) != nr_pages)
> > > > +                       return ERR_PTR(-EINVAL);
> > > > +
> >
> > Hi Barry,
> >
> > > We're doing this before swapcache_prepare() for mTHP swapin. Why does it
> > > happen after swapcache_prepare() in the shmem case?
> >
> > `non_swapcache_batch(entry, nr_pages) != nr_pages` is unlikely, that's
> > the reason why no one noticed this issue so far, so moving it after
> > swapcache_prepare can help avoid overhead caused by it in the common
> > case. swapcache_prepare already implies this check, but
> > swapcache_prepare can fall for multiple reasons, and shmem should and
> > only should fallback to order 0 swapin if it's caused by an existing
> > cache. (currently shmem unconditionally retry)
>
> Maybe it's because people are running it on systems with plenty of memory?
> Once we run it on a system with limited memory, we might see more failures
> allocating large folios and fall back to order-0 more often?
> For example, what if there's a 50% chance of failing to allocate large
> folios?

When under memory pressure the swap cache pages will get reclaimed, so
it will also be less likely to hit this issue, it has to be in a
situation where some swapin falls back to order 0 while memory
pressure is not too high to cause swap cache reclaim. So in most cases
it's unlikely to run into conflicting small folios, but chance is high
enough to trigger real issues, the reproducer shell script can trigger
this within 5 minutes on my machine.

>
> >
> > And non_swapcache_batch might not be the best solution here, it also
> > might have false positives, we can add a full filemap lookup here, but
> > might be overkill for a corner case like this. I still think merge
> > swap cache with swap_map using swap table is the long term solution.
> >
> > Maybe I'm premature optimizing it, I can use the easier to review
> > implementation (same way with anon mTHP) and do a quick benchmark, if
> > there is no obvious performance change I'll use that style in V2.
>
> Right, the current approach is a bit hard to follow, since we ultimately
> change the return value from -EEXIST to -EINVAL. It does feel like there’s
> some back-and-forth. But anyway, let’s look at the data—if the current
> approach yields better results, we can refine the code comments to make
> it easier to understand.

After more testing the performance change seems trivial, and
considering checking SWAP_HAS_CACHE before swapcache_prepare could
help avoid a ci locking in certain workloads, let me use that way in
V2 later today.

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