Message-ID: <CAMgjq7AsKFz7UN+seR5atznE_RBTDC9qjDmwN5saMe+KL3b1mQ@mail.gmail.com>
Date: Tue, 10 Jun 2025 20:09:42 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...il.com>
Cc: linux-mm@...ck.org, stable@...r.kernel.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 v2] mm/shmem, swap: fix softlockup with mTHP swapin

On Tue, Jun 10, 2025 at 5:17 AM Barry Song <21cnbao@...il.com> wrote:
>
> On Tue, Jun 10, 2025 at 5:24 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > On Tue, Jun 10, 2025 at 1:18 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
> > > with shmem (test for 12 times):
> > > Before:  2.47s
> > > After:   2.48s
> > >
> > > Fixes: 1dd44c0af4fa1 ("mm: shmem: skip swapcache for swapin of synchronous swap device")
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
>
> Reviewed-by: Barry Song <baohua@...nel.org>
>
> > >
> > > ---
> > >
> > > V1: https://lore.kernel.org/linux-mm/20250608192713.95875-1-ryncsn@gmail.com/
> > > Updates:
> > > - Move non_swapcache_batch check before swapcache_prepare, I was
> > >   expecting this could improve the performance, turns out it barely
> > >   helps and may even cause more overhead in some cases. [ Barry Song ]
> > > - Remove zero map check, no need to do that for shmem [ Barry Song,
> > >   Baolin Wang ]
> > > - Fix build bot error.
> > >
> > >  mm/memory.c | 20 --------------------
> > >  mm/shmem.c  |  4 +++-
> > >  mm/swap.h   | 23 +++++++++++++++++++++++
> > >  3 files changed, 26 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..a4fdfbd086f1 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -2256,6 +2256,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> > >         folio = swap_cache_get_folio(swap, NULL, 0);
> > >         order = xa_get_order(&mapping->i_pages, index);
> > >         if (!folio) {
> > > +               int nr_pages = 1 << order;
> > >                 bool fallback_order0 = false;
> > >
> > >                 /* Or update major stats only when swapin succeeds?? */
> > > @@ -2271,7 +2272,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> > >                  * to swapin order-0 folio, as well as for zswap case.
> > >                  */
> > >                 if (order > 0 && ((vma && unlikely(userfaultfd_armed(vma))) ||
> > > -                                 !zswap_never_enabled()))
> > > +                                 !zswap_never_enabled() ||
> > > +                                 non_swapcache_batch(swap, nr_pages) != nr_pages))
> > >                         fallback_order0 = true;
> > >
> > >                 /* Skip swapcache for synchronous device. */
> > > diff --git a/mm/swap.h b/mm/swap.h
> > > index e87a0f19a0ee..911ad5ff0f89 100644
> > > --- a/mm/swap.h
> > > +++ b/mm/swap.h
> > > @@ -108,6 +108,25 @@ static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr,
> > >                 return find_next_bit(sis->zeromap, end, start) - start;
> > >  }
> > >
> > > +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 mTHP swapin, we need to
> > > +        * ensure all entries are not cached, otherwise, the mTHP folio will
> > > +        * be in conflict with the folio in swap cache.
> > > +        */
> > > +       for (i = 0; i < max_nr; i++) {
> > > +               if ((si->swap_map[offset + i] & SWAP_HAS_CACHE))
> > > +                       return i;
> > > +       }
> > > +
> > > +       return i;
> > > +}
>
> Nit:
> This was previously under CONFIG_TRANSPARENT_HUGEPAGE, but is now
> guarded by CONFIG_SWAP. I assume there's no performance impact, since
> if (order > 0) would never be true on platforms where THP is
> disabled, meaning non_swapcache_batch() would never be called?

Yes, there is no performance overhead, order is always 0. But some
clean up can be done later.

The whole `order = xa_get_order` and related checks are not needed at
all for !CONFIG_TRANSPARENT_HUGEPAGE. It's hard to optimize it out
without shuffle too many code here, simply wrap non_swapcache_batch
with CONFIG_TRANSPARENT_HUGEPAGE may cause more trouble for
understanding the code and doesn't help much here, it's really the
caller that should avoid the whole thing. That's not in the scope of a
bugfix.

I have a shmem xarray lookup optimization patch in the swap table
series, now looking at it I found it might be useful for the clean up
here. I'll check if I can send a clean up & optimize series first.

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