Message-ID: <CAGsJ_4xSRdjVac_UdUKBk2-5upeHMEEow3_Q-F_VSa8HYj0mtw@mail.gmail.com>
Date: Mon, 9 Jun 2025 11:57:50 +1200
From: Barry Song <21cnbao@...il.com>
To: Kairui Song <kasong@...cent.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 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);
> +

We're doing this before swapcache_prepare() for mTHP swapin. Why does it
happen after swapcache_prepare() in the shmem case?

>                 return ERR_PTR(-EEXIST);
>         }
>
> @@ -2256,6 +2264,7 @@ 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 +2280,8 @@ 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() ||
> +                                 nr_pages != swap_zeromap_batch(swap, nr_pages, NULL)))
>                         fallback_order0 = true;

I mean, why don't we reject large folios at this point instead?
Because if we do the check here, we might end up with a small folio in
swapcache afterward?

>
>                 /* Skip swapcache for synchronous device. */
> diff --git a/mm/swap.h b/mm/swap.h
> index e87a0f19a0ee..2d8ce1102153 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;
> +}
> +
>  #else /* CONFIG_SWAP */
>  struct swap_iocb;
>  static inline void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
> --
> 2.49.0
>

Thanks
Barry

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