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Message-ID: <66d52892-9f95-45c8-93cd-8182a4743af9@linux.alibaba.com>
Date: Wed, 11 Jun 2025 14:19:18 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins
<hughd@...gle.com>, Kemeng Shi <shikemeng@...weicloud.com>,
Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>,
Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>,
Usama Arif <usamaarif642@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/shmem, swap: fix softlockup with mTHP swapin
On 2025/6/11 02:16, Kairui Song 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)ยท
> 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>
> Acked-by: Nhat Pham <nphamcs@...il.com>
LGTM. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>
> 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.
>
> V2: https://lore.kernel.org/linux-mm/20250609171751.36305-1-ryncsn@gmail.com/
> Updates:
> - Minor comment adjustment [ Nhat Pham ]
>
> mm/memory.c | 20 --------------------
> mm/shmem.c | 6 +++++-
> mm/swap.h | 23 +++++++++++++++++++++++
> 3 files changed, 28 insertions(+), 21 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 8eba595056fe..b0cda5aab398 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4315,26 +4315,6 @@ 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 0c5fb4ffa03a..3a5a65b1f41a 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2259,6 +2259,7 @@ 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?? */
> @@ -2272,9 +2273,12 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> * If uffd is active for the vma, we need per-page fault
> * fidelity to maintain the uffd semantics, then fallback
> * to swapin order-0 folio, as well as for zswap case.
> + * Any existing sub folio in the swap cache also blocks
> + * mTHP swapin.
> */
> 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 2269eb9df0af..9096082a915e 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -106,6 +106,25 @@ 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)
> @@ -199,6 +218,10 @@ static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr,
> return 0;
> }
>
> +static inline int non_swapcache_batch(swp_entry_t entry, int max_nr)
> +{
> + return 0;
> +}
> #endif /* CONFIG_SWAP */
>
> /**
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