Re: [PATCH v3] mm/shmem, swap: fix softlockup with mTHP swapin

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/6/11 02:16, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.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@tencent.com>
> Reviewed-by: Barry Song <baohua@kernel.org>
> Acked-by: Nhat Pham <nphamcs@gmail.com>

LGTM. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@linux.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 */
>   
>   /**