Message-ID: <36f52466-071a-4efb-adc2-8514b11f120c@linux.alibaba.com>
Date: Mon, 9 Jun 2025 16:27:03 +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] mm/shmem, swap: fix softlockup with mTHP swapin



On 2025/6/9 03:27, 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)·
> 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.

If swapcache_prepare() fails and retries, the folio's order (order 0) 
getting from swapcache will be different from the order stored in the 
shmem mapping, so we will split the large swap entry by the following 
logic in shmem_swapin_folio(). So I am not sure why causing a softlockup?

	} else if (order != folio_order(folio)) {
		/*
		 * Swap readahead may swap in order 0 folios into swapcache
		 * asynchronously, while the shmem mapping can still stores
		 * large swap entries. In such cases, we should split the
		 * large swap entry to prevent possible data corruption.
		 */
		split_order = shmem_split_large_entry(inode, index, swap, gfp);
		if (split_order < 0) {
			error = split_order;
			goto failed;
		}

		/*
		 * If the large swap entry has already been split, it is
		 * necessary to recalculate the new swap entry based on
		 * the old order alignment.
		 */
		if (split_order > 0) {
			pgoff_t offset = index - round_down(index, 1 << split_order);

			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
		}
	}

> 
> 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).

I don't understand why adding zeromap changes, and should explain this 
explicitly.

> 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);
> +
>   		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;
>   
>   		/* 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)

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