Re: [PATCH v2] mm/migrate: fix deadlock in migrate_pages_batch() on large folios

[David Hildenbrand <david@redhat.com>]

On 29.07.24 04:13, Gao Xiang wrote:
> Currently, migrate_pages_batch() can lock multiple locked folios
> with an arbitrary order.  Although folio_trylock() is used to avoid
> deadlock as commit 2ef7dbb26990 ("migrate_pages: try migrate in batch
> asynchronously firstly") mentioned, it seems try_split_folio() is
> still missing.
> 
> It was found by compaction stress test when I explicitly enable EROFS
> compressed files to use large folios, which case I cannot reproduce with
> the same workload if large folio support is off (current mainline).
> Typically, filesystem reads (with locked file-backed folios) could use
> another bdev/meta inode to load some other I/Os (e.g. inode extent
> metadata or caching compressed data), so the locking order will be:
> 
>    file-backed folios  (A)
>       bdev/meta folios (B)
> 
> The following calltrace shows the deadlock:
>     Thread 1 takes (B) lock and tries to take folio (A) lock
>     Thread 2 takes (A) lock and tries to take folio (B) lock
> 
> [Thread 1]
> INFO: task stress:1824 blocked for more than 30 seconds.
>        Tainted: G           OE      6.10.0-rc7+ #6
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:stress          state:D stack:0     pid:1824  tgid:1824  ppid:1822   flags:0x0000000c
> Call trace:
>   __switch_to+0xec/0x138
>   __schedule+0x43c/0xcb0
>   schedule+0x54/0x198
>   io_schedule+0x44/0x70
>   folio_wait_bit_common+0x184/0x3f8
> 			<-- folio mapping ffff00036d69cb18 index 996  (**)
>   __folio_lock+0x24/0x38
>   migrate_pages_batch+0x77c/0xea0	// try_split_folio (mm/migrate.c:1486:2)
> 					// migrate_pages_batch (mm/migrate.c:1734:16)
> 		<--- LIST_HEAD(unmap_folios) has
> 			..
> 			folio mapping 0xffff0000d184f1d8 index 1711;   (*)
> 			folio mapping 0xffff0000d184f1d8 index 1712;
> 			..
>   migrate_pages+0xb28/0xe90
>   compact_zone+0xa08/0x10f0
>   compact_node+0x9c/0x180
>   sysctl_compaction_handler+0x8c/0x118
>   proc_sys_call_handler+0x1a8/0x280
>   proc_sys_write+0x1c/0x30
>   vfs_write+0x240/0x380
>   ksys_write+0x78/0x118
>   __arm64_sys_write+0x24/0x38
>   invoke_syscall+0x78/0x108
>   el0_svc_common.constprop.0+0x48/0xf0
>   do_el0_svc+0x24/0x38
>   el0_svc+0x3c/0x148
>   el0t_64_sync_handler+0x100/0x130
>   el0t_64_sync+0x190/0x198
> 
> [Thread 2]
> INFO: task stress:1825 blocked for more than 30 seconds.
>        Tainted: G           OE      6.10.0-rc7+ #6
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:stress          state:D stack:0     pid:1825  tgid:1825  ppid:1822   flags:0x0000000c
> Call trace:
>   __switch_to+0xec/0x138
>   __schedule+0x43c/0xcb0
>   schedule+0x54/0x198
>   io_schedule+0x44/0x70
>   folio_wait_bit_common+0x184/0x3f8
> 			<-- folio = 0xfffffdffc6b503c0 (mapping == 0xffff0000d184f1d8 index == 1711) (*)
>   __folio_lock+0x24/0x38
>   z_erofs_runqueue+0x384/0x9c0 [erofs]
>   z_erofs_readahead+0x21c/0x350 [erofs]       <-- folio mapping 0xffff00036d69cb18 range from [992, 1024] (**)
>   read_pages+0x74/0x328
>   page_cache_ra_order+0x26c/0x348
>   ondemand_readahead+0x1c0/0x3a0
>   page_cache_sync_ra+0x9c/0xc0
>   filemap_get_pages+0xc4/0x708
>   filemap_read+0x104/0x3a8
>   generic_file_read_iter+0x4c/0x150
>   vfs_read+0x27c/0x330
>   ksys_pread64+0x84/0xd0
>   __arm64_sys_pread64+0x28/0x40
>   invoke_syscall+0x78/0x108
>   el0_svc_common.constprop.0+0x48/0xf0
>   do_el0_svc+0x24/0x38
>   el0_svc+0x3c/0x148
>   el0t_64_sync_handler+0x100/0x130
>   el0t_64_sync+0x190/0x198
> 
> Fixes: 5dfab109d519 ("migrate_pages: batch _unmap and _move")
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb