Btrfs: fix deadlock between direct IO write and defrag/readpages
If readpages() (triggered by defrag or buffered reads) is called while a direct IO write is in progress, we have a small time window where we can deadlock, resulting in traces like the following being generated: [84723.212993] INFO: task fio:2849 blocked for more than 120 seconds. [84723.214310] Tainted: G W 4.3.0-rc5-btrfs-next-17+ #1 [84723.215640] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [84723.217313] fio D ffff88023ec75218 0 2849 2835 0x00000000 [84723.218778] ffff880122dfb6e8 0000000000000092 0000000000000000 ffff88023ec75200 [84723.220458] ffff88000e05d2c0 ffff880122dfc000 ffff88023ec75200 7fffffffffffffff [84723.230597] 0000000000000002 ffffffff8147891a ffff880122dfb700 ffffffff8147856a [84723.232085] Call Trace: [84723.232625] [<ffffffff8147891a>] ? bit_wait+0x3c/0x3c [84723.233529] [<ffffffff8147856a>] schedule+0x7d/0x95 [84723.234398] [<ffffffff8147baa3>] schedule_timeout+0x43/0x10b [84723.235384] [<ffffffff810f82eb>] ? time_hardirqs_on+0x15/0x28 [84723.236426] [<ffffffff8108a23d>] ? trace_hardirqs_on+0xd/0xf [84723.237502] [<ffffffff810af8a3>] ? read_seqcount_begin.constprop.20+0x57/0x6d [84723.238807] [<ffffffff8108a09b>] ? trace_hardirqs_on_caller+0x16/0x1ab [84723.242012] [<ffffffff8108a23d>] ? trace_hardirqs_on+0xd/0xf [84723.243064] [<ffffffff810af2ad>] ? timekeeping_get_ns+0xe/0x33 [84723.244116] [<ffffffff810afa2e>] ? ktime_get+0x41/0x52 [84723.245029] [<ffffffff81477cff>] io_schedule_timeout+0xb7/0x12b [84723.245942] [<ffffffff81477cff>] ? io_schedule_timeout+0xb7/0x12b [84723.246596] [<ffffffff81478953>] bit_wait_io+0x39/0x45 [84723.247503] [<ffffffff81478b93>] __wait_on_bit_lock+0x49/0x8d [84723.248540] [<ffffffff8111684f>] __lock_page+0x66/0x68 [84723.249558] [<ffffffff81081c9b>] ? autoremove_wake_function+0x3a/0x3a [84723.250844] [<ffffffff81124a04>] lock_page+0x2c/0x2f [84723.251871] [<ffffffff81124afc>] invalidate_inode_pages2_range+0xf5/0x2aa [84723.253274] [<ffffffff81117c34>] ? filemap_fdatawait_range+0x12d/0x146 [84723.254757] [<ffffffff81118191>] ? filemap_fdatawrite_range+0x13/0x15 [84723.256378] [<ffffffffa05139a2>] btrfs_get_blocks_direct+0x1b0/0x664 [btrfs] [84723.258556] [<ffffffff8119e3f9>] ? submit_page_section+0x7b/0x111 [84723.260064] [<ffffffff8119eb90>] do_blockdev_direct_IO+0x658/0xbdb [84723.261479] [<ffffffffa05137f2>] ? btrfs_page_exists_in_range+0x1a9/0x1a9 [btrfs] [84723.262961] [<ffffffffa050a8a6>] ? btrfs_writepage_start_hook+0xce/0xce [btrfs] [84723.264449] [<ffffffff8119f144>] __blockdev_direct_IO+0x31/0x33 [84723.265614] [<ffffffff8119f144>] ? __blockdev_direct_IO+0x31/0x33 [84723.266769] [<ffffffffa050a8a6>] ? btrfs_writepage_start_hook+0xce/0xce [btrfs] [84723.268264] [<ffffffffa050935d>] btrfs_direct_IO+0x1b9/0x259 [btrfs] [84723.270954] [<ffffffffa050a8a6>] ? btrfs_writepage_start_hook+0xce/0xce [btrfs] [84723.272465] [<ffffffff8111878c>] generic_file_direct_write+0xb3/0x128 [84723.273734] [<ffffffffa051955c>] btrfs_file_write_iter+0x228/0x404 [btrfs] [84723.275101] [<ffffffff8116ca6f>] __vfs_write+0x7c/0xa5 [84723.276200] [<ffffffff8116cfab>] vfs_write+0xa0/0xe4 [84723.277298] [<ffffffff8116d79d>] SyS_write+0x50/0x7e [84723.278327] [<ffffffff8147cd97>] entry_SYSCALL_64_fastpath+0x12/0x6f [84723.279595] INFO: lockdep is turned off. [84723.379035] INFO: task btrfs:2923 blocked for more than 120 seconds. [84723.380323] Tainted: G W 4.3.0-rc5-btrfs-next-17+ #1 [84723.381608] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [84723.383003] btrfs D ffff88023ed75218 0 2923 2859 0x00000000 [84723.384277] ffff88001311f860 0000000000000082 ffff88001311f840 ffff88023ed75200 [84723.385748] ffff88012c6751c0 ffff880013120000 ffff88012042fe68 ffff88012042fe30 [84723.387152] ffff880221571c88 0000000000000001 ffff88001311f878 ffffffff8147856a [84723.388620] Call Trace: [84723.389105] [<ffffffff8147856a>] schedule+0x7d/0x95 [84723.391882] [<ffffffffa051da32>] btrfs_start_ordered_extent+0x161/0x1fa [btrfs] [84723.393718] [<ffffffff81081c61>] ? signal_pending_state+0x31/0x31 [84723.395659] [<ffffffffa0522c5b>] __do_contiguous_readpages.constprop.21+0x81/0xdc [btrfs] [84723.397383] [<ffffffffa050ac96>] ? btrfs_submit_direct+0x3f0/0x3f0 [btrfs] [84723.398852] [<ffffffffa0522da3>] __extent_readpages.constprop.20+0xed/0x100 [btrfs] [84723.400561] [<ffffffff81123f6c>] ? __lru_cache_add+0x5d/0x72 [84723.401787] [<ffffffffa0523896>] extent_readpages+0x111/0x1a7 [btrfs] [84723.403121] [<ffffffffa050ac96>] ? btrfs_submit_direct+0x3f0/0x3f0 [btrfs] [84723.404583] [<ffffffffa05088fa>] btrfs_readpages+0x1f/0x21 [btrfs] [84723.406007] [<ffffffff811226df>] __do_page_cache_readahead+0x168/0x1f4 [84723.407502] [<ffffffff81122988>] ondemand_readahead+0x21d/0x22e [84723.408937] [<ffffffff81122988>] ? ondemand_readahead+0x21d/0x22e [84723.410487] [<ffffffff81122af1>] page_cache_sync_readahead+0x3d/0x3f [84723.411710] [<ffffffffa0535388>] btrfs_defrag_file+0x419/0xaaf [btrfs] [84723.413007] [<ffffffffa0531db0>] ? kzalloc+0xf/0x11 [btrfs] [84723.414085] [<ffffffffa0535b43>] btrfs_ioctl_defrag+0x125/0x14e [btrfs] [84723.415307] [<ffffffffa0536753>] btrfs_ioctl+0x746/0x24c6 [btrfs] [84723.416532] [<ffffffff81087481>] ? arch_local_irq_save+0x9/0xc [84723.417731] [<ffffffff8113ad61>] ? __might_fault+0x4c/0xa7 [84723.418699] [<ffffffff8113ad61>] ? __might_fault+0x4c/0xa7 [84723.421532] [<ffffffff8113adba>] ? __might_fault+0xa5/0xa7 [84723.422629] [<ffffffff81171139>] ? cp_new_stat+0x15d/0x174 [84723.423712] [<ffffffff8117c610>] do_vfs_ioctl+0x427/0x4e6 [84723.424801] [<ffffffff81171175>] ? SYSC_newfstat+0x25/0x2e [84723.425968] [<ffffffff8118574d>] ? __fget_light+0x4d/0x71 [84723.427063] [<ffffffff8117c726>] SyS_ioctl+0x57/0x79 [84723.428138] [<ffffffff8147cd97>] entry_SYSCALL_64_fastpath+0x12/0x6f Consider the following logical and physical file layout: logical: ... [ prealloc extent A ] [ prealloc extent B ] [ extent C ] ... 4K 8K 16K physical: ... 12853248 12857344 1103101952 ... (= 12853248 + 4K) Extents A and B are physically adjacent. The following diagram shows a sequence of events that lead to the deadlock when we attempt to do a direct IO write against the file range [4K, 16K[ and a defrag is triggered simultaneously. CPU 1 CPU 2 btrfs_direct_IO() btrfs_get_blocks_direct() creates ordered extent A, covering the 4k prealloc extent A (range [4K, 8K[) btrfs_defrag_file() page_cache_sync_readahead([0K, 1M[) btrfs_readpages() extent_readpages() locks all pages in the file range [0K, 128K[ through calls to add_to_page_cache_lru() __do_contiguous_readpages() finds ordered extent A waits for it to complete btrfs_get_blocks_direct() called again lock_extent_direct(range [8K, 16K[) finds a page in range [8K, 16K[ through btrfs_page_exists_in_range() invalidate_inode_pages2_range([8K, 16K[) --> tries to lock pages that are already locked by the task at CPU 2 --> our task, running __blockdev_direct_IO(), hangs waiting to lock the pages and the submit bio callback, btrfs_submit_direct(), ends up never being called, resulting in the ordered extent A never completing (because a corresponding bio is never submitted) and CPU 2 will wait for it forever while holding the pages locked ---> deadlock! Fix this by removing the page invalidation approach when attempting to lock the range for IO from the callback btrfs_get_blocks_direct() and falling back buffered IO. This was a rare case anyway and well behaved applications do not mix concurrent direct IO writes with buffered reads anyway, being a concurrent defrag the only normal case that could lead to the deadlock. Signed-off-by: Filipe Manana <fdmanana@suse.com>
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@ -7411,25 +7411,21 @@ static int lock_extent_direct(struct inode *inode, u64 lockstart, u64 lockend,
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btrfs_start_ordered_extent(inode, ordered, 1);
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btrfs_put_ordered_extent(ordered);
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} else {
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/* Screw you mmap */
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ret = btrfs_fdatawrite_range(inode, lockstart, lockend);
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if (ret)
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break;
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ret = filemap_fdatawait_range(inode->i_mapping,
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lockstart,
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lockend);
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if (ret)
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break;
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/*
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* If we found a page that couldn't be invalidated just
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* fall back to buffered.
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* We could trigger writeback for this range (and wait
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* for it to complete) and then invalidate the pages for
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* this range (through invalidate_inode_pages2_range()),
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* but that can lead us to a deadlock with a concurrent
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* call to readpages() (a buffered read or a defrag call
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* triggered a readahead) on a page lock due to an
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* ordered dio extent we created before but did not have
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* yet a corresponding bio submitted (whence it can not
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* complete), which makes readpages() wait for that
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* ordered extent to complete while holding a lock on
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* that page.
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*/
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ret = invalidate_inode_pages2_range(inode->i_mapping,
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lockstart >> PAGE_CACHE_SHIFT,
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lockend >> PAGE_CACHE_SHIFT);
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if (ret)
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break;
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ret = -ENOTBLK;
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break;
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}
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cond_resched();
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