xfs: don't allow recursion into fs under write_begin

Michael Monnerie reported this fantastic stack overflow:

[21877.948005] BUG: scheduling while atomic: rsync/2345/0xffff8800
[21877.948005] Modules linked in: af_packet nfs lockd fscache nfs_acl auth_rpcgss sunrpc ipv6 ramzswap xvmalloc lzo_decompress lzo_compress loop dm_mod reiserfs xfs exportfs xennet xenblk cdrom
[21877.948005] Pid: 2345, comm: rsync Not tainted 2.6.31.12-0.2-xen #1
[21877.948005] Call Trace:
[21877.949649]  [<ffffffff800119b9>] try_stack_unwind+0x189/0x1b0
[21877.949659]  [<ffffffff8000f466>] dump_trace+0xa6/0x1e0
[21877.949666]  [<ffffffff800114c4>] show_trace_log_lvl+0x64/0x90
[21877.949676]  [<ffffffff80011513>] show_trace+0x23/0x40
[21877.949684]  [<ffffffff8046b92c>] dump_stack+0x81/0x9e
[21877.949695]  [<ffffffff8003f398>] __schedule_bug+0x78/0x90
[21877.949702]  [<ffffffff8046c97c>] thread_return+0x1d7/0x3fb
[21877.949709]  [<ffffffff8046cf85>] schedule_timeout+0x195/0x200
[21877.949717]  [<ffffffff8046be2b>] wait_for_common+0x10b/0x230
[21877.949726]  [<ffffffff8046c09b>] wait_for_completion+0x2b/0x50
[21877.949768]  [<ffffffffa009e741>] xfs_buf_iowait+0x31/0x80 [xfs]
[21877.949894]  [<ffffffffa009ea30>] _xfs_buf_read+0x70/0x80 [xfs]
[21877.949992]  [<ffffffffa009ef8b>] xfs_buf_read_flags+0x8b/0xd0 [xfs]
[21877.950089]  [<ffffffffa0091ab9>] xfs_trans_read_buf+0x1e9/0x320 [xfs]
[21877.950174]  [<ffffffffa005b278>] xfs_btree_read_buf_block+0x68/0xe0 [xfs]
[21877.950232]  [<ffffffffa005b99e>] xfs_btree_lookup_get_block+0x8e/0x110 [xfs]
[21877.950281]  [<ffffffffa005c0af>] xfs_btree_lookup+0xdf/0x4d0 [xfs]
[21877.950329]  [<ffffffffa0042b77>] xfs_alloc_lookup_eq+0x27/0x50 [xfs]
[21877.950361]  [<ffffffffa0042f09>] xfs_alloc_fixup_trees+0x249/0x370 [xfs]
[21877.950397]  [<ffffffffa0044c30>] xfs_alloc_ag_vextent_near+0x4e0/0x9a0 [xfs]
[21877.950432]  [<ffffffffa00451f5>] xfs_alloc_ag_vextent+0x105/0x160 [xfs]
[21877.950471]  [<ffffffffa0045bb4>] xfs_alloc_vextent+0x3b4/0x4b0 [xfs]
[21877.950504]  [<ffffffffa0058da8>] xfs_bmbt_alloc_block+0xf8/0x210 [xfs]
[21877.950550]  [<ffffffffa005e3b7>] xfs_btree_split+0xc7/0x720 [xfs]
[21877.950597]  [<ffffffffa005ef8c>] xfs_btree_make_block_unfull+0x15c/0x1c0 [xfs]
[21877.950643]  [<ffffffffa005f3ff>] xfs_btree_insrec+0x40f/0x5c0 [xfs]
[21877.950689]  [<ffffffffa005f651>] xfs_btree_insert+0xa1/0x1b0 [xfs]
[21877.950748]  [<ffffffffa005325e>] xfs_bmap_add_extent_delay_real+0x82e/0x12a0 [xfs]
[21877.950787]  [<ffffffffa00540f4>] xfs_bmap_add_extent+0x424/0x450 [xfs]
[21877.950833]  [<ffffffffa00573f3>] xfs_bmapi+0xda3/0x1320 [xfs]
[21877.950879]  [<ffffffffa007c248>] xfs_iomap_write_allocate+0x1d8/0x3f0 [xfs]
[21877.950953]  [<ffffffffa007d089>] xfs_iomap+0x2c9/0x300 [xfs]
[21877.951021]  [<ffffffffa009a1b8>] xfs_map_blocks+0x38/0x60 [xfs]
[21877.951108]  [<ffffffffa009b93a>] xfs_page_state_convert+0x3fa/0x720 [xfs]
[21877.951204]  [<ffffffffa009bde4>] xfs_vm_writepage+0x84/0x160 [xfs]
[21877.951301]  [<ffffffff800e3603>] pageout+0x143/0x2b0
[21877.951308]  [<ffffffff800e514e>] shrink_page_list+0x26e/0x650
[21877.951314]  [<ffffffff800e5803>] shrink_inactive_list+0x2d3/0x7c0
[21877.951320]  [<ffffffff800e5d4b>] shrink_list+0x5b/0x110
[21877.951325]  [<ffffffff800e5f71>] shrink_zone+0x171/0x250
[21877.951330]  [<ffffffff800e60d3>] shrink_zones+0x83/0x120
[21877.951336]  [<ffffffff800e620e>] do_try_to_free_pages+0x9e/0x380
[21877.951342]  [<ffffffff800e6607>] try_to_free_pages+0x77/0xa0
[21877.951349]  [<ffffffff800dbfa3>] __alloc_pages_slowpath+0x2d3/0x5c0
[21877.951355]  [<ffffffff800dc3e1>] __alloc_pages_nodemask+0x151/0x160
[21877.951362]  [<ffffffff800d44b7>] __page_cache_alloc+0x27/0x50
[21877.951368]  [<ffffffff800d68ca>] grab_cache_page_write_begin+0x9a/0xe0
[21877.951376]  [<ffffffff8014bdfe>] block_write_begin+0xae/0x120
[21877.951396]  [<ffffffffa009ac24>] xfs_vm_write_begin+0x34/0x50 [xfs]
[21877.951482]  [<ffffffff800d4b31>] generic_perform_write+0xc1/0x1f0
[21877.951489]  [<ffffffff800d5d00>] generic_file_buffered_write+0x90/0x160
[21877.951512]  [<ffffffffa00a4711>] xfs_write+0x521/0xb60 [xfs]
[21877.951624]  [<ffffffffa009fb80>] xfs_file_aio_write+0x70/0xa0 [xfs]
[21877.951711]  [<ffffffff80118c42>] do_sync_write+0x102/0x160
[21877.951718]  [<ffffffff80118fc8>] vfs_write+0xd8/0x1c0
[21877.951723]  [<ffffffff8011995b>] sys_write+0x5b/0xa0
[21877.951729]  [<ffffffff8000c868>] system_call_fastpath+0x16/0x1b
[21877.951736]  [<00007fc41b0fab10>] 0x7fc41b0fab10
[21877.951750] BUG: unable to handle kernel paging request at 0000000108743280
[21877.951755] IP: [<ffffffff80034832>] dequeue_task+0x72/0x110
[21877.951766] PGD 31c6f067 PUD 0 
[21877.951770] Thread overran stack, or stack corrupted

I don't think we can afford to let write_begin recurse into the fs, so we can set AOP_FLAG_NOFS ... is this too big a hammer?

---

(resend with linux-fsdevel and linux-mm per Christoph's request)

index 57c3827..3fb2cc8 100644
@@ -1678,6 +1678,8 @@ xfs_vm_write_begin(
 	void			**fsdata)
 {
 	*pagep = NULL;
+	/* can't afford to recurse into fs due to stack reasons */
+	flags |= AOP_FLAG_NOFS;
 	return block_write_begin(file, mapping, pos, len, flags, pagep, fsdata,
 								xfs_get_blocks);
 }

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I don't really like it. There's nothing XFS-specific here - it's the same problem with direct reclaim calling back into the FS and causing massive amounts of problems. If we want to fix this class of problems we just need to do the same thing ext4 and btrfs already do and refuse to call the allocator from reclaim context.

Just curious, how much stack does the path up to generic_perform_write use?

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