hello, On 07/06/2016 08:27 PM, Holger Hoffstätte wrote:
On 07/06/16 12:37, Wang Xiaoguang wrote:Below test scripts can reproduce this false ENOSPC: #!/bin/bash dd if=/dev/zero of=fs.img bs=$((1024*1024)) count=128 dev=$(losetup --show -f fs.img) mkfs.btrfs -f -M $dev mkdir /tmp/mntpoint mount /dev/loop0 /tmp/mntpoint cd mntpoint xfs_io -f -c "falloc 0 $((40*1024*1024))" testfileAbove fallocate(2) operation will fail for ENOSPC reason, but indeed fs still has free space to satisfy this request. The reason is btrfs_fallocate() dose not decrease btrfs_space_info's bytes_may_use just in time, and it calls btrfs_free_reserved_data_space_noquota() in the end of btrfs_fallocate(), which is too late and have already added false unnecessary pressure to enospc system. See call graph: btrfs_fallocate() |-> btrfs_alloc_data_chunk_ondemand() It will add btrfs_space_info's bytes_may_use accordingly. |-> btrfs_prealloc_file_range() It will call btrfs_reserve_extent(), but note that alloc type is RESERVE_ALLOC_NO_ACCOUNT, so btrfs_update_reserved_bytes() will only increase btrfs_space_info's bytes_reserved accordingly, but will not decrease btrfs_space_info's bytes_may_use, then obviously we have overestimated real needed disk space, and it'll impact other processes who do write(2) or fallocate(2) operations, also can impact metadata reservation in mixed mode, and bytes_max_use will only be decreased in the end of btrfs_fallocate(). To fix this false ENOSPC, we need to decrease btrfs_space_info's bytes_may_use in btrfs_prealloc_file_range() in time, as what we do in cow_file_range(), See call graph in : cow_file_range() |-> extent_clear_unlock_delalloc() |-> clear_extent_bit() |-> btrfs_clear_bit_hook() |-> btrfs_free_reserved_data_space_noquota() This function will decrease bytes_may_use accordingly. So this patch choose to call btrfs_free_reserved_data_space() in __btrfs_prealloc_file_range() for both successful and failed path. Also this patch removes some old and useless comments. Signed-off-by: Wang Xiaoguang <wangxg.f...@cn.fujitsu.com>Verified that the reproducer script indeed fails (with btrfs ~4.7) and the patch (on top of 1/2) fixes it. Also ran a bunch of other fallocating things without problem. Free space also still seems sane, as far as I could tell. So for both patches: Tested-by: Holger Hoffstätte <hol...@applied-asynchrony.com>
Thanks very much :) Regards, Xiaoguang Wang
cheers, Holger
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