Re: [PATCH 2/2] btrfs: fix false ENOSPC for btrfs_fallocate()

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

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))" testfile
> 
> Above 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.fnst@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 <holger@applied-asynchrony.com>

cheers,
Holger