Re: [BUG] during balance operation, WARNING: at fs/btrfs/relocation.c:1624 replace_file_extents+0x74b/0x7e0 [btrfs]()

[Liu Bo <bo.li.liu@oracle.com>]

On Tue, Mar 05, 2013 at 12:59:05PM +0100, Stefan Behrens wrote:
> On Mon, 4 Mar 2013 14:31:37 -0500, Chris Mason wrote:
> > On Mon, Mar 04, 2013 at 10:24:39AM -0700, Stefan Behrens wrote:
> >> Just ran the following command sequence and got lots of WARNINGs.
> >> The issue is reproducible.
> >> The box was running the cmason/for-linus that made it into Linux 3.9 RC1.
> >>
> >> #!/bin/sh
> >> mkfs.btrfs -f /dev/sdl /dev/sdk -m raid1 -d raid1 -l 16384
> >> mount /dev/sdl /mnt
> >> dd if=/dev/urandom of=/mnt/urandom.1GB bs=10M count=100 &
> >> dd if=/dev/zero of=/mnt/zero.4GB bs=10M count=400 &
> >> (cd ~/kernel-src; tar cf - fs) | (cd /mnt && tar xf -)
> >> wait
> >>
> >> ((cd ~/kernel-src; tar cf - drivers) | (cd /mnt && tar xf -)) &
> >> sleep 5
> >> btrfs fi balance start /mnt
> > 
> > This doesn't look new, are you able to trigger it with an older kernel?
> > 
> 
> git bisect identifies the following post v3.8 commit to be the one:

This bisect can explain a lot :)

Relocating file extents writes their data into reloc inode and then later
updates file extent pointer from reloc inode back to the original block
pointers.

Now prealloc gets the file extent's size smaller than the original one, so here
comes those warnings.

thanks,
liubo

> 
> commit 24542bf7ea5e4fdfdb5157ff544c093fa4dcb536
> Author: Zach Brown <zab@redhat.com>
> Date:   Fri Nov 16 00:04:43 2012 +0000
> 
>     btrfs: limit fallocate extent reservation to 256MB
> 
>     Very large fallocate requests are cpu bound and result in extents with a
>     repeating pattern of ever decreasing size:
>     
>     $ time fallocate -l 1T file
>     real	0m13.039s
>     
>     ( an excerpt of the extents from btrfs-debug-tree: )
>       prealloc data disk byte 1536292564992 nr 397312
>       prealloc data disk byte 1536292962304 nr 196608
>       prealloc data disk byte 1536293158912 nr 98304
>       prealloc data disk byte 1536293257216 nr 49152
>       prealloc data disk byte 1536293306368 nr 24576
>       prealloc data disk byte 1536293330944 nr 12288
>       prealloc data disk byte 1536293343232 nr 8192
>       prealloc data disk byte 1536293351424 nr 4096
>       prealloc data disk byte 1536293355520 nr 4096
>       prealloc data disk byte 1536293359616 nr 4096
>     
>     The excessive cpu use comes from __btrfs_prealloc_file_range() trying to
>     allocate the entire remaining size after each extent is allocated.
>     btrfs_reserve_extent() repeatedly cuts this requested size in half until
>     it gets down to the size that the allocators can return.  We limit the
>     problem for now by capping each reservation at 256 meg.
>     
>     The small extents come from a masking bug when decreasing the requested
>     reservation size.  The high 32bits are cleared and the remaining low
>     bits might happen to reserve a small size.   Fix this by using
>     round_down() which properly casts the mask.
>     
>     After these fixes huge fallocate requests are fast and result in nice
>     large extents:
>     
>     $ time fallocate -l 1T file
>     real	0m0.082s
>     
>       prealloc data disk byte 1112425889792 nr 268435456
>       prealloc data disk byte 1112694325248 nr 268435456
>       prealloc data disk byte 1112962760704 nr 268435456
>     
>     Reported-by: Eric Sandeen <sandeen@redhat.com>
>     Signed-off-by: Zach Brown <zab@redhat.com>
>     Signed-off-by: Chris Mason <chris.mason@fusionio.com>
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index b3ecca4..d2b3a5e 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -6143,7 +6143,7 @@ again:
>  	if (ret == -ENOSPC) {
>  		if (!final_tried) {
>  			num_bytes = num_bytes >> 1;
> -			num_bytes = num_bytes & ~(root->sectorsize - 1);
> +			num_bytes = round_down(num_bytes, root->sectorsize);
>  			num_bytes = max(num_bytes, min_alloc_size);
>  			if (num_bytes == min_alloc_size)
>  				final_tried = true;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 4e6a11c..3bc62b1 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -7894,8 +7894,9 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
>  			}
>  		}
>  
> -		ret = btrfs_reserve_extent(trans, root, num_bytes, min_size,
> -					   0, *alloc_hint, &ins, 1);
> +		ret = btrfs_reserve_extent(trans, root,
> +					   min(num_bytes, 256ULL * 1024 * 1024),
> +					   min_size, 0, *alloc_hint, &ins, 1);
>  		if (ret) {
>  			if (own_trans)
>  				btrfs_end_transaction(trans, root);
> 
> 
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