Re: [PATCH 3/6] xfs: invalidate allocbt blocks moved to the free list

Mark Tinguely
2012-11-02T21:21:48Z

On 11/01/12 19:38, Dave Chinner wrote:
> From: Dave Chinner<dchinner@redhat.com>
>
> When we free a block from the alloc btree tree, we move it to the
> freelist held in the AGFL and mark it busy in the busy extent tree.
> This typically happens when we merge btree blocks.
>
> Once the transaction is committed and checkpointed, the block can
> remain on the free list for an indefinite amount of time. Now, this
> isn't the end of the world at this point - if the free list is
> shortened, the buffer is invalidated in the transaction that moves
> it back to free space. If the buffer is allocated as metadata from
> the free list, then all the modifications getted logged, and we have
> no issues, either. And if it gets allocated as userdata direct from
> the freelist, it gets invalidated and so will never get written.
>
> However, during the time it sits on the free list, pressure on the
> log can cause the AIL to be pushed and the buffer that covers the
> block gets pushed for write. IOWs, we end up writing a freed
> metadata block to disk. Again, this isn't the end of the world
> because we know from the above we are only writing to free space.
>
> The problem, however, is for validation callbacks. If the block was
> on old btree root block, then the level of the block is going to be
> higher than the current tree root, and so will fail validation.
> There may be other inconsistencies in the block as well, and
> currently we don't care because the block is in free space. Shutting
> down the filesystem because a freed block doesn't pass write
> validation, OTOH, is rather unfriendly.
>
> So, make sure we always invalidate buffers as they move from the
> free space trees to the free list so that we guarantee they never
> get written to disk while on the free list.
>
> Signed-off-by: Dave Chinner<dchinner@redhat.com>
> Reviewed-by: Christoph Hellwig<hch@lst.de>

Looks good.

Reviewed-by: Mark Tinguely <tinguely@sgi.com>