Re: [PATCH 2/2] Prevent shutdown on inode allocation failure

[Christoph Hellwig <hch@infradead.org>]

On Thu, Mar 20, 2008 at 04:21:00PM +1100, David Chinner wrote:
> At ENOSPC, we can get a filesystem shutdown due to a cancelling a
> dirty transaction in xfs_mkdir or xfs_create. This is due to the
> initial allocation attempt not taking into inode alignment and hence
> we can prepare the AGF freelist for allocation when it's not actually
> possible to do an allocation. This results in inode allocation returning
> ENOSPC with a dirty transaction, and hence we shut down the filesystem.
> 
> Because the first allocation is an exact allocation attempt, we must tell
> the allocator that the alignment does not affect the allocation attempt.
> i.e. we will accept any extent alignment as long as the extent starts
> at the block we want. Unfortunately, this means that if the longest
> free extent is less than the length + alignment necessary for fallback
> allocation attempts but is long enough to attempt a non-aligned allocation,
> we will modify the free list.
> 
> If we then have the exact allocation fail, all other allocation attempts
> will also fail due to the alignment constraint being taken into account.
> Hence the initial attempt needs to set the "alignment slop" field so
> that alignment, while not required, must be taken into account when
> determining if there is enough space left in the AG to do the allocation.
> 
> That means if the exact allocation fails, we will not dirty the freelist
> if there is not enough space available fo a subsequent allocation to
> succeed. Hence we get an ENOSPC error back to userspace without shutting
> down the filesystem.

Looks good.