iomap infrastructure and multipage writes V5

[Dave Chinner]

On Sun, Jul 31, 2016 at 09:19:00PM +0200, Christoph Hellwig wrote:
> Now after spending this much time I've started wondering why we even
> reserve blocks in xfs_iomap_write_allocate - after all we've reserved
> space for the actual data blocks and the indlen worst case in
> xfs_bmapi_reserve_delalloc.  And in fact a little hack to drop that
> reservation seems to solve both the root cause (depleted reserved pool)
> and the cleanup mess.  I just haven't spend enought time to convince
> myself that it's actually safe, and in fact looking at the allocator
> makes me thing it only works by accident currently despite generally
> postive test results.
> 
> Here is the quick patch if anyone wants to chime in:
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 620fc91..67c317f 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -717,7 +717,7 @@ xfs_iomap_write_allocate(
>  
>  		nimaps = 0;
>  		while (nimaps == 0) {
> -			nres = XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
> +			nres = 0; // XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
>  
>  			error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, nres,
>  					0, XFS_TRANS_RESERVE, &tp);
> 

This solves the problem for me, and from history appears to be the
right thing to do. Christoph, can you send a proper patch for this?

Cheers,

Dave.

-- 
Dave Chinner
david at fromorbit.com