xfs_iext_realloc_indirect and "XFS: possible memory allocation deadlock"

[Dave Chinner]

On Thu, Jul 23, 2015 at 05:39:28PM +0200, Alex Lyakas wrote:
> Hi Dave,
> Just for completeness,  XFS speculative preallocation (that is based
> now upon the size of the last extent) can still grow up to 4Gb-8Gb
> (depending on which patches we are pulling). As a result, xfs_iozero
> can still sometimes trigger 1-2GB writes of zeros in one shot. This
> turns out to be a bit unfriendly to the drives in some
> configurations. So we have applied a custom patch to limit the
> speculative preallocation to 32Mb.

It would be much better to change xfs_zero_eof() to convert extents
beyond EOF to unwritten extents rather than zero them. That way you
still get the benefits of the large speculative prealloc but without
the zeroing overhead that wierd sparse write patterns can trigger.

I just haven't got around to doing this because it hasn't been
reported as a significant problem for anyone until now.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com