Hey Andreas,

On Mon, Jun 18, 2012 at 12:25:37PM -0600, Andreas Dilger wrote:

On 2012-06-18, at 6:08 AM, Christoph Hellwig wrote:

quoted

May saw the release of Linux 3.4, including a decent sized XFS update.
Remarkable XFS features in Linux 3.4 include moving over all metadata
updates to use transactions, the addition of a work queue for the
low-level allocator code to avoid stack overflows due to extreme stack
use in the Linux VM/VFS call chain,

This is essentially a workaround for too-small stacks in the kernel,
which we've had to do at times as well, by doing work in a separate
thread (with a new stack) and waiting for the results?  This is a
generic problem that any reasonably-complex filesystem will have when
running under memory pressure on a complex storage stack (e.g. LVM +
iSCSI), but causes unnecessary context switching.

Any thoughts on a better way to handle this, or will there continue
to be a 4kB stack limit and hack around this with repeated kmalloc
on callpaths for any struct over a few tens of bytes, implementing
memory pools all over the place, and "forking" over to other threads
to continue the stack consumption for another 4kB to work around
the small stack limit?

FWIW, I think your characterization of the problem as a 'workaround for
too-small stacks in the kernel' is about right.  I don't think any of the XFS
folk were very happy about having to do this, but in the near term it doesn't
seem that we have a good alternative.  I'm glad to see that there are others
with the same pain, so maybe we can build some support for upping the stack
limit.

Regards,
	Ben

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