On Fri, Dec 18, 2015 at 12:51:07PM +1100, NeilBrown wrote:

quoted

if the reclaim thread doesn't have anything to reclaim,
r5l_run_no_space_stripes isn't called. we might miss the retry.

so something like this:

that looks fine to me.  I'll give a spin on my QA setup.

quoted

I'm a little worrying about the GFP_ATOMIC allocation. In the first try,
GFP_NOWAIT is better. And on the other hand, why sleep is bad here? We
could use GFP_NOIO | __GFP_NORETRY, there is no deadlock risk.

In the retry, GFP_NOIO looks better. No deadlock too, since it's not
called from raid5d (maybe we shouldn't call from reclaim thread if using
GFP_NOIO, a workqueue is better). Otherwise we could keep retring but do
nothing.

I did wonder a little bit about that.
GFP_ATOMIC is (__GFP_HIGH)
GFP_NOIO | __GFP_NORETRY is  (__GFP_WAIT | __GFP_NORETRY)

It isn't clear that we need 'HIGH', and WAIT with NORETRY should be OK.
It allows __alloc_pages_direct_reclaim, but only once and never waits
for other IO.

In general we go for HIGH on non-sleeping allocation to avoid having
the stalled.  This is especially important in the I/O path.

WAIT means we will reclaim and wait for it, which looks a little dangerous
to me.  In general I'd prefer not to use obscure gfp flag combination
unless there is a real need, and it's clearly documented.