On Thu, 28 Jul 2016, Michal Hocko wrote:

quoted

quoted

quoted

I think we'd end up with cleaner code if we removed the cute-hacks.  And
we'd be able to use 6 more GFP flags!!  (though I do wonder if we really
need all those 26).

Well, maybe we are able to remove those hacks, I wouldn't definitely
be opposed.  But right now I am not even convinced that the mempool
specific gfp flags is the right way to go.

I'm not suggesting a mempool-specific gfp flag.  I'm suggesting a
transient-allocation gfp flag, which would be quite useful for mempool.

Can you give more details on why using a gfp flag isn't your first choice
for guiding what happens when the system is trying to get a free page
:-?

If we get rid of throttle_vm_writeout then I guess it might turn out to
be unnecessary. There are other places which will still throttle but I
believe those should be kept regardless of who is doing the allocation
because they are helping the LRU scanning sane. I might be wrong here
and bailing out from the reclaim rather than waiting would turn out
better for some users but I would like to see whether the first approach
works reasonably well.

If we are swapping to a dm-crypt device, the dm-crypt device is congested 
and the underlying block device is not congested, we should not throttle 
mempool allocations made from the dm-crypt workqueue. Not even a little 
bit.

So, I think, mempool_alloc should set PF_NO_THROTTLE (or 
__GFP_NO_THROTTLE).

Mikulas

-- 
Michal Hocko
SUSE Labs

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