[dm-devel] [RFC PATCH 1/2] mempool: do not consume memory reserves from the reclaim path

[Johannes Weiner]

On Thu, Jul 21, 2016 at 10:52:03AM +0200, Michal Hocko wrote:
> Look, there are
> $ git grep mempool_alloc | wc -l
> 304
> 
> many users of this API and we do not want to flip the default behavior
> which is there for more than 10 years. So far you have been arguing
> about potential deadlocks and haven't shown any particular path which
> would have a direct or indirect dependency between mempool and normal
> allocator and it wouldn't be a bug. As the matter of fact the change
> we are discussing here causes a regression. If you want to change the
> semantic of mempool allocator then you are absolutely free to do so. In
> a separate patch which would be discussed with IO people and other
> users, though. But we _absolutely_ want to fix the regression first
> and have a simple fix for 4.6 and 4.7 backports. At this moment there
> are revert and patch 1 on the table.  The later one should make your
> backtrace happy and should be only as a temporal fix until we find out
> what is actually misbehaving on your systems. If you are not interested
> to pursue that way I will simply go with the revert.

+1

It's very unlikely that decade-old mempool semantics are suddenly a
fundamental livelock problem, when all the evidence we have is one
hang and vague speculation. Given that the patch causes regressions,
and that the bug is most likely elsewhere anyway, a full revert rather
than merely-less-invasive mempool changes makes the most sense to me.