On Thu, 4 Sep 2014 15:54:27 +0200 Michal Hocko <mhocko@xxxxxxx> wrote: > [Sorry for jumping in so late - I've been busy last days] > > On Wed 27-08-14 16:36:44, Mel Gorman wrote: > > On Tue, Aug 26, 2014 at 08:00:20PM -0400, Trond Myklebust wrote: > > > On Tue, Aug 26, 2014 at 7:51 PM, Trond Myklebust > > > <trond.myklebust@xxxxxxxxxxxxxxx> wrote: > > > > On Tue, Aug 26, 2014 at 7:19 PM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > [...] > > > >> wait_on_page_writeback() is a hammer, and we need to be better about > > > >> this once we have per-memcg dirty writeback and throttling, but I > > > >> think that really misses the point. Even if memcg writeback waiting > > > >> were smarter, any length of time spent waiting for yourself to make > > > >> progress is absurd. We just shouldn't be solving deadlock scenarios > > > >> through arbitrary timeouts on one side. If you can't wait for IO to > > > >> finish, you shouldn't be passing __GFP_IO. > > Exactly! This is overly simplistic. The code that cannot wait may be further up the call chain and not in a position to avoid passing __GFP_IO. In many case it isn't that "you can't wait for IO" in general, but that you cannot wait for one specific IO request. wait_on_page_writeback() waits for a specific IO and so is dangerous. congestion_wait() or similar waits for IO in general and so is much safer. NeilBrown
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