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Date: Wed, 25 Jan 2017 14:22:45 -0500 From: Johannes Weiner <hannes@...xchg.org> To: Michal Hocko <mhocko@...nel.org> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v6] mm: Add memory allocation watchdog kernel thread. On Wed, Jan 25, 2017 at 07:45:49PM +0100, Michal Hocko wrote: > On Wed 25-01-17 13:11:50, Johannes Weiner wrote: > [...] > > >From 6420cae52cac8167bd5fb19f45feed2d540bc11d Mon Sep 17 00:00:00 2001 > > From: Johannes Weiner <hannes@...xchg.org> > > Date: Wed, 25 Jan 2017 12:57:20 -0500 > > Subject: [PATCH] mm: page_alloc: __GFP_NOWARN shouldn't suppress stall > > warnings > > > > __GFP_NOWARN, which is usually added to avoid warnings from callsites > > that expect to fail and have fallbacks, currently also suppresses > > allocation stall warnings. These trigger when an allocation is stuck > > inside the allocator for 10 seconds or longer. > > > > But there is no class of allocations that can get legitimately stuck > > in the allocator for this long. This always indicates a problem. > > > > Always emit stall warnings. Restrict __GFP_NOWARN to alloc failures. > > Tetsuo has already suggested something like this and I didn't really > like it because it makes the semantic of the flag confusing. The mask > says to not warn while the kernel log might contain an allocation splat. > You are right that stalling for 10s seconds means a problem on its own > but on the other hand I can imagine somebody might really want to have > clean logs and the last thing we want is to have another gfp flag for > that purpose. I don't think it's confusing. __GFP_NOWARN tells the allocator whether an allocation failure can be handled or whether it constitutes a bug. If we agree that stalling for 10s is a bug, then we should emit the warnings. Tying this to whether the caller can handle an allocation failure is non-sensical. Not warning about a bug because the user would prefer clean logs is... somewhat out there.
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