Re: [PATCH v6] mm: Add memory allocation watchdog kernel thread.

[Johannes Weiner <hannes@cmpxchg.org>]

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@cmpxchg.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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