Re: dm bufio: Reduce dm_bufio_lock contention

From: Michal Hocko <mhocko@kernel.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: jing xia <jing.xia.mail@gmail.com>,
	Mike Snitzer <snitzer@redhat.com>,
	agk@redhat.com, dm-devel@redhat.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: dm bufio: Reduce dm_bufio_lock contention
Date: Tue, 19 Jun 2018 12:43:12 +0200	[thread overview]
Message-ID: <20180619104312.GD13685@dhcp22.suse.cz> (raw)
In-Reply-To: <alpine.LRH.2.02.1806181003560.4201@file01.intranet.prod.int.rdu2.redhat.com>

On Mon 18-06-18 18:11:26, Mikulas Patocka wrote:
[...]
> I grepped the kernel for __GFP_NORETRY and triaged them. I found 16 cases 
> without a fallback - those are bugs that make various functions randomly 
> return -ENOMEM.

Well, maybe those are just optimistic attempts to allocate memory and
have a fallback somewhere else. So I would be careful calling them
outright bugs. But maybe you are right.

> Most of the callers provide callback.
> 
> There is another strange flag - __GFP_RETRY_MAYFAIL - it provides two 
> different functions - if the allocation is larger than 
> PAGE_ALLOC_COSTLY_ORDER, it retries the allocation as if it were smaller. 
> If the allocations is smaller than PAGE_ALLOC_COSTLY_ORDER, 
> __GFP_RETRY_MAYFAIL will avoid the oom killer (larger order allocations 
> don't trigger the oom killer at all).

Well, the primary purpose of this flag is to provide a consistent
failure behavior for all requests regardless of the size.

> So, perhaps __GFP_RETRY_MAYFAIL could be used instead of __GFP_NORETRY in 
> the cases where the caller wants to avoid trigerring the oom killer (the 
> problem is that __GFP_NORETRY causes random failure even in no-oom 
> situations but __GFP_RETRY_MAYFAIL doesn't).

myabe yes.

> So my suggestion is - fix these obvious bugs when someone allocates memory 
> with __GFP_NORETRY without any fallback - and then, __GFP_NORETRY could be 
> just changed to return NULL instead of sleeping.

No real objection to fixing wrong __GFP_NORETRY usage. But __GFP_NORETRY
can sleep. Nothing will really change in that regards.  It does a
reclaim and that _might_ sleep.

But seriously, isn't the best way around the throttling issue to use
PF_LESS_THROTTLE?
-- 
Michal Hocko
SUSE Labs