linux-kernel - Re: dm bufio: Reduce dm_bufio

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Message-ID: <20180619104312.GD13685@dhcp22.suse.cz>
Date:   Tue, 19 Jun 2018 12:43:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     jing xia <jing.xia.mail@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, agk@...hat.com,
        dm-devel@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: dm bufio: Reduce dm_bufio_lock contention

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