linux-kernel - Re: [PATCH 1/1] mm/page_alloc: add a warning about high order allocations

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Date:   Thu, 27 Dec 2018 17:50:55 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Konstantin Khorenko <khorenko@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 1/1] mm/page_alloc: add a warning about high order
 allocations

On Thu 27-12-18 16:05:18, Konstantin Khorenko wrote:
> On 12/26/2018 11:40 AM, Michal Hocko wrote:
> > Appart from general comments as a reply to the cover (btw. this all
> > should be in the changelog because this is the _why_ part of the
> > justification which should be _always_ part of the changelog).
> 
> Thank you, will add in the next version of the patch alltogether
> with other changes if any.
> 
> > On Tue 25-12-18 18:39:27, Konstantin Khorenko wrote:
> > [...]
> >> +config WARN_HIGH_ORDER
> >> +	bool "Enable complains about high order memory allocations"
> >> +	depends on !LOCKDEP
> >
> > Why?
> 
> LOCKDEP makes structures big, so if we see a high order allocation warning
> on a debug kernel with lockdep, it does not give us a lot - lockdep enabled
> kernel performance is not our target.
> i can remove !LOCKDEP dependence here, but then need to adjust default
> warning level i think, or logs will be spammed.

OK, I see but this just points to how this is not really a suitable
solution for the problem you are looking for.

> >> +static __always_inline void warn_high_order(int order, gfp_t gfp_mask)
> >> +{
> >> +	static atomic_t warn_count = ATOMIC_INIT(32);
> >> +
> >> +	if (order >= warn_order && !(gfp_mask & __GFP_NOWARN))
> >> +		WARN(atomic_dec_if_positive(&warn_count) >= 0,
> >> +		     "order %d >= %d, gfp 0x%x\n",
> >> +		     order, warn_order, gfp_mask);
> >> +}
> >
> > We do have ratelimit functionality, so why cannot you use it?
> 
> Well, my idea was to really shut up the warning after some number of messages
> (if a node is in production and its uptime, say, a year, i don't want to see
> many warnings in logs, first several is enough - let's fix them first).

OK, but it is quite likely that the system is perfectly healthy and
unfragmented after fresh boot when doing a large order allocations is
perfectly fine. Note that it is smaller order allocations that generate
fragmentation in general.
-- 
Michal Hocko
SUSE Labs