linux-kernel - Re: [PATCH] mm: respect the __GFP_NOWARN flag when warning about stalls

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Date:   Mon, 11 Sep 2017 10:26:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: respect the __GFP_NOWARN flag when warning about
 stalls

On Mon 11-09-17 02:52:53, Mikulas Patocka wrote:
> I am occasionally getting these warnings in khugepaged. It is an old 
> machine with 550MHz CPU and 512 MB RAM.
> 
> Note that khugepaged has nice value 19, so when the machine is loaded with 
> some work, khugepaged is stalled and this stall produces warning in the 
> allocator.
> 
> khugepaged does allocations with __GFP_NOWARN, but the flag __GFP_NOWARN
> is masked off when calling warn_alloc. This patch removes the masking of
> __GFP_NOWARN, so that the warning is suppressed.
> 
> khugepaged: page allocation stalls for 10273ms, order:10, mode:0x4340ca(__GFP_HIGHMEM|__GFP_IO|__GFP_FS|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_HARDWALL|__GFP_MOVABLE|__GFP_DIRECT_RECLAIM), nodemask=(null)
> CPU: 0 PID: 3936 Comm: khugepaged Not tainted 4.12.3 #1
> Hardware name: System Manufacturer Product Name/VA-503A, BIOS 4.51 PG 08/02/00
> Call Trace:
>  ? warn_alloc+0xb9/0x140
>  ? __alloc_pages_nodemask+0x724/0x880
>  ? arch_irq_stat_cpu+0x1/0x40
>  ? detach_if_pending+0x80/0x80
>  ? khugepaged+0x10a/0x1d40
>  ? pick_next_task_fair+0xd2/0x180
>  ? wait_woken+0x60/0x60
>  ? kthread+0xcf/0x100
>  ? release_pte_page+0x40/0x40
>  ? kthread_create_on_node+0x40/0x40
>  ? ret_from_fork+0x19/0x30
> 
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> Cc: stable@...r.kernel.org
> Fixes: 63f53dea0c98 ("mm: warn about allocations which stall for too long")

This patch hasn't introduced this behavior. It deliberately skipped
warning on __GFP_NOWARN. This has been introduced later by 822519634142
("mm: page_alloc: __GFP_NOWARN shouldn't suppress stall warnings"). I
disagreed [1] but overall consensus was that such a warning won't be
harmful. Could you be more specific why do you consider it wrong,
please?

[1] http://lkml.kernel.org/r/20170125184548.GB32041@dhcp22.suse.cz

> 
> ---
>  mm/page_alloc.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6/mm/page_alloc.c
> ===================================================================
> --- linux-2.6.orig/mm/page_alloc.c
> +++ linux-2.6/mm/page_alloc.c
> @@ -3923,7 +3923,7 @@ retry:
>  
>  	/* Make sure we know about allocations which stall for too long */
>  	if (time_after(jiffies, alloc_start + stall_timeout)) {
> -		warn_alloc(gfp_mask & ~__GFP_NOWARN, ac->nodemask,
> +		warn_alloc(gfp_mask, ac->nodemask,
>  			"page allocation stalls for %ums, order:%u",
>  			jiffies_to_msecs(jiffies-alloc_start), order);
>  		stall_timeout += 10 * HZ;

-- 
Michal Hocko
SUSE Labs