Re: readahead and oom

[Minchan Kim]

Hi Wu,

On Tue, Apr 26, 2011 at 3:34 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> On Tue, Apr 26, 2011 at 02:29:15PM +0800, Dave Young wrote:
>> On Tue, Apr 26, 2011 at 2:25 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>> > On Tue, Apr 26, 2011 at 02:07:17PM +0800, Dave Young wrote:
>> >> On Tue, Apr 26, 2011 at 2:05 PM, Dave Young <hidave.darkstar@xxxxxxxxx> wrote:
>> >> > On Tue, Apr 26, 2011 at 1:55 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>> >> >> On Tue, Apr 26, 2011 at 01:49:25PM +0800, Dave Young wrote:
>> >> >>> Hi,
>> >> >>>
>> >> >>> When memory pressure is high, readahead could cause oom killing.
>> >> >>> IMHO we should stop readaheading under such circumstancesãIf it's true
>> >> >>> how to fix it?
>> >> >>
>> >> >> Good question. Before OOM there will be readahead thrashings, which
>> >> >> can be addressed by this patch:
>> >> >>
>> >> >> http://lkml.org/lkml/2010/2/2/229
>> >> >
>> >> > Hi, I'm not clear about the patch, could be regard as below cases?
>> >> > 1) readahead alloc fail due to low memory such as other large allocation
>> >>
>> >> For example vm balloon allocate lots of memory, then readahead could
>> >> fail immediately and then oom
>> >
>> > If true, that would be the problem of vm balloon. It's not good to
>> > consume lots of memory all of a sudden, which will likely impact lots
>> > of kernel subsystems.
>> >
>> > btw readahead page allocations are completely optional. They are OK to
>> > fail and in theory shall not trigger OOM on themselves. We may
>> > consider passing __GFP_NORETRY for readahead page allocations.
>>
>> Good idea, care to submit a patch?
>
> Here it is :)
>
> Thanks,
> Fengguang
> ---
> readahead: readahead page allocations is OK to fail
>
> Pass __GFP_NORETRY for readahead page allocations.
>
> readahead page allocations are completely optional. They are OK to
> fail and in particular shall not trigger OOM on themselves.
>
> Reported-by: Dave Young <hidave.darkstar@xxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> Âinclude/linux/pagemap.h | Â Â5 +++++
> Âmm/readahead.c     Â|  Â2 +-
> Â2 files changed, 6 insertions(+), 1 deletion(-)
>
> --- linux-next.orig/include/linux/pagemap.h   2011-04-26 14:27:46.000000000 +0800
> +++ linux-next/include/linux/pagemap.h Â2011-04-26 14:29:31.000000000 +0800
> @@ -219,6 +219,11 @@ static inline struct page *page_cache_al
> Â Â Â Âreturn __page_cache_alloc(mapping_gfp_mask(x)|__GFP_COLD);
> Â}
>
> +static inline struct page *page_cache_alloc_cold_noretry(struct address_space *x)
> +{
> + Â Â Â return __page_cache_alloc(mapping_gfp_mask(x)|__GFP_COLD|__GFP_NORETRY);

It makes sense to me but it could make a noise about page allocation
failure. I think it's not desirable.
How about adding __GFP_NOWARAN?


-- 
Kind regards,
Minchan Kim
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