Re: [PATCH 3/4] mm: vmscan: Evaluate the watermarks against the correct classzone

Minchan Kim
2011-06-28T23:23:16Z

On Tue, Jun 28, 2011 at 9:52 PM, Mel Gorman <mgorman@suse.de> wrote:
> On Mon, Jun 27, 2011 at 03:53:04PM +0900, Minchan Kim wrote:
>> On Fri, Jun 24, 2011 at 11:44 PM, Mel Gorman <mgorman@suse.de> wrote:
>> > When deciding if kswapd is sleeping prematurely, the classzone is
>> > taken into account but this is different to what balance_pgdat() and
>> > the allocator are doing. Specifically, the DMA zone will be checked
>> > based on the classzone used when waking kswapd which could be for a
>> > GFP_KERNEL or GFP_HIGHMEM request. The lowmem reserve limit kicks in,
>> > the watermark is not met and kswapd thinks its sleeping prematurely
>> > keeping kswapd awake in error.
>>
>>
>> I thought it was intentional when you submitted a patch firstly.
>
> It was, it also wasn't right.
>
>> "Kswapd makes sure zones include enough free pages(ie, include reserve
>> limit of above zones).
>> But you seem to see DMA zone can't meet above requirement forever in
>> some situation so that kswapd doesn't sleep.
>> Right?
>>
>
> Right.
>
>> >
>> > Reported-and-tested-by: Pádraig Brady <P@draigBrady.com>
>> > Signed-off-by: Mel Gorman <mgorman@suse.de>
>> > ---
>> >  mm/vmscan.c |    2 +-
>> >  1 files changed, 1 insertions(+), 1 deletions(-)
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index 9cebed1..a76b6cc2 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -2341,7 +2341,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
>> >                }
>> >
>> >                if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
>> > -                                                       classzone_idx, 0))
>> > +                                                       i, 0))
>>
>> Isn't it  better to use 0 instead of i?
>>
>
> I considered it but went with i to compromise between making sure zones
> included enough free pages without requiring that ZONE_DMA meet an
> almost impossible requirement when under continual memory pressure.

I see.
Thanks, Mel.

--
Kind regards,
Minchan Kim