linux-kernel - RE: [PATCH] page_alloc: avoid the negative free for meminfo available

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Message-ID: <20230103075054epcms1p308aa577d10429a2e0e6167de510be6b6@epcms1p3>
Date:   Tue, 03 Jan 2023 16:50:54 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>
Subject: RE: [PATCH] page_alloc: avoid the negative free for meminfo
 available

>--------- Original Message ---------
>Sender : Lorenzo Stoakes?<lstoakes@...il.com>
>Date : 2023-01-03 16:35 (GMT+9)
>Title : Re: [PATCH] page_alloc: avoid the negative free for meminfo available
>?
>On Tue, Jan 03, 2023 at 04:28:07PM +0900, Jaewon Kim wrote:
>> The totalreserve_pages could be higher than the free because of
>> watermark high or watermark boost. Handle this situation and fix it to 0
>> free size.
>>
>> Signed-off-by: Jaewon Kim <jaewon31.kim@...sung.com>
>> ---
>> ?mm/page_alloc.c | 2 ++
>> ?1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 218b28ee49ed..e510ae83d5f3 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5948,6 +5948,8 @@ long si_mem_available(void)
>> ? ? ? ? ? * without causing swapping or OOM.
>> ? ? ? ? ? */
>> ? ? ? ? ?available = global_zone_page_state(NR_FREE_PAGES) - totalreserve_pages;
>> + ? ? ? ?if (available < 0)
>> + ? ? ? ? ? ? ? ?available = 0;
>>
>> ? ? ? ? ?/*
>> ? ? ? ? ? * Not all the page cache can be freed, otherwise the system will
>> --
>> 2.17.1
>>
>
>We already reset to zero at the end of the function, wouldn't resetting to zero
>here potentially skew the result?
>

Hello

I did not mean the negative of the final available, we should account the actual size
by removing some improper portion of it. The free should be not negative in that perspective.
If negative, other parts like pagecache an reclailable would be decreased.

Actually pagecache and reclaimable are caculated with min, so I think reseting to zero
at the end the function is not necessary.

br