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Subject: RE: (2) [PATCH] page_alloc: avoid the negative free for meminfo
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To:     Michal Hocko <mhocko@suse.com>
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Date:   Tue, 03 Jan 2023 18:22:32 +0900
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>> >On Tue 03-01-23 16:28:07, 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.
>> >
>> >What is the actual problem you are trying to address by this change?
>> 
>> Hello
>> 
>> As described on the original commit,
>>   34e431b0ae39 /proc/meminfo: provide estimated available memory
>> mm is tring to provide the avaiable memory to user space.
>> 
>> But if free is negative, the available memory shown to userspace
>> would be shown smaller thatn the actual available size. The userspace
>> may do unwanted memory shrinking actions like process kills.
>
>Do you have any specific example? Have you seen this happening in
>practice or is this based on the code inspection?

I found this from a device using v5.10 based kernel.
Actually the log was printed by user space in its format after reading /proc/meminfo.

MemFree          38220 KB
MemAvailable     90008 KB
Active(file)    137116 KB
Inactive(file)  124128 KB
SReclaimable    100960 KB

Here's /proc/zoneinfo for wmark info.

------ ZONEINFO (/proc/zoneinfo) ------
Node 0, zone    DMA32
  pages free     17059
        min      862
        low      9790
        high     18718
        spanned  524288
        present  497920
        managed  413348
Node 0, zone   Normal
  pages free     12795
        min      1044
        low      11855
        high     22666
        spanned  8388608
        present  524288
        managed  500548

The pagecache at this time, seems to be 174,664 KB.
  pagecache -= min(pagecache / 2, wmark_low)
We also need to add the reclaimable and the actual free on it to be MemAvaiable.

The MemAvailable should be bigger at leat this 174,664 KB, but it was 90,008 KB only
because the big wmark high 165,536 seems to be used.

>
>Also does this patch actually fix anything? Say the system is really
>struggling and we are under min watermark. Shouldn't that lead to
>Available to be reported as 0 without even looking at other counters?
>

Sorry but I did not understand, this mis-calculation can be happened
above the min watermark. Do you think the wmark high should be extracted
all the time even if the free is negative?


>> I think the logic sholud account the positive size only.
>> 
>> BR
>> 
>> >
>> >> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.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
>> >
>> >-- 
>> >Michal Hocko
>> >SUSE Labs
>> 
>> 
>> 
>>  
>> --------- Original Message ---------
>> Sender : Michal Hocko <mhocko@suse.com>
>> Date : 2023-01-03 17:03 (GMT+9)
>> Title : Re: [PATCH] page_alloc: avoid the negative free for meminfo available
>>  
>> On Tue 03-01-23 16:28:07, 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.
>> 
>> What is the actual problem you are trying to address by this change?
>> 
>> > Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.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
>> 
>> -- 
>> Michal Hocko
>> SUSE Labs
>>