Re: [PATCH] mm: Add nr_free_highatomimic to fix incorrect watermatk routine

Re: [PATCH] mm: Add nr_free_highatomimic to fix incorrect watermatk routine

[Michal Hocko]

On Fri 30-08-19 18:25:53, Sangwoo wrote:
> The highatomic migrate block can be increased to 1% of Total memory.
> And, this is for only highorder ( > 0 order). So, this block size is
> excepted during check watermark if allocation type isn't alloc_harder.
> 
> It has problem. The usage of highatomic is already calculated at NR_FREE_PAGES.
> So, if we except total block size of highatomic, it's twice minus size of allocated
> highatomic.
> It's cause allocation fail although free pages enough.
> 
> We checked this by random test on my target(8GB RAM).
> 
> 	Binder:6218_2: page allocation failure: order:0, mode:0x14200ca(GFP_HIGHUSER_MOVABLE), nodemask=(null)
> 	Binder:6218_2 cpuset=background mems_allowed=0

How come this order-0 sleepable allocation fails? The upstream kernel
doesn't fail those allocations unless the process context is killed by
the oom killer.

Also please note that atomic reserves are released when the memory
pressure is high and we cannot reclaim any other memory. Have a look at
unreserve_highatomic_pageblock called from should_reclaim_retry.
-- 
Michal Hocko
SUSE Labs

RE: Re: [PATCH] mm: Add nr_free_highatomimic to fix incorrect watermatk routine

[박상우]

[-- Attachment #1: Type: text/html, Size: 5150 bytes --]

Re: Re: [PATCH] mm: Add nr_free_highatomimic to fix incorrect watermatk routine

[Michal Hocko]

On Mon 02-09-19 13:34:54, 박상우 wrote:
> >On Fri 30-08-19 18:25:53, Sangwoo wrote:
> >> The highatomic migrate block can be increased to 1% of Total memory.
> >> And, this is for only highorder ( > 0 order). So, this block size is
> >> excepted during check watermark if allocation type isn't alloc_harder.
> >>
> >> It has problem. The usage of highatomic is already calculated at
> NR_FREE_PAGES.
> >> So, if we except total block size of highatomic, it's twice minus size of
> allocated
> >> highatomic.
> >> It's cause allocation fail although free pages enough.
> >>
> >> We checked this by random test on my target(8GB RAM).
> >>
> >>  Binder:6218_2: page allocation failure: order:0, mode:0x14200ca
> (GFP_HIGHUSER_MOVABLE), nodemask=(null)
> >>  Binder:6218_2 cpuset=background mems_allowed=0
> >
> >How come this order-0 sleepable allocation fails? The upstream kernel
> >doesn't fail those allocations unless the process context is killed by
> >the oom killer.
> 
> Most calltacks are zsmalloc, as shown below.

What makes those allocations special so that they fail unlike any other
normal order-0 requests? Also do you see the same problem with the
current upstream kernel? Is it possible this is an Android specific
issue?

>  Call trace:
>   dump_backtrace+0x0/0x1f0
>   show_stack+0x18/0x20
>   dump_stack+0xc4/0x100
>   warn_alloc+0x100/0x198
>   __alloc_pages_nodemask+0x116c/0x1188
>   do_swap_page+0x10c/0x6f0
>   handle_pte_fault+0x12c/0xfe0
>   handle_mm_fault+0x1d0/0x328
>   do_page_fault+0x2a0/0x3e0
>   do_translation_fault+0x44/0xa8
>   do_mem_abort+0x4c/0xd0
>   el1_da+0x24/0x84
>   __arch_copy_to_user+0x5c/0x220
>   binder_ioctl+0x20c/0x740
>   compat_SyS_ioctl+0x128/0x248
>   __sys_trace_return+0x0/0x4
-- 
Michal Hocko
SUSE Labs

Re: [PATCH] mm: Add nr_free_highatomimic to fix incorrect watermatk routine

[Vlastimil Babka]

On 8/30/19 11:25 AM, Sangwoo wrote:
> The highatomic migrate block can be increased to 1% of Total memory.
> And, this is for only highorder ( > 0 order). So, this block size is
> excepted during check watermark if allocation type isn't alloc_harder.
> 
> It has problem. The usage of highatomic is already calculated at NR_FREE_PAGES.
> So, if we except total block size of highatomic, it's twice minus size of allocated
> highatomic.
> It's cause allocation fail although free pages enough.

This is known, the comment in __zone_watermark_order says "This will
over-estimate the size of the atomic reserve but it avoids a search."
It was discussed during review and wasn't considered a large issue
thanks to unreserving on demand before OOM happens.

> @@ -919,6 +923,9 @@ static inline void __free_one_page(struct page *page,
>  	VM_BUG_ON(migratetype == -1);
>  	if (likely(!is_migrate_isolate(migratetype)))
>  		__mod_zone_freepage_state(zone, 1 << order, migratetype);
> +	if (is_migrate_highatomic(migratetype) ||
> +		is_migrate_highatomic_page(page))
> +		__mod_zone_page_state(zone, NR_FREE_HIGHATOMIC_PAGES, 1 << order);

I suspect the counter will eventually get imbalanced, at the least due
to merging a highatomic pageblock and non-highatomic pageblock. To get
it right, it would have to be complicated in a similar way that we
handle MIGRATE_ISOLATED and MIGRATE_CMA. It wasn't considered serious
enough to warrant these complications.