mm: Add nr_free_highatomimic to fix incorrect watermatk routine

PATCH New Park Sangwoo 1 participant 1 message Sep 5, 2019 Memory management

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Park Sangwoo

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2019-09-05 01:46 UTC #0 lore raw

PATCH 1/1 mm: Add nr_free_highatomimic to fix incorrect watermatk routine

Quoted 55 lines · Park Sangwoo

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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.
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So, if we except total block size of highatomic, it's twice minus size of
allocated
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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)
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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?
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There is the other case of fail order-0 fail.
----
hvdcp_opti: page allocation failure: order:0, mode:0x1004000(GFP_NOWAIT|__GFP_COMP), nodemask=(null)
This is an atomic allocation and failing that one is not a problem usually. High atomic reservations might prevent GFP_NOWAIT allocation from suceeding but I do not see that as a problem. This is the primary purpose of the reservation.
Thanks, your answer helped me. However, my suggestion is not to modify the use and management of the high atomic region, but to calculate the exact free size of the highatomic so that fail does not occur for previously shared cases.
In __zone_water_mark_ok(...) func, if it is not atomic allocation, high atomic size is excluded.
bool __zone_watermark_ok(struct zone *z,
...
{
    ...
    if (likely(!alloc_harder)) {
        free_pages -= z->nr_reserved_highatomic;
    ...
}

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However, free_page excludes the size already allocated by hiahtomic. If highatomic block is small(Under 4GB RAM), it could be no problem. But, the larger the memory size, the greater the chance of problems. (Becasue highatomic size can be increased up to 1% of memory)
I still do not understand. NR_FREE_PAGES should include the amount of hhighatomic reserves, right. So reducing the free_pages for normal allocations just makes sense. Or what do I miss?

You are right. But z->nr_reserved_highatomic value is total size of highatomic migrate type per zone.

nr_reserved_highatomic = (# of allocated of highatomic) + (# of free list of highatomic).

And (# of allocated of hiagatomic) is already excluded at NR_FREE_PAGES. So, if reducing nr_reserved_highatomic at NR_FREE_PAGES,
the (# of allocated of highatomic) is double reduced.

So I proposal that only (# of free list of highatomic) is reduced at NR_FREE_PAGE.

    if (likely(!alloc_harder)) {
-       free_pages -= z->nr_reserved_highatomic;
+       free_pages -= zone_page_state(z, NR_FREE_HIGHATOMIC_PAGES);
    } else {

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I am sorry but I find your reasoning really hard to follow.