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Date:   Fri, 12 Jun 2020 16:34:54 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Jaewon Kim <jaewon31.kim@...sung.com>, mgorman@...hsingularity.net,
        minchan@...nel.org, mgorman@...e.de, hannes@...xchg.org,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jaewon31.kim@...il.com, ytk.lee@...sung.com,
        cmlaika.kim@...sung.com
Subject: Re: [PATCH v2] page_alloc: consider highatomic reserve in wmartermark
 fast

On 6/13/20 4:51 AM, Jaewon Kim wrote:
> zone_watermark_fast was introduced by commit 48ee5f3696f6 ("mm,
> page_alloc: shortcut watermark checks for order-0 pages"). The commit
> simply checks if free pages is bigger than watermark without additional
> calculation such like reducing watermark.
> 
> It considered free cma pages but it did not consider highatomic
> reserved. This may incur exhaustion of free pages except high order
> atomic free pages.
> 
> Assume that reserved_highatomic pageblock is bigger than watermark min,
> and there are only few free pages except high order atomic free. Because
> zone_watermark_fast passes the allocation without considering high order
> atomic free, normal reclaimable allocation like GFP_HIGHUSER will
> consume all the free pages. Then finally order-0 atomic allocation may
> fail on allocation.

I don't understand why order-0 atomic allocation will fail. Is it because of
watermark check, or finding no suitable pages?
- watermark check should be OK as atomic allocations can use reserves
- suitable pages should be OK, even if all free pages are in the highatomic
reserves, because rmqueue() contains:

if (alloc_flags & ALLOC_HARDER)
	page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);

So what am I missing?

> This means watermark min is not protected against non-atomic allocation.
> The order-0 atomic allocation with ALLOC_HARDER unwantedly can be
> failed. Additionally the __GFP_MEMALLOC allocation with
> ALLOC_NO_WATERMARKS also can be failed.
> 
> To avoid the problem, zone_watermark_fast should consider highatomic
> reserve. If the actual size of high atomic free is counted accurately
> like cma free, we may use it. On this patch just use
> nr_reserved_highatomic. Additionally introduce
> __zone_watermark_unusable_free to factor out common parts between
> zone_watermark_fast and __zone_watermark_ok.
> 
> This is trace log which shows GFP_HIGHUSER consumes free pages right
> before ALLOC_NO_WATERMARKS.
> 
>   <...>-22275 [006] ....   889.213383: mm_page_alloc: page=00000000d2be5665 pfn=970744 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213385: mm_page_alloc: page=000000004b2335c2 pfn=970745 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213387: mm_page_alloc: page=00000000017272e1 pfn=970278 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213389: mm_page_alloc: page=00000000c4be79fb pfn=970279 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213391: mm_page_alloc: page=00000000f8a51d4f pfn=970260 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213393: mm_page_alloc: page=000000006ba8f5ac pfn=970261 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213395: mm_page_alloc: page=00000000819f1cd3 pfn=970196 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
>   <...>-22275 [006] ....   889.213396: mm_page_alloc: page=00000000f6b72a64 pfn=970197 order=0 migratetype=0 nr_free=3650 gfp_flags=GFP_HIGHUSER|__GFP_ZERO
> kswapd0-1207  [005] ...1   889.213398: mm_page_alloc: page= (null) pfn=0 order=0 migratetype=1 nr_free=3650 gfp_flags=GFP_NOWAIT|__GFP_HIGHMEM|__GFP_NOWARN|__GFP_MOVABLE
> 
> This is an example of ALLOC_HARDER allocation failure.
> 
> <4>[ 6207.637280]  [3:  Binder:9343_3:22875] Binder:9343_3: page allocation failure: order:0, mode:0x480020(GFP_ATOMIC), nodemask=(null)
> <4>[ 6207.637311]  [3:  Binder:9343_3:22875] Call trace:
> <4>[ 6207.637346]  [3:  Binder:9343_3:22875] [<ffffff8008f40f8c>] dump_stack+0xb8/0xf0
> <4>[ 6207.637356]  [3:  Binder:9343_3:22875] [<ffffff8008223320>] warn_alloc+0xd8/0x12c
> <4>[ 6207.637365]  [3:  Binder:9343_3:22875] [<ffffff80082245e4>] __alloc_pages_nodemask+0x120c/0x1250
> <4>[ 6207.637374]  [3:  Binder:9343_3:22875] [<ffffff800827f6e8>] new_slab+0x128/0x604
> <4>[ 6207.637381]  [3:  Binder:9343_3:22875] [<ffffff800827b0cc>] ___slab_alloc+0x508/0x670
> <4>[ 6207.637387]  [3:  Binder:9343_3:22875] [<ffffff800827ba00>] __kmalloc+0x2f8/0x310
> <4>[ 6207.637396]  [3:  Binder:9343_3:22875] [<ffffff80084ac3e0>] context_struct_to_string+0x104/0x1cc
> <4>[ 6207.637404]  [3:  Binder:9343_3:22875] [<ffffff80084ad8fc>] security_sid_to_context_core+0x74/0x144
> <4>[ 6207.637412]  [3:  Binder:9343_3:22875] [<ffffff80084ad880>] security_sid_to_context+0x10/0x18
> <4>[ 6207.637421]  [3:  Binder:9343_3:22875] [<ffffff800849bd80>] selinux_secid_to_secctx+0x20/0x28
> <4>[ 6207.637430]  [3:  Binder:9343_3:22875] [<ffffff800849109c>] security_secid_to_secctx+0x3c/0x70
> <4>[ 6207.637442]  [3:  Binder:9343_3:22875] [<ffffff8008bfe118>] binder_transaction+0xe68/0x454c
> <4>[ 6207.637569]  [3:  Binder:9343_3:22875] Mem-Info:
> <4>[ 6207.637595]  [3:  Binder:9343_3:22875] active_anon:102061 inactive_anon:81551 isolated_anon:0
> <4>[ 6207.637595]  [3:  Binder:9343_3:22875]  active_file:59102 inactive_file:68924 isolated_file:64
> <4>[ 6207.637595]  [3:  Binder:9343_3:22875]  unevictable:611 dirty:63 writeback:0 unstable:0
> <4>[ 6207.637595]  [3:  Binder:9343_3:22875]  slab_reclaimable:13324 slab_unreclaimable:44354
> <4>[ 6207.637595]  [3:  Binder:9343_3:22875]  mapped:83015 shmem:4858 pagetables:26316 bounce:0
> <4>[ 6207.637595]  [3:  Binder:9343_3:22875]  free:2727 free_pcp:1035 free_cma:178
> <4>[ 6207.637616]  [3:  Binder:9343_3:22875] Node 0 active_anon:408244kB inactive_anon:326204kB active_file:236408kB inactive_file:275696kB unevictable:2444kB isolated(anon):0kB isolated(file):256kB mapped:332060kB dirty:252kB writeback:0kB shmem:19432kB writeback_tmp:0kB unstable:0kB all_unreclaimable? no
> <4>[ 6207.637627]  [3:  Binder:9343_3:22875] Normal free:10908kB min:6192kB low:44388kB high:47060kB active_anon:409160kB inactive_anon:325924kB active_file:235820kB inactive_file:276628kB unevictable:2444kB writepending:252kB present:3076096kB managed:2673676kB mlocked:2444kB kernel_stack:62512kB pagetables:105264kB bounce:0kB free_pcp:4140kB local_pcp:40kB free_cma:712kB
> <4>[ 6207.637632]  [3:  Binder:9343_3:22875] lowmem_reserve[]: 0 0
> <4>[ 6207.637637]  [3:  Binder:9343_3:22875] Normal: 505*4kB (H) 357*8kB (H) 201*16kB (H) 65*32kB (H) 1*64kB (H) 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 10236kB

OK this shows we are well above min watermark, and indeed only free pages are
highatomic. Why doesn't the rmqueue() part quoted above work as expected and
allow this allocation to use those highatomic blocks?

> <4>[ 6207.637655]  [3:  Binder:9343_3:22875] 138826 total pagecache pages
> <4>[ 6207.637663]  [3:  Binder:9343_3:22875] 5460 pages in swap cache
> <4>[ 6207.637668]  [3:  Binder:9343_3:22875] Swap cache stats: add 8273090, delete 8267506, find 1004381/4060142
> 
> This is an example of ALLOC_NO_WATERMARKS allocation failure.
> 
> <6>[  156.701551]  [4:        kswapd0: 1209] kswapd0 cpuset=/ mems_allowed=0
> <4>[  156.701563]  [4:        kswapd0: 1209] CPU: 4 PID: 1209 Comm: kswapd0 Tainted: G        W       4.14.113-18113966 #1
> <4>[  156.701572]  [4:        kswapd0: 1209] Call trace:
> <4>[  156.701605]  [4:        kswapd0: 1209] [<0000000000000000>] dump_stack+0x68/0x90
> <4>[  156.701612]  [4:        kswapd0: 1209] [<0000000000000000>] warn_alloc+0x104/0x198
> <4>[  156.701617]  [4:        kswapd0: 1209] [<0000000000000000>] __alloc_pages_nodemask+0xdc0/0xdf0
> <4>[  156.701623]  [4:        kswapd0: 1209] [<0000000000000000>] zs_malloc+0x148/0x3d0
> <4>[  156.701630]  [4:        kswapd0: 1209] [<0000000000000000>] zram_bvec_rw+0x250/0x568
> <4>[  156.701634]  [4:        kswapd0: 1209] [<0000000000000000>] zram_rw_page+0x8c/0xe0
> <4>[  156.701640]  [4:        kswapd0: 1209] [<0000000000000000>] bdev_write_page+0x70/0xbc
> <4>[  156.701645]  [4:        kswapd0: 1209] [<0000000000000000>] __swap_writepage+0x58/0x37c
> <4>[  156.701649]  [4:        kswapd0: 1209] [<0000000000000000>] swap_writepage+0x40/0x4c
> <4>[  156.701654]  [4:        kswapd0: 1209] [<0000000000000000>] shrink_page_list+0xc3c/0xf54
> <4>[  156.701659]  [4:        kswapd0: 1209] [<0000000000000000>] shrink_inactive_list+0x2b0/0x61c
> <4>[  156.701664]  [4:        kswapd0: 1209] [<0000000000000000>] shrink_node_memcg+0x23c/0x618
> <4>[  156.701668]  [4:        kswapd0: 1209] [<0000000000000000>] shrink_node+0x1c8/0x304
> <4>[  156.701673]  [4:        kswapd0: 1209] [<0000000000000000>] kswapd+0x680/0x7c4
> <4>[  156.701679]  [4:        kswapd0: 1209] [<0000000000000000>] kthread+0x110/0x120
> <4>[  156.701684]  [4:        kswapd0: 1209] [<0000000000000000>] ret_from_fork+0x10/0x18
> <4>[  156.701689]  [4:        kswapd0: 1209] Mem-Info:
> <4>[  156.701712]  [4:        kswapd0: 1209] active_anon:88690 inactive_anon:88630 isolated_anon:0
> <4>[  156.701712]  [4:        kswapd0: 1209]  active_file:99173 inactive_file:169305 isolated_file:32
> <4>[  156.701712]  [4:        kswapd0: 1209]  unevictable:48292 dirty:538 writeback:38 unstable:0
> <4>[  156.701712]  [4:        kswapd0: 1209]  slab_reclaimable:15131 slab_unreclaimable:47762
> <4>[  156.701712]  [4:        kswapd0: 1209]  mapped:274654 shmem:2824 pagetables:25088 bounce:0
> <4>[  156.701712]  [4:        kswapd0: 1209]  free:2489 free_pcp:444 free_cma:3
> <4>[  156.701728]  [4:        kswapd0: 1209] Node 0 active_anon:354760kB inactive_anon:354520kB active_file:396692kB inactive_file:677220kB unevictable:193168kB isolated(anon):0kB isolated(file):128kB mapped:1098616kB dirty:2152kB writeback:152kB shmem:11296kB writeback_tmp:0kB unstable:0kB all_unreclaimable? no
> <4>[  156.701738]  [4:        kswapd0: 1209] Normal free:9956kB min:7428kB low:93440kB high:97032kB active_anon:355176kB inactive_anon:354580kB active_file:396196kB inactive_file:677284kB unevictable:193168kB writepending:2304kB present:4081664kB managed:3593324kB mlocked:193168kB kernel_stack:55008kB pagetables:100352kB bounce:0kB free_pcp:1776kB local_pcp:656kB free_cma:12kB
> <4>[  156.701741]  [4:        kswapd0: 1209] lowmem_reserve[]: 0 0
> <4>[  156.701747]  [4:        kswapd0: 1209] Normal: 196*4kB (H) 141*8kB (H) 109*16kB (H) 63*32kB (H) 20*64kB (H) 8*128kB (H) 2*256kB (H) 1*512kB (H) 0*1024kB 0*2048kB 0*4096kB = 9000kB

Same here, although here AFAICS ALLOC_NO_WATERMARKS doesn't imply ALLOC_HARDER,
so the rmqueue() check wouldn't kick in? That would be something to fix... but
doesn't explain the GFP_ATOMIC case above.

...

> @@ -3598,9 +3604,12 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
        /*
         * Fast check for order-0 only. If this fails then the reserves
         * need to be calculated. There is a corner case where the check
         * passes but only the high-order atomic reserve are free. If
>  	 * the caller is !atomic then it'll uselessly search the free
>  	 * list. That corner case is then slower but it is harmless.
>  	 */

The comment stops being true after this patch? It also suggests that Mel
anticipated this corner case, but that it should only cause a false positive
zone_watermark_fast() and then rmqueue() fails for !ALLOC_HARDER as it cannot
use MIGRATE_HIGHATOMIC blocks. It expects atomic order-0 still works. So what's
going on?

> -	if (!order && (free_pages - cma_pages) >
> -				mark + z->lowmem_reserve[highest_zoneidx])
> -		return true;
> +	if (!order) {
> +		long fast_free = free_pages - unusable_free;
> +
> +		if (fast_free > mark + z->lowmem_reserve[highest_zoneidx])
> +			return true;
> +	}
>  
>  	return __zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
>  					free_pages);
> 

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