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Date:   Tue, 12 Jan 2021 11:48:55 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Rokudo Yan <wu-yan@....com>, mgorman@...hsingularity.net
Cc:     aarcange@...hat.com, akpm@...ux-foundation.org, haiwang.fu@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        rientjes@...gle.com, tang.ding@....com, xiushui.ye@....com
Subject: Re: [PATCH] mm, compaction: move high_pfn to the for loop scope.

On 1/12/21 10:47 AM, Rokudo Yan wrote:
> In fast_isolate_freepages, high_pfn will be used if a prefered one(PFN >= low_fn) not found. But the high_pfn
> is not reset before searching an free area, so when it was used as freepage, it may from another free area searched before.
> And move_freelist_head(freelist, freepage) will have unexpected behavior(eg. corrupt the MOVABLE freelist)
> 
> Unable to handle kernel paging request at virtual address dead000000000200
> Mem abort info:
>   ESR = 0x96000044
>   Exception class = DABT (current EL), IL = 32 bits
>   SET = 0, FnV = 0
>   EA = 0, S1PTW = 0
> Data abort info:
>   ISV = 0, ISS = 0x00000044
>   CM = 0, WnR = 1
> [dead000000000200] address between user and kernel address ranges
> 
> -000|list_cut_before(inline)
> -000|move_freelist_head(inline)
> -000|fast_isolate_freepages(inline)
> -000|isolate_freepages(inline)
> -000|compaction_alloc(?, ?)
> -001|unmap_and_move(inline)
> -001|migrate_pages([NSD:0xFFFFFF80088CBBD0] from = 0xFFFFFF80088CBD88, [NSD:0xFFFFFF80088CBBC8] get_new_p
> -002|__read_once_size(inline)
> -002|static_key_count(inline)
> -002|static_key_false(inline)
> -002|trace_mm_compaction_migratepages(inline)
> -002|compact_zone(?, [NSD:0xFFFFFF80088CBCB0] capc = 0x0)
> -003|kcompactd_do_work(inline)
> -003|kcompactd([X19] p = 0xFFFFFF93227FBC40)
> -004|kthread([X20] _create = 0xFFFFFFE1AFB26380)
> -005|ret_from_fork(asm)
> ---|end of frame
> 
> The issue was reported on an smart phone product with 6GB ram and 3GB zram as swap device.
> 
> This patch fixes the issue by reset high_pfn before searching each free area, which ensure
> freepage and freelist match when call move_freelist_head in fast_isolate_freepages().
> 
> Link: http://lkml.kernel.org/r/20190118175136.31341-12-mgorman@techsingularity.net
> Fixes: 5a811889de10f1eb ("mm, compaction: use free lists to quickly locate a migration target")

Sounds serious enough. Wonder how it wasn't reported sooner.

Cc: <stable@...r.kernel.org>
Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/compaction.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index cc1a7f600a86..75f0e550b18f 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1303,7 +1303,7 @@ fast_isolate_freepages(struct compact_control *cc)
>  {
>  	unsigned int limit = min(1U, freelist_scan_limit(cc) >> 1);
>  	unsigned int nr_scanned = 0;
> -	unsigned long low_pfn, min_pfn, high_pfn = 0, highest = 0;
> +	unsigned long low_pfn, min_pfn, highest = 0;
>  	unsigned long nr_isolated = 0;
>  	unsigned long distance;
>  	struct page *page = NULL;
> @@ -1348,6 +1348,7 @@ fast_isolate_freepages(struct compact_control *cc)
>  		struct page *freepage;
>  		unsigned long flags;
>  		unsigned int order_scanned = 0;
> +		unsigned long high_pfn = 0;
>  
>  		if (!area->nr_free)
>  			continue;
> 

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