linux-kernel - Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

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Date:   Thu, 3 Sep 2020 15:50:20 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mhocko@...e.com, linux-mm@...ck.org
Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

On 9/1/20 2:46 PM, Pavel Tatashin wrote:
> There is a race during page offline that can lead to infinite loop:
> a page never ends up on a buddy list and __offline_pages() keeps
> retrying infinitely or until a termination signal is received.
> 
> Thread#1 - a new process:
> 
> load_elf_binary
>  begin_new_exec
>   exec_mmap
>    mmput
>     exit_mmap
>      tlb_finish_mmu
>       tlb_flush_mmu
>        release_pages
>         free_unref_page_list
>          free_unref_page_prepare
>           set_pcppage_migratetype(page, migratetype);
>              // Set page->index migration type below  MIGRATE_PCPTYPES
> 
> Thread#2 - hot-removes memory
> __offline_pages
>   start_isolate_page_range
>     set_migratetype_isolate
>       set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>         Set migration type to MIGRATE_ISOLATE-> set
>         drain_all_pages(zone);
>              // drain per-cpu page lists to buddy allocator.
> 
> Thread#1 - continue
>          free_unref_page_commit
>            migratetype = get_pcppage_migratetype(page);
>               // get old migration type
>            list_add(&page->lru, &pcp->lists[migratetype]);
>               // add new page to already drained pcp list
> 
> Thread#2
> Never drains pcp again, and therefore gets stuck in the loop.
> 
> The fix is to try to drain per-cpu lists again after
> check_pages_isolated_cb() fails.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: stable@...r.kernel.org

Fixes: ?
Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks.