linux-kernel - Re: [PATCH v3 3/3] mm: fs: Invalidate BH LRU during page migration

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Date:   Fri, 12 Mar 2021 10:33:48 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        joaodias@...gle.com, surenb@...gle.com, cgoldswo@...eaurora.org,
        willy@...radead.org, mhocko@...e.com, vbabka@...e.cz,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] mm: fs: Invalidate BH LRU during page migration

On 12.03.21 10:03, David Hildenbrand wrote:
> On 10.03.21 17:14, Minchan Kim wrote:
>> ffer_head LRU caches will be pinned and thus cannot be migrated.
>> This can prevent CMA allocations from succeeding, which are often used
>> on platforms with co-processors (such as a DSP) that can only use
>> physically contiguous memory. It can also prevent memory
>> hot-unplugging from succeeding, which involves migrating at least
>> MIN_MEMORY_BLOCK_SIZE bytes of memory, which ranges from 8 MiB to 1
>> GiB based on the architecture in use.
> 
> Actually, it's memory_block_size_bytes(), which can be even bigger
> (IIRC, 128MiB..2 GiB on x86-64) that fails to get offlined. But that
> will prevent bigger granularity (e.g., a whole DIMM) from getting unplugged.
> 
>>
>> Correspondingly, invalidate the BH LRU caches before a migration
>> starts and stop any buffer_head from being cached in the LRU caches,
>> until migration has finished.
> 
> Sounds sane to me.
> 

Diving a bit into the code, I am wondering:


a) Are these buffer head pages marked as movable?

IOW, are they either PageLRU() or __PageMovable()?


b) How do these pages end up on ZONE_MOVABLE or MIGRATE_CMA?

I assume these pages come via
alloc_page_buffers()->alloc_buffer_head()->kmem_cache_zalloc(GFP_NOFS | 
__GFP_ACCOUNT)



-- 
Thanks,

David / dhildenb