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

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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






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