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

[David Hildenbrand]

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