FROMLIST: mm: fs: Invalidate BH LRU during page migration

Pages containing buffer_heads that are in one of the per-CPU
buffer_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.

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.

Bug: 180018981
Link: https://lore.kernel.org/linux-mm/20210310161429.399432-3-minchan@kernel.org/


Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Minchan Kim <minchan@google.com>
Change-Id: I7ac085c2ec14a81c3c4d7b65a7eeedb0cfba4ea6