Hi there,
ext4_aops and ext4_da_aops both have a migratepage callback, whereas
ext4_journalled_aops lacks such a callback. Why is this so? I’m asking
this due to the following: when a page containing EXT4 journal buffer
heads ends up being migrated, fallback_migrate_page() is used, which
eventually calls try_to_free_buffers(), which will call drop_buffers().
Drop buffers() can fail for a page if that page is on the LRU list (see
https://elixir.bootlin.com/linux/v5.8.14/source/fs/buffer.c#L3225).
Now, if buffer_migrate_page() was supplied as the migratepage callback
for the journaled aops, this wouldn’t be problem since we ignore the LRU
lists altogether.
Resolving this issue will benefit CMA allocations, which might have to
migrate movable pages that were allocated from a CMA region (the
assumption is that these pages can be migrated once the memory backing
these pages is needed).
Thanks,
Chris.
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