Question regarding ext4_journalled_aops: lack of migrate_page

From: Chris Goldsworthy
Date: Fri Oct 09 2020 - 02:42:31 EST


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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