linux-ext4 - Question regarding ext4_journalled_aops: lack of migrate

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Date:   Thu, 08 Oct 2020 23:42:16 -0700
From:   Chris Goldsworthy <cgoldswo@...eaurora.org>
To:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Question regarding ext4_journalled_aops: lack of migrate_page

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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