linux-kernel - [QUESTION] cachefiles: Recovery concerns with on-demand loading after unexpected power loss

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Message-Id: <20250528080759.105178-1-wozizhi@huaweicloud.com>
Date: Wed, 28 May 2025 16:07:59 +0800
From: Zizhi Wo <wozizhi@...weicloud.com>
To: netfs@...ts.linux.dev,
	dhowells@...hat.com,
	jlayton@...nel.org,
	brauner@...nel.org
Cc: hsiangkao@...ux.alibaba.com,
	jefflexu@...ux.alibaba.com,
	zhujia.zj@...edance.com,
	linux-erofs@...ts.ozlabs.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	wozizhi@...wei.com,
	libaokun1@...wei.com,
	yangerkun@...wei.com,
	houtao1@...wei.com,
	yukuai3@...wei.com
Subject: [QUESTION] cachefiles: Recovery concerns with on-demand loading after unexpected power loss

Currently, in on-demand loading mode, cachefiles first calls
cachefiles_create_tmpfile() to generate a tmpfile, and only during the exit
process does it call cachefiles_commit_object->cachefiles_commit_tmpfile to
create the actual dentry and making it visible to users.

If the cache write is interrupted unexpectedly (e.g., by system crash or
power loss), during the next startup process, cachefiles_look_up_object()
will determine that no corresponding dentry has been generated and will
recreate the tmpfile and pull the complete data again!

The current implementation mechanism appears to provide per-file atomicity.
For scenarios involving large image files (where significant amount of
cache data needs to be written), this re-pulling process after an
interruption seems considerable overhead?

In previous kernel versions, cache dentry were generated during the
LOOK_UP_OBJECT process of the object state machine. Even if power was lost
midway, the next startup process could continue pulling data based on the
previously downloaded cache data on disk.

What would be the recommended way to handle this situation? Or am I
thinking about this incorrectly? Would appreciate any feedback and guidance
from the community.

Thanks,
Zizhi Wo