linux-kernel - [PATCH v3 0/2] fuse: improve crash recovery for backing files

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Message-ID: <20260119083750.2055-1-luochunsheng@ustc.edu>
Date: Mon, 19 Jan 2026 16:37:47 +0800
From: Chunsheng Luo <luochunsheng@...c.edu>
To: miklos@...redi.hu
Cc: amir73il@...il.com,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Chunsheng Luo <luochunsheng@...c.edu>
Subject: [PATCH v3 0/2] fuse: improve crash recovery for backing files

Hello maintainers and community,

This v3 series addresses crash recovery issues for FUSE daemons
with passthrough support, following up on the previous RFC
discussion [1].

Problem Summary
===============
To simplify FUSE daemon crash recovery and reduce performance
overhead, passthrough backing_id information is not persisted.
However, this design choice introduces two issues when the daemon
restarts:

1. Non-persistent backing_ids prevent proper resource cleanup during
   daemon restart, potentially causing resource leaks.

2. New backing_ids allocated for the same backing file trigger -EBUSY
   errors due to strict pointer validation in
   fuse_inode_uncached_io_start(), even though the backing inodes
   are identical.

Changes from v2
===============
Based on Amir Goldstein's review feedback, this v3 series updates:

Patch 1/2: "fuse: add ioctl to cleanup all backing files"
  - Changed backing_files_map to RCU-protected pointer for safe concurrent
    access, using rcu_replace_pointer() instead of swap().
  - Added synchronize_rcu() to prevent use-after-free with in-flight lookups.
  - Made fuse_backing_files_init() return int to handle allocation failures.

Patch 2/2: "fuse: Relax backing file validation to compare backing inodes"
  - No changes from v2.

Changes from RFC v1 [2]
=======================
v2 series implemented:
  - Replaced magic -1 value with dedicated FUSE_DEV_IOC_BACKING_CLOSE_ALL
    ioctl for comprehensive cleanup during daemon restart.
  - Dropped FOPEN_PASSTHROUGH_INODE_CACHE flag approach.
  - Modified fuse_inode_uncached_io_start() to compare backing inodes
    instead of fuse_backing pointers, allowing safe reuse of identical
    backing files after recovery.

Testing
=======
The patches have been tested with FUSE daemon crash and recovery
scenarios on kernel 6.19.0-rc4, successfully resolving the -EIO
errors and backing file resource leaks.

This approach maintains the simplicity of non-persistent backing_ids
while ensuring proper recovery semantics and thread safety.

Thank you for your review and guidance.

[1] https://lore.kernel.org/linux-fsdevel/20260115072032.402-1-luochunsheng@ustc.edu/
[2] https://lore.kernel.org/linux-fsdevel/20260116142845.422-1-luochunsheng@ustc.edu/T/#t

Chunsheng Luo (2):
  fuse: add ioctl to cleanup all backing files
  fuse: Relax backing file validation to compare backing inodes

 fs/fuse/backing.c         | 76 +++++++++++++++++++++++++++++++++++----
 fs/fuse/dev.c             | 16 +++++++++
 fs/fuse/fuse_i.h          |  5 +--
 fs/fuse/inode.c           | 11 +++---
 fs/fuse/iomode.c          |  2 +-
 include/uapi/linux/fuse.h |  1 +
 6 files changed, 95 insertions(+), 16 deletions(-)

-- 
2.41.0