Message-ID: <2264748f-58f7-490e-be0b-257db08a761d@ustc.edu>
Date: Sun, 18 Jan 2026 19:47:48 +0800
From: Chunsheng Luo <luochunsheng@...c.edu>
To: Amir Goldstein <amir73il@...il.com>
Cc: miklos@...redi.hu, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] fuse: add ioctl to cleanup all backing files



On 1/18/26 1:00 AM, Amir Goldstein wrote:
> On Sat, Jan 17, 2026 at 5:14 PM Chunsheng Luo <luochunsheng@...c.edu> wrote:
>>
>>
>>
>> On 1/16/26 11:39 PM, Amir Goldstein wrote:
>>> On Fri, Jan 16, 2026 at 3:28 PM Chunsheng Luo <luochunsheng@...c.edu> wrote:
>>>>
>>>> To simplify crash recovery and reduce performance impact, backing_ids
>>>> are not persisted across daemon restarts. After crash recovery, this
>>>> may lead to resource leaks if backing file resources are not properly
>>>> cleaned up.
>>>>
>>>> Add FUSE_DEV_IOC_BACKING_CLOSE_ALL ioctl to release all backing_ids
>>>> and put backing files. When the FUSE daemon restarts, it can use this
>>>> ioctl to cleanup all backing file resources.
>>>>
>>>> Signed-off-by: Chunsheng Luo <luochunsheng@...c.edu>
>>>> ---
>>>>    fs/fuse/backing.c         | 19 +++++++++++++++++++
>>>>    fs/fuse/dev.c             | 16 ++++++++++++++++
>>>>    fs/fuse/fuse_i.h          |  1 +
>>>>    include/uapi/linux/fuse.h |  1 +
>>>>    4 files changed, 37 insertions(+)
>>>>
>>>> diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
>>>> index 4afda419dd14..e93d797a2cde 100644
>>>> --- a/fs/fuse/backing.c
>>>> +++ b/fs/fuse/backing.c
>>>> @@ -166,6 +166,25 @@ int fuse_backing_close(struct fuse_conn *fc, int backing_id)
>>>>           return err;
>>>>    }
>>>>
>>>> +static int fuse_backing_close_one(int id, void *p, void *data)
>>>> +{
>>>> +       struct fuse_conn *fc = data;
>>>> +
>>>> +       fuse_backing_close(fc, id);
>>>> +
>>>> +       return 0;
>>>> +}
>>>> +
>>>> +int fuse_backing_close_all(struct fuse_conn *fc)
>>>> +{
>>>> +       if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
>>>> +               return -EPERM;
>>>> +
>>>> +       idr_for_each(&fc->backing_files_map, fuse_backing_close_one, fc);
>>>> +
>>>> +       return 0;
>>>> +}
>>>> +
>>>
>>> This is not safe and not efficient.
>>> For safety from racing with _open/_close, iteration needs at least
>>> rcu_read_lock(),
>>
>> Yes, you're absolutely right. Additionally, calling idr_remove within
>> idr_for_each maybe presents safety risks.
>>
>>> but I think it will be much more efficient to zap the entire map with
>>> fuse_backing_files_free()/fuse_backing_files_init().
>>>
>>> This of course needs to be synchronized with concurrent _open/_close/_lookup.
>>> This could be done by making c->backing_files_map a struct idr __rcu *
>>> and replace the old and new backing_files_map under spin_lock(&fc->lock);
>>>
>>> Then you can call fuse_backing_files_free() on the old backing_files_map
>>> without a lock.
>>>
>>> As a side note, fuse_backing_files_free() iteration looks like it may need
>>> cond_resched() if there are a LOT of backing ids, but I am not sure and
>>> this is orthogonal to your change.
>>>
>>> Thanks,
>>> Amir.
>>>
>>>
>>
>> Thank you for your helpful suggestions. However, it cannot use
>> fuse_backing_files_free() in the close_all implementation because it
>> directly frees backing files without respecting reference counts. This
>> function requires that no one is actively using the backing file (it
>> even has WARN_ON_ONCE(refcount_read(&fb->count) != 1)), which cannot be
>> guaranteed after a crash recovery scenario where backing files may still
>> be in use.
> 
> Right.
> 
>>
>> Instead, the implementation uses fuse_backing_put() to safely decrement
>> the reference count and allow the backing file to be freed when no
>> longer in use.
> 
> OK.
> 
>>
>> Additionally, the implementation addresses two race conditions:
>>
>> - Race between idr_for_each and lookup: Uses synchronize_rcu() to ensure
>> all concurrent RCU readers (i.e., in-flight fuse_backing_lookup() calls)
>> complete before releasing backing files, preventing use-after-free issues.
> 
> Almost. See below.
> 
>>
>> - Race with open/close operations: Uses fc->lock to atomically swap the
>> old and new IDR maps, ensuring consistency with concurrent
>> fuse_backing_open() and fuse_backing_close() operations.
>>
>> This approach provides the same as the RCU pointer suggestion, but with
>> less code and no changes to the struct fuse_conn data structures.
>>
>> I've updated it and verified the implementation. Could you please review it?
>>
>>
>> diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
>> index 4afda419dd14..047d373684f9 100644
>> --- a/fs/fuse/backing.c
>> +++ b/fs/fuse/backing.c
>> @@ -166,6 +166,45 @@ int fuse_backing_close(struct fuse_conn *fc, int
>> backing_id)
>>           return err;
>>    }
>>
>> +static int fuse_backing_release_one(int id, void *p, void *data)
>> +{
>> +       struct fuse_backing *fb = p;
>> +
>> +       fuse_backing_put(fb);
>> +
>> +       return 0;
>> +}
>> +
>> +int fuse_backing_close_all(struct fuse_conn *fc)
>> +{
>> +       struct idr old_map;
>> +
>> +       if (!fc->passthrough || !capable(CAP_SYS_ADMIN))
>> +               return -EPERM;
>> +
>> +       /*
>> +        * Swap out the old backing_files_map with a new empty one under
>> lock,
>> +        * then release all backing files outside the lock. This avoids long
>> +        * lock hold times and potential races with concurrent open/close
>> +        * operations.
>> +        */
>> +       idr_init(&old_map);
>> +       spin_lock(&fc->lock);
>> +       swap(fc->backing_files_map, old_map);
>> +       spin_unlock(&fc->lock);
>> +
>> +       /*
>> +        * Ensure all concurrent RCU readers complete before releasing
>> backing
>> +        * files, so any in-flight lookups can safely take references.
>> +        */
>> +       synchronize_rcu();
>> +
>> +       idr_for_each(&old_map, fuse_backing_release_one, NULL);
>> +       idr_destroy(&old_map);
>> +
>> +       return 0;
>> +}
>> +
> 
> That's almost safe but not enough.
> This lookup code is not safe against the swap():
> 
>    rcu_read_lock();
>    fb = idr_find(&fc->backing_files_map, backing_id);
> 
> That is the reason you need to make fc->backing_files_map
> an rcu referenced ptr.
> 
> Instead of swap() you use xchg() to atomically exchange the
> old and new struct idr pointers and for lookup:
> 
>    rcu_read_lock();
>    fb = idr_find(rcu_dereference(fc->backing_files_map), backing_id);
> 
> Thanks,
> Amir.
> 
> 

Yes, swap() isn't atomic, it's just copying structs, so it's not safe 
when racing with lookup.

I've updated the version to make fc->backing_files_map an rcu referenced 
ptr. Please review the attached patch.

Thanks,
Chunsheng Luo.
View attachment "0001-fuse-add-ioctl-to-cleanup-all-backing-files.patch" of type "text/plain" (7935 bytes)

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