Re: [PATCH v5 1/5] libfs: Return ENOSPC when the directory offset range is exhausted

[Chuck Lever <chuck.lever@oracle.com>]

On 12/16/24 8:39 AM, Pratyush Yadav wrote:
> On Sun, Dec 15 2024, David Laight wrote:
> 
>> From: cel@kernel.org
>>> Sent: 15 December 2024 18:58
>>>
>>> From: Chuck Lever <chuck.lever@oracle.com>
>>>
>>> Testing shows that the EBUSY error return from mtree_alloc_cyclic()
>>> leaks into user space. The ERRORS section of "man creat(2)" says:
>>>
>>>> 	EBUSY	O_EXCL was specified in flags and pathname refers
>>>> 		to a block device that is in use by the system
>>>> 		(e.g., it is mounted).
>>>
>>> ENOSPC is closer to what applications expect in this situation.
>>>
>>> Note that the normal range of simple directory offset values is
>>> 2..2^63, so hitting this error is going to be rare to impossible.
>>>
>>> Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
>>> Cc: <stable@vger.kernel.org> # v6.9+
>>> Reviewed-by: Jeff Layton <jlayton@kernel.org>
>>> Reviewed-by: Yang Erkun <yangerkun@huawei.com>
>>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>>> ---
>>>   fs/libfs.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/libfs.c b/fs/libfs.c
>>> index 748ac5923154..f6d04c69f195 100644
>>> --- a/fs/libfs.c
>>> +++ b/fs/libfs.c
>>> @@ -292,7 +292,9 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
>>>
>>>   	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
>>>   				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
>>> -	if (ret < 0)
>>> +	if (unlikely(ret == -EBUSY))
>>> +		return -ENOSPC;
>>> +	if (unlikely(ret < 0))
>>>   		return ret;
>>
>> You've just added an extra comparison to a hot path.
>> Doing:
>> 	if (ret < 0)
>> 		return ret == -EBUSY ? -ENOSPC : ret;
>> would be better.
> 
> This also has two comparisons: one for ret < 0 and another for ret ==
> -EBUSY. So I don't see a difference. I was curious to see if compilers
> can somehow optimize one or the other, so I ran the two on godbolt and I
> see no real difference between the two: https://godbolt.org/z/9Gav6b6Mf

In my version, both comparisons are done every time through this flow.
David's version changes it so that only one comparison is done unless
@ret is less than zero (which is rare).

I've updated simple_offset_add() in my tree to use David's version.


-- 
Chuck Lever