Re: [PATCH v2] erofs: accept source file descriptor via fsconfig

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2026/7/14 14:36, Giuseppe Scrivano wrote:
> Gao Xiang <hsiangkao@linux.alibaba.com> writes:
> 
>> On 2026/7/14 08:49, Aleksa Sarai wrote:
>>> On 2026-07-13, Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>>>> thanks for the hints.
>>>>
>>>> I'll prepare a v3 if you are fine with the version below:
>>> No worries, and this seems more reasonable at a first glance.
>>>
>>>> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
>>>> index 86fa5c6a0c70..72c85cc53085 100644
>>>> --- a/fs/erofs/super.c
>>>> +++ b/fs/erofs/super.c
>>> ...
>>>> @@ -437,6 +439,38 @@ static bool erofs_fc_set_dax_mode(struct fs_context *fc, unsigned int mode)
>>>>    	return false;
>>>>    }
>>>>    +static int erofs_fc_parse_source(struct fs_context *fc,
>>>> +				 struct fs_parameter *param)
>>>> +{
>>>> +	struct erofs_sb_info *sbi = fc->s_fs_info;
>>>> +
>>>> +	if (fc->source || sbi->dif0.file)
>>>> +		return invalf(fc, "Multiple sources");
>>>> +
>>>> +	switch (param->type) {
>>>> +	case fs_value_is_string:
>>>> +		fc->source = param->string;
>>>> +		param->string = NULL;
>>>> +		return 0;
>>>> +	case fs_value_is_file: {
>>>> +		char *buf, *p;
>>>> +
>>>> +		sbi->dif0.file = get_file(param->file);
>>> A very minor nit, but you can actually steal the file reference here
>>> with
>>> 		sbi->dif0.file = no_free_ptr(param->file);
>>> A few other places do this. (You'll also need to change the
>>> param->file
>>> reference below.)
>>>
>>>> +		buf = kmalloc(PATH_MAX, GFP_KERNEL);
>>>> +		if (!buf)
>>>> +			return -ENOMEM;
>>>> +		p = file_path(param->file, buf, PATH_MAX);
>>>> +		fc->source = kstrdup(IS_ERR(p) ? "(fd)" : p, GFP_KERNEL);
>>> I think that /proc/self/fd/%d would be a more useful name for
>>> debugging
>>> if file_path() fails (not that it is really possible here AFAICS). But
>>> I'm not really too fussed.
>>
>> Not quite sure if we should get in agreement with the format of this
>> one in advance (IOWs, users use source_fd and how fc->source looks like;
>> since other fses may follow the same practice if source_fd becomes common
>> later) since it's a user-visible field and I believe we shouldn't treat
>> this one as a dontcare field as some pseudo fses (since those fses don't
>> rely on `fc->source` by design but typically EROFS can rely on.)
>>
>> I hope Christian and others could share move thought on this part too
>> before I land this feature for the next cycle.
> 
> would it be better to just return the error from file_path without any
> fallback?

I hope Christian or other vfs folks can decide how to handle fc->source
string here, since in the long term, how to deal with source_fd should
be unique among different fses: just our current short-term
implementation lands into erofs directly for file-backed mounts to
fulfill composefs needs.

Thanks,
Gao Xiang

> 
> Thanks,
> Giuseppe