[PATCH RFC] erofs: file-backed mount supports direct io

[Hongbo Li]

On 2025/1/20 11:10, Gao Xiang wrote:
> 
> 
> On 2025/1/20 11:02, Hongbo Li wrote:
>>
>>
> 
> ...
> 
>>>>   }
>>>> +static int erofs_fileio_scan_iter(struct erofs_fileio *io, struct 
>>>> kiocb *iocb,
>>>> +                  struct iov_iter *iter)
>>>
>>> I wonder if it's possible to just extract a folio from
>>> `struct iov_iter` and reuse erofs_fileio_scan_folio() logic.
>> Thanks for reviewing. Ok, I'll think about reusing the 
>> erofs_fileio_scan_folio logic in later version.
> 
> Thanks.
> 
>>
>> Additionally, for the file-backed mount case, can we consider removing 
>> the erofs's page cache and just using the backend file's page cache? 
>> If in this way, it will use buffer io for reading the backend's 
>> mounted files in default, and it also can decrease the memory overhead.
> 
> I think it's too hacky for upstreaming, since EROFS can only
> operate its own page cache, otherwise it should only support
> overlayfs-like per-inode sharing.
> 
> Per-extent sharing among different filesystems is too hacky
It just like the dax mode of erofs (but instead of the dax devices, it's 
a backend file). It does not share the page among the different 
filesystems, because there is only the backend file's page cache. I 
found the whole io path is similar to this direct io mode.

Thanks,
Hongbo

> on the MM side, but if you have some detailed internal
> requirement, you could implement downstream.
> 
> Thanks,
> Gao Xiang
> 
>>
>> This is just my initial idea, for uncompressed mode, this should make 
>> sense. But for compressed layout, it needs to be verified.
>>
>> Thanks,
>> Hongbo
>>
>>>
>>> It simplifies the codebase a lot, and I think the performance
>>> is almost the same.
>>>
>>> Otherwise currently it looks good to me.
>>>
>>> Thanks,
>>> Gao Xiang
>