Re: [PATCH] f2fs: f2fs supports uncached buffered I/O read

[Chao Yu <chao@kernel.org>]

On 7/30/25 23:20, Jens Axboe wrote:
> On 7/28/25 2:28 AM, hanqi wrote:
>> ? 2025/7/28 16:07, Chao Yu ??:
>>> On 7/28/25 16:03, hanqi wrote:
>>>> ? 2025/7/28 15:38, Chao Yu ??:
>>>>
>>>>> On 7/25/25 15:53, Qi Han wrote:
>>>>>> Jens has already completed the development of uncached buffered I/O
>>>>>> in git [1], and in f2fs, uncached buffered I/O read can be enabled
>>>>>> simply by setting the FOP_DONTCACHE flag in f2fs_file_operations.
>>>>> IIUC, we may suffer lock issue when we call pwritev(.. ,RWF_DONTCACHE)?
>>>>> as Jen mentioned in below path, right?
>>>>>
>>>>> soft-irq
>>>>> - folio_end_writeback()
>>>>>    - filemap_end_dropbehind_write()
>>>>>     - filemap_end_dropbehind()
>>>>>      - folio_unmap_invalidate()
>>>>>       - lock i_lock
>>>>>
>>>>> Thanks,
>>>> That's how I understand it.
>>> So I guess we need to wait for the support RWF_DONTCACHE on write path, unless
>>> you can walk around for write path in this patch.
>>>
>>> Thanks,
>>
>> I think the read and write paths can be submitted separately.
>> Currently, uncached buffered I/O write requires setting the
>> FGP_DONTCACHE flag when the filesystem allocates a folio. In
>> f2fs, this is done in the following path:
>>
>> - write_begin
>>  - f2fs_write_begin
>>   - __filemap_get_folio
>>   As I understand it, if we don't set the FGP_DONTCACHE flag here, this
>> issue shouldn't occur.
> 
> It won't cause an issue, but it also won't work in the sense that the
> intent is that if the file system doesn't support DONTCACHE, it would
> get errored at submission time. Your approach would just ignore the flag
> for writes, rather than return -EOPNOTSUPP as would be expected.

Jens,

Do you mean like what we have done in kiocb_set_rw_flags()?

	if (flags & RWF_DONTCACHE) {
		/* file system must support it */
		if (!(ki->ki_filp->f_op->fop_flags & FOP_DONTCACHE))
			return -EOPNOTSUPP;
...
	}

IIUC, it's better to have this in original patch, let me know if I'm
missing something.

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 9b8d24097b7a..7f09cad6b6d7 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -5185,6 +5185,11 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		goto out;
 	}

+	if (iocb->ki_flags & IOCB_DONTCACHE) {
+		ret = -EOPNOTSUPP;
+		goto out;
+	}
+
 	if (!f2fs_is_compress_backend_ready(inode)) {
 		ret = -EOPNOTSUPP;
 		goto out;
-- 

Thanks,

> 
> You could potentially make it work just on the read side by having the
> f2fs write submit side check DONTCACHE on the write side and error them
> out.
>