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

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Message-ID: <d3ced6d7-4037-4962-a2d4-382af0aad4cd@vivo.com>
Date: Thu, 31 Jul 2025 09:58:13 +0800
From: hanqi <hanqi@...o.com>
To: Jens Axboe <axboe@...nel.dk>, Chao Yu <chao@...nel.org>,
 jaegeuk@...nel.org
Cc: linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] f2fs: f2fs supports uncached buffered I/O read


在 2025/7/30 23:20, Jens Axboe 写道:
> 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.
>
> 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.

Hi Jens,

Thank you for your suggestions. I am currently working on modifying
F2FS to handle the dontcache unmap operation in a workqueue. I expect
to submit the patch soon, after which F2FS should also support uncached
buffer I/O writes.

Thanks,

>