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

[hanqi <hanqi@vivo.com>]

在 2025/7/15 22:28, Jens Axboe 写道:
> On 7/14/25 9:10 PM, Qi Han wrote:
>> Jens has already completed the development of uncached buffered I/O
>> in git [1], and in f2fs, the feature can be enabled simply by setting
>> the FOP_DONTCACHE flag in f2fs_file_operations.
> You need to ensure that for any DONTCACHE IO that the completion is
> routed via non-irq context, if applicable. I didn't verify that this is
> the case for f2fs. Generally you can deduce this as well through
> testing, I'd say the following cases would be interesting to test:
>
> 1) Normal DONTCACHE buffered read
> 2) Overwrite DONTCACHE buffered write
> 3) Append DONTCACHE buffered write
>
> Test those with DEBUG_ATOMIC_SLEEP set in your config, and it that
> doesn't complain, that's a great start.
>
> For the above test cases as well, verify that page cache doesn't grow as
> IO is performed. A bit is fine for things like meta data, but generally
> you want to see it remain basically flat in terms of page cache usage.
>
> Maybe this is all fine, like I said I didn't verify. Just mentioning it
> for completeness sake.

Hi, Jens
Thanks for your suggestion. As I mentioned earlier in [1], in f2fs,
the regular buffered write path invokes folio_end_writeback from a
softirq context. Therefore, it seems that f2fs may not be suitable
for DONTCACHE I/O writes.

I’d like to ask a question: why is DONTCACHE I/O write restricted to
non-interrupt context only? Is it because dropping the page might be
too time-consuming to be done safely in interrupt context? This might
be a naive question, but I’d really appreciate your clarification.
Thanks in advance.

[1] https://lore.kernel.org/all/137c0a07-ea0a-48fa-acc4-3e0ec63681f4@vivo.com/

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