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

[hanqi]

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,

>