Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Buffered atomic writes

[Ojaswin Mujoo <ojaswin@linux.ibm.com>]

On Wed, Feb 18, 2026 at 06:42:05PM +0100, Jan Kara wrote:
> On Tue 17-02-26 22:50:17, Ojaswin Mujoo wrote:
> > On Mon, Feb 16, 2026 at 10:52:35AM +0100, Pankaj Raghav wrote:
> > > Hmm, IIUC, postgres will write their dirty buffer cache by combining multiple DB
> > > pages based on `io_combine_limit` (typically 128kb). So immediately writing them
> > > might be ok as long as we don't remove those pages from the page cache like we do in
> > > RWF_UNCACHED.
> > 
> > Yep, and Ive not looked at the code path much but I think if we really
> > care about the user not changing the data b/w write and writeback then
> > we will probably need to start the writeback while holding the folio
> > lock, which is currently not done in RWF_UNCACHED.
> 
> That isn't enough. submit_bio() returning isn't enough to guaranteed DMA
> to the device has happened. And until it happens, modifying the pagecache
> page means modifying the data the disk will get. The best is probably to
> transition pages to writeback state and deal with it as with any other
> requirement for stable pages.

Yes true, looking at the code, it does seem like we would also need to
depend on the stable page mechanism to ensure nobody changes the buffers
till the IO has actually finished.

I think the right way to go would be to first start with an
implementation of RWF_WRITETHOUGH and then utilize that and stable pages
to enable RWF_ATOMIC for buffered IO.

Regards,
ojaswin

> 
> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR