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

[Jan Kara <jack@suse.cz>]

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.

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