Re: O_DIRECT vs BLK_FEAT_STABLE_WRITES, was Re: [PATCH] btrfs: never trust the bio from direct IO

[Jan Kara <jack@suse.cz>]

On Tue 21-10-25 09:57:08, David Hildenbrand wrote:
> On 21.10.25 09:49, Christoph Hellwig wrote:
> > On Mon, Oct 20, 2025 at 09:00:50PM +0200, David Hildenbrand wrote:
> > > Just FYI, because it might be interesting in this context.
> > > 
> > > For anonymous memory we have this working by only writing the folio out if
> > > it is completely unmapped and there are no unexpected folio references/pins
> > > (see pageout()), and only allowing to write to such a folio ("reuse") if
> > > SWP_STABLE_WRITES is not set (see do_swap_page()).
> > > 
> > > So once we start writeback the folio has no writable page table mappings
> > > (unmapped) and no GUP pins. Consequently, when trying to write to it we can
> > > just fallback to creating a page copy without causing trouble with GUP pins.
> > 
> > Yeah.  But anonymous is the easy case, the pain is direct I/O to file
> > mappings.  Mapping the right answer is to just fail pinning them and fall
> > back to (dontcache) buffered I/O.
> 
> Right, I think the rules could likely be
> 
> a) Don't start writeback to such devices if there may be GUP pins (o
> writeble PTEs)
> 
> b) Don't allow FOLL_WRITE GUP pins if there is writeback to such a device
> 
> Regarding b), I would have thought that GUP would find the PTE to not be
> writable and consequently trigger a page fault first to make it writable?
> And I'd have thought that we cannot make such a PTE writable while there is
> writeback to such a device going on (otherwise the CPU could just cause
> trouble).

See some of the cases in my reply to Christoph. It is also stuff like:

c) Don't allow FOLL_WRITE GUP pins or writeable mapping if there are *any*
pins to the page.

And we'd have to write-protect the page in the page tables at the moment we
obtain the FOLL_WRITE GUP pin to make sure the pin owner is the only thread
able to modify that page contents while the DIO is running.

								Honza

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