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

[David Hildenbrand <david@redhat.com>]

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).

-- 
Cheers

David / dhildenb