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 11:33, Jan Kara wrote:
> 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.

Oh that's nasty, but yeah I understood the problem now, thanks.

-- 
Cheers

David / dhildenb