Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page

[David Hildenbrand]

On 02.03.22 21:59, Jason Gunthorpe wrote:
> On Wed, Mar 02, 2022 at 09:38:09PM +0100, David Hildenbrand wrote:
> 
>> (a) I want a R/O pin to observe file modifications.
>> (b) I want the R/O pin to *not* observe file modifications but observe
>>     my (eventual? if any) private modifications,
> 
> A scenario I know that motivated this is fairly straightfoward:
> 
>    static char data[] = {};
> 
>    ibv_reg_mr(data, READ_ONLY)
>    data[0] = 1
>    .. go touch data via DMA ..
> 
> We want to reliably observe the '1'
> 
> What is happening under the covers is that 'data' is placed in the
> .data segment and becomes a file backed MAP_PRIVATE page. The write
> COWs that page
> 
> It would work OK if it was in .bss instead
> 
> I think the FOLL_FORCE is there because otherwise the trick doesn't
> work on true RO pages and the API becomes broken.

Thanks for the nice example, it matches what John brought up in respect
to MAP_PRIVATE semantics.

-- 
Thanks,

David / dhildenb