Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

["Thomas Hellström (Intel)" <thomas_os@shipmail.org>]

On 3/25/21 6:55 PM, Jason Gunthorpe wrote:
> On Thu, Mar 25, 2021 at 06:51:26PM +0100, Thomas Hellström (Intel) wrote:
>> On 3/24/21 9:25 PM, Dave Hansen wrote:
>>> On 3/24/21 1:22 PM, Thomas Hellström (Intel) wrote:
>>>>> We also have not been careful at *all* about how _PAGE_BIT_SOFTW* are
>>>>> used.  It's quite possible we can encode another use even in the
>>>>> existing bits.
>>>>>
>>>>> Personally, I'd just try:
>>>>>
>>>>> #define _PAGE_BIT_SOFTW5        57      /* available for programmer */
>>>>>
>>>> OK, I'll follow your advise here. FWIW I grepped for SW1 and it seems
>>>> used in a selftest, but only for PTEs AFAICT.
>>>>
>>>> Oh, and we don't care about 32-bit much anymore?
>>> On x86, we have 64-bit PTEs when running 32-bit kernels if PAE is
>>> enabled.  IOW, we can handle the majority of 32-bit CPUs out there.
>>>
>>> But, yeah, we don't care about 32-bit. :)
>> Hmm,
>>
>> Actually it makes some sense to use SW1, to make it end up in the same dword
>> as the PSE bit, as from what I can tell, reading of a 64-bit pmd_t on 32-bit
>> PAE is not atomic, so in theory a huge pmd could be modified while reading
>> the pmd_t making the dwords inconsistent.... How does that work with fast
>> gup anyway?
> It loops to get an atomic 64 bit value if the arch can't provide an
> atomic 64 bit load

Hmm, ok, I see a READ_ONCE() in gup_pmd_range(), and then the resulting 
pmd is dereferenced either in try_grab_compound_head() or 
__gup_device_huge(), before the pmd is compared to the value the pointer 
is currently pointing to. Couldn't those dereferences be on invalid 
pointers?

/Thomas

>
> Jason