On 01/09/2018 04:45 PM, Thomas Gleixner wrote:
> On Mon, 8 Jan 2018, Andrea Arcangeli wrote:
>> On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote:
>> Did my best to do the cleanest patch for tip, but I now figured Dave's
>> original comment was spot on: a _PAGE_NX clear then becomes necessary
>> also after pud_alloc not only after p4d_alloc.
>>
>> pmd_alloc would run into the same with x86 32bit non-PAE too.

non-PAE doesn't have an NX bit. :)

But we #define _PAGE_NX down to 0 there so it's harmless.

>> So there are two choices, either going back to one single _PAGE_NX
>> clear from the original Dave's original patch as below, or to add
>> multiple clear after each level which was my objective and is more
>> robust, but it may be overkill in this case. As long as it was one
>> line it looked a clear improvement.
>>
>> Considering the caller in both cases is going to abort I guess we can
>> use the one liner approach as Dave and Jiri did originally.
> 
> Dave ?

I agree with Andrea.  The patch in -tip potentially misses the pgd
clearing if pud_alloc() sets a PGD.  It would also be nice to have that
comment back.

Note that the -tip commit probably works in *practice* because for two
adjacent calls to map_tboot_page() that share a PGD entry, the first
will clear NX, *then* allocate and set the PGD (without NX clear).  The
second call will *not* allocate but will clear the NX bit.

The patch I think we want is attached.