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SubjectRe: [PATCH v2] x86/mm/pti: remove dead logic during user pagetable population
On Sun, Jan 07, 2018 at 06:33:17PM +0800, Jike Song wrote:
> Look at one of the code snippets:
>
> 162 if (pgd_none(*pgd)) {
> 163 unsigned long new_p4d_page = __get_free_page(gfp);
> 164 if (!new_p4d_page)
> 165 return NULL;
> 166
> 167 if (pgd_none(*pgd)) {
> 168 set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
> 169 new_p4d_page = 0;
> 170 }
> 171 if (new_p4d_page)
> 172 free_page(new_p4d_page);
> 173 }
>
> There can't be any difference between two pgd_none(*pgd) at L162 and L167,
> so it's always false at L171.

I think this is a remnant from the kaiser version which did this:

if (pud_none(*pud)) {
unsigned long new_pmd_page = __get_free_page(gfp);
if (!new_pmd_page)
return NULL;
spin_lock(&shadow_table_allocation_lock);
if (pud_none(*pud))
set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
else
free_page(new_pmd_page);
spin_unlock(&shadow_table_allocation_lock);
}

I was wondering too, why the duplicated checks.

Which has this explanation about the need for the locking:

/*
* At runtime, the only things we map are some things for CPU
* hotplug, and stacks for new processes. No two CPUs will ever
* be populating the same addresses, so we only need to ensure
* that we protect between two CPUs trying to allocate and
* populate the same page table page.
*
* Only take this lock when doing a set_p[4um]d(), but it is not
* needed for doing a set_pte(). We assume that only the *owner*
* of a given allocation will be doing this for _their_
* allocation.
*
* This ensures that once a system has been running for a while
* and there have been stacks all over and these page tables
* are fully populated, there will be no further acquisitions of
* this lock.
*/
static DEFINE_SPINLOCK(shadow_table_allocation_lock);

Now I have my suspicions why that's not needed anymore upstream but I'd
let tglx explain better.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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