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.
To quote Dave Hansen:
> Yes, the double-test was part of an optimization where we attempted to
> avoid using a global spinlock in the fork() path. We would check for
> unallocated mid-level page tables without the lock. The lock was only
> taken it when we needed to *make* an entry to avoid collisions.
>
> Now that it is all single-threaded, there is no chance of a collision,
> no need for a lock, and no need for the re-check.
v3: mark functions __init; add first-hand explanation
v2: add commit message.
Signed-off-by: Jike Song <albcamus@xxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Koshina <jikos@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Andi Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
arch/x86/mm/pti.c | 32 ++++++--------------------------
1 file changed, 6 insertions(+), 26 deletions(-)
diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 43d4a4a29037..ce38f165489b 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -149,7 +149,7 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
*
* Returns a pointer to a P4D on success, or NULL on failure.
*/
-static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
+static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
{
pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
@@ -164,12 +164,7 @@ static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
if (!new_p4d_page)
return NULL;
- if (pgd_none(*pgd)) {
- set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
- new_p4d_page = 0;
- }
- if (new_p4d_page)
- free_page(new_p4d_page);
+ set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
}
BUILD_BUG_ON(pgd_large(*pgd) != 0);
@@ -182,7 +177,7 @@ static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
*
* Returns a pointer to a PMD on success, or NULL on failure.
*/
-static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
+static __init pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
{
gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
@@ -194,12 +189,7 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
if (!new_pud_page)
return NULL;
- if (p4d_none(*p4d)) {
- set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
- new_pud_page = 0;
- }
- if (new_pud_page)
- free_page(new_pud_page);
+ set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
}
pud = pud_offset(p4d, address);
@@ -213,12 +203,7 @@ static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
if (!new_pmd_page)
return NULL;
- if (pud_none(*pud)) {
- set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
- new_pmd_page = 0;
- }
- if (new_pmd_page)
- free_page(new_pmd_page);
+ set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
}
return pmd_offset(pud, address);
@@ -251,12 +236,7 @@ static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
if (!new_pte_page)
return NULL;
- if (pmd_none(*pmd)) {
- set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
- new_pte_page = 0;
- }
- if (new_pte_page)
- free_page(new_pte_page);
+ set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
}
pte = pte_offset_kernel(pmd, address);
--
2.14.3