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@gmail.com Cc: David Woodhouse dwmw@amazon.co.uk Cc: Alan Cox gnomes@lxorguk.ukuu.org.uk Cc: Jiri Koshina jikos@kernel.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Tim Chen tim.c.chen@linux.intel.com Cc: Andi Lutomirski luto@amacapital.net Cc: Andi Kleen ak@linux.intel.com Cc: Peter Zijlstra peterz@infradead.org Cc: Paul Turner pjt@google.com Cc: Tom Lendacky thomas.lendacky@amd.com Cc: Greg KH gregkh@linux-foundation.org Cc: Dave Hansen dave.hansen@intel.com Cc: Kees Cook keescook@google.com Cc: Borislav Petkov bp@alien8.de Cc: stable@vger.kernel.org --- 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);