[PATCH v3 2/5] x86/mm: avoid static_protection() checking if not whole large page attr change

Bin Yang
2018-08-21T01:16:45Z

The range check whether the address is aligned to the large page and
covers the full large page (1G or 2M) is obvious to do _before_
static_protection() check, because if the requested range does not fit
and has a different pgprot_val() then it will decide to split after the
check anyway.

The approach and some of the comments came from Thomas Gleixner's
email example for how to do this

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Bin Yang <bin.yang@intel.com>
---
arch/x86/mm/pageattr.c | 35 ++++++++++++++++-------------------
1 file changed, 16 insertions(+), 19 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 68613fd..091f1d3 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -645,11 +645,21 @@ try_preserve_large_page(pte_t *kpte, unsigned long address,
}

/*
+ * If the requested address range is not aligned to the start of
+ * the large page or does not cover the full range, split it up.
+ * No matter what the static_protections() check below does, it
+ * would anyway result in a split after doing all the check work
+ * for nothing.
+ */
+ addr = address & pmask;
+ if (address != addr || cpa->numpages != numpages)
+ goto out_unlock;
+
+ /*
* We need to check the full range, whether
* static_protection() requires a different pgprot for one of
* the pages in the range we try to preserve:
*/
- addr = address & pmask;
pfn = old_pfn;
for (i = 0; i < (psize >> PAGE_SHIFT); i++, addr += PAGE_SIZE, pfn++) {
pgprot_t chk_prot = static_protections(req_prot, addr, pfn);
@@ -659,24 +669,11 @@ try_preserve_large_page(pte_t *kpte, unsigned long address,
}

- /*
- * We need to change the attributes. Check, whether we can
- * change the large page in one go. We request a split, when
- * the address is not aligned and the number of pages is
- * smaller than the number of pages in the large page. Note
- * that we limited the number of possible pages already to
- * the number of pages in the large page.
- */
- if (address == (address & pmask) && cpa->numpages == (psize >> PAGE_SHIFT)) {
- /*
- * The address is aligned and the number of pages
- * covers the full page.
- */
- new_pte = pfn_pte(old_pfn, new_prot);
- __set_pmd_pte(kpte, address, new_pte);
- cpa->flags |= CPA_FLUSHTLB;
- do_split = 0;
- }
+ /* All checks passed. Just change the large mapping entry */
+ new_pte = pfn_pte(old_pfn, new_prot);
+ __set_pmd_pte(kpte, address, new_pte);
+ cpa->flags |= CPA_FLUSHTLB;
+ do_split = 0;

out_unlock:
spin_unlock(&pgd_lock);
--
2.7.4