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

[Bin Yang]

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@xxxxxxxxxxxxx>
Signed-off-by: Bin Yang <bin.yang@xxxxxxxxx>
---
 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