[PATCH v29 3/6] fs/proc/task_mmu: Add fast paths to get/clear PAGE_IS_WRITTEN flag

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Adding fast code paths to handle specifically only get and/or clear
operation of PAGE_IS_WRITTEN, increases its performance by 0-35%.
The results of some test cases are given below:

Test-case-1
t1 = (Get + WP) time
t2 = WP time
                       t1            t2
Without this patch:    140-170mcs    90-115mcs
With this patch:       110mcs        80mcs
Worst case diff:       35% faster    30% faster

Test-case-2
t3 = atomic Get and WP
                      t3
Without this patch:   120-140mcs
With this patch:      100-110mcs
Worst case diff:      21% faster

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
The test to measure the performance can be found: https://is.gd/FtSKcD
8 8192 3 1 0 and 8 8192 3 1 1 arguments have been used to produce the
above mentioned results.

Changes in v29:
- Minor updates in flush logic following the original patch
---
 fs/proc/task_mmu.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 5271c634d2dc8..0ea0ed9ec704e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -2113,6 +2113,41 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
 		return 0;
 	}
 
+	if (!p->vec_out) {
+		/* Fast path for performing exclusive WP */
+		for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
+			if (pte_uffd_wp(ptep_get(pte)))
+				continue;
+			make_uffd_wp_pte(vma, addr, pte);
+			if (!flush_end)
+				start = addr;
+			flush_end = addr + PAGE_SIZE;
+		}
+		goto flush_and_return;
+	}
+
+	if (!p->arg.category_anyof_mask && !p->arg.category_inverted &&
+	    p->arg.category_mask == PAGE_IS_WRITTEN &&
+	    p->arg.return_mask == PAGE_IS_WRITTEN) {
+		for (addr = start; addr < end; pte++, addr += PAGE_SIZE) {
+			unsigned long next = addr + PAGE_SIZE;
+
+			if (pte_uffd_wp(ptep_get(pte)))
+				continue;
+			ret = pagemap_scan_output(p->cur_vma_category | PAGE_IS_WRITTEN,
+						  p, addr, &next);
+			if (next == addr)
+				break;
+			if (~p->arg.flags & PM_SCAN_WP_MATCHING)
+				continue;
+			make_uffd_wp_pte(vma, addr, pte);
+			if (!flush_end)
+				start = addr;
+			flush_end = next;
+		}
+		goto flush_and_return;
+	}
+
 	for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
 		unsigned long categories = p->cur_vma_category |
 					   pagemap_page_category(p, vma, addr, ptep_get(pte));
@@ -2136,6 +2171,7 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
 		flush_end = next;
 	}
 
+flush_and_return:
 	if (flush_end)
 		flush_tlb_range(vma, start, addr);
 
-- 
2.40.1