fs/proc/task_mmu: add fast paths to get/clear PAGE_IS_WRITTEN flag

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

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muhammad Usama Anjum <[e-mail]>
Cc: Alex Sierra <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Andrei Vagin <[e-mail]>
Cc: Axel Rasmussen <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Cyrill Gorcunov <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Gustavo A. R. Silva <[e-mail]>
Cc: "Liam R. Howlett" <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Michal Miroslaw <[e-mail]>
Cc: Michał Mirosław <[e-mail]>
Cc: Mike Rapoport (IBM) <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: Pasha Tatashin <[e-mail]>
Cc: Paul Gofman <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Yang Shi <[e-mail]>
Cc: Yun Zhou <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/proc/task_mmu.c

···
 		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));
···
 		flush_end = next;
 	}
 
+flush_and_return:
 	if (flush_end)
 		flush_tlb_range(vma, start, addr);