WIP: Performance improvements

[Muhammad Usama Anjum]

We are optimizing for more performance. Please find the change-set below
for easy review before next revision and post your comments:

- Replace memcpy() with direct copy as it proving to be very expensive
- Don't check if PAGE_IS_FILE if no mask needs it as it is very
  expensive to check per pte
- Add question in comment for discussion purpose
- Add fast path for exclusive WP for ptes
---
 fs/proc/task_mmu.c | 54 ++++++++++++++++++++++++++++++++++++----------
 1 file changed, 43 insertions(+), 11 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 7e92c33635cab..879baf896ed0b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1757,37 +1757,51 @@ static int pagemap_release(struct inode *inode,
struct file *file)
 				 PAGE_IS_HUGE)
 #define PM_SCAN_FLAGS		(PM_SCAN_WP_MATCHING | PM_SCAN_CHECK_WPASYNC)

+#define MASKS_OF_INTEREST(a)	(a.category_inverted | a.category_mask | \
+				 a.category_anyof_mask | a.return_mask)
+
 struct pagemap_scan_private {
 	struct pm_scan_arg arg;
+	unsigned long masks_of_interest;
 	unsigned long cur_vma_category;
 	struct page_region *vec_buf, cur_buf;
 	unsigned long vec_buf_len, vec_buf_index, found_pages, end_addr;
 	struct page_region __user *vec_out;
 };

-static unsigned long pagemap_page_category(struct vm_area_struct *vma,
+static unsigned long pagemap_page_category(struct pagemap_scan_private *p,
+					   struct vm_area_struct *vma,
 					   unsigned long addr, pte_t pte)
 {
 	unsigned long categories = 0;

 	if (pte_present(pte)) {
-		struct page *page = vm_normal_page(vma, addr, pte);
+		struct page *page;

 		categories |= PAGE_IS_PRESENT;
 		if (!pte_uffd_wp(pte))
 			categories |= PAGE_IS_WRITTEN;
-		if (page && !PageAnon(page))
-			categories |= PAGE_IS_FILE;
+
+		if (p->masks_of_interest & PAGE_IS_FILE) {
+			page = vm_normal_page(vma, addr, pte);
+			if (page && !PageAnon(page))
+				categories |= PAGE_IS_FILE;
+		}
+
 		if (is_zero_pfn(pte_pfn(pte)))
 			categories |= PAGE_IS_PFNZERO;
 	} else if (is_swap_pte(pte)) {
-		swp_entry_t swp = pte_to_swp_entry(pte);
+		swp_entry_t swp;

 		categories |= PAGE_IS_SWAPPED;
 		if (!pte_swp_uffd_wp_any(pte))
 			categories |= PAGE_IS_WRITTEN;
-		if (is_pfn_swap_entry(swp) && !PageAnon(pfn_swap_entry_to_page(swp)))
-			categories |= PAGE_IS_FILE;
+
+		if (p->masks_of_interest & PAGE_IS_FILE) {
+			swp = pte_to_swp_entry(pte);
+			if (is_pfn_swap_entry(swp) && !PageAnon(pfn_swap_entry_to_page(swp)))
+				categories |= PAGE_IS_FILE;
+		}
 	}

 	return categories;
@@ -1957,9 +1971,7 @@ static bool pagemap_scan_push_range(unsigned long
categories,
 		if (p->vec_buf_index >= p->vec_buf_len)
 			return false;

-		memcpy(&p->vec_buf[p->vec_buf_index], cur_buf,
-		       sizeof(*p->vec_buf));
-		++p->vec_buf_index;
+		p->vec_buf[p->vec_buf_index++] = *cur_buf;
 	}

 	cur_buf->start = addr;
@@ -2095,9 +2107,24 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
unsigned long start,
 		return 0;
 	}

+	if (!p->vec_buf) {
+		/* 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) {
+				start = addr;
+				flush = true;
+			}
+		}
+		ret = 0;
+		goto flush_and_return;
+	}
+
 	for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
 		unsigned long categories = p->cur_vma_category |
-					   pagemap_page_category(vma, addr, ptep_get(pte));
+					   pagemap_page_category(p, vma, addr, ptep_get(pte));
 		unsigned long next = addr + PAGE_SIZE;

 		ret = pagemap_scan_output(categories, p, addr, &next);
@@ -2119,6 +2146,7 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
unsigned long start,
 		}
 	}

+flush_and_return:
 	if (flush)
 		flush_tlb_range(vma, start, addr);

@@ -2284,6 +2312,9 @@ static int pagemap_scan_init_bounce_buffer(struct
pagemap_scan_private *p)
 	 * consecutive ranges that have the same categories returned across
 	 * walk_page_range() calls.
 	 */
+	// Question: Increasing the vec_buf_len increases the execution speed.
+	// But it'll increase the memory needed to run the IOCTL. Can we do
something here?
+	// Right now only have space for 512 entries of page_region
 	p->vec_buf_len = min_t(size_t, PAGEMAP_WALK_SIZE >> PAGE_SHIFT,
 			       p->arg.vec_len - 1);
 	p->vec_buf = kmalloc_array(p->vec_buf_len, sizeof(*p->vec_buf),
@@ -2329,6 +2360,7 @@ static long do_pagemap_scan(struct mm_struct *mm,
unsigned long uarg)
 	if (ret)
 		return ret;

+	p.masks_of_interest = MASKS_OF_INTEREST(p.arg);
 	ret = pagemap_scan_init_bounce_buffer(&p);
 	if (ret)
 		return ret;
-- 
2.39.2