Re: [PATCH v7 6/7] mm/vmalloc: map contiguous pages in batches for vmap() if possible

[Wen Jiang]

Hi David and Dev,

The updated diff is blow.

Subject: [PATCH] mm/vmalloc: simplify vmap batching helper and limit scan by PFN alignment

Pass the current page pointer directly to the batching helper, and
limit the contiguous scan by PFN alignment before checking for a batch
size. This keeps the helper interface simpler and avoids scanning more
pages than can actually be mapped together.

Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Signed-off-by: Wen Jiang <jiangwen6@xxxxxxxxxx>

---
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 92f9cd5e9def5..2f7e6128cbd7c 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3558,7 +3558,7 @@ static inline unsigned int vm_shift(pgprot_t prot, unsigned long size)
 }
 
 static inline int get_vmap_batch_order(struct page **pages,
-		pgprot_t prot, unsigned int max_steps, unsigned int idx)
+		pgprot_t prot, unsigned int nr_pages)
 {
 	unsigned long pfn;
 	unsigned int nr_contig;
@@ -3567,16 +3567,16 @@ static inline int get_vmap_batch_order(struct page **pages,
 	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP))
 		return 0;
 
-	nr_contig = num_pages_contiguous(&pages[idx], max_steps);
+	/* Limit nr_pages by pfn alignment */
+	pfn = page_to_pfn(*pages);
+	if (pfn > 0)
+		nr_pages = min_t(unsigned int, nr_pages, 1UL << __ffs(pfn));
+
+	nr_contig = num_pages_contiguous(pages, nr_pages);
 	if (nr_contig < 2)
 		return 0;
 
 	order = ilog2(nr_contig);
-	pfn = page_to_pfn(pages[idx]);
-
-	/* Limit order by pfn alignment */
-	if (pfn > 0)
-		order = min_t(int, order, __ffs(pfn));
 
 	if (vm_shift(prot, PAGE_SIZE << order) == PAGE_SHIFT)
 		return 0;
@@ -3587,8 +3587,8 @@ static inline int get_vmap_batch_order(struct page **pages,
 static int vmap_pages_range_batched(unsigned long addr, unsigned long end,
 		pgprot_t prot, struct page **pages)
 {
-	unsigned int count = (end - addr) >> PAGE_SHIFT;
-	unsigned int prev_shift = 0, idx = 0;
+	const unsigned int nr_pages = (end - addr) >> PAGE_SHIFT;
+	unsigned int prev_shift = 0, batch_start = 0;
 	unsigned long map_addr = addr, batch_end = addr;
 	int err;
 
@@ -3597,26 +3597,26 @@ static int vmap_pages_range_batched(unsigned long addr, unsigned long end,
 	if (err)
 		goto out;
 
-	for (unsigned int i = 0; i < count; ) {
+	for (unsigned int i = 0; i < nr_pages; ) {
 		unsigned int shift = PAGE_SHIFT +
-			get_vmap_batch_order(pages, prot, count - i, i);
+			get_vmap_batch_order(pages + i, prot, nr_pages - i);
 
 		if (!i)
 			prev_shift = shift;
 
 		if (shift != prev_shift) {
 			err = vmap_pages_range_noflush_walk(map_addr, batch_end,
-					prot, pages + idx, prev_shift);
+					prot, pages + batch_start, prev_shift);
 			if (err)
 				goto out;
 			prev_shift = shift;
 			map_addr = batch_end;
-			idx = i;
+			batch_start = i;
 		}
 
 		/*
-		 * Once small pages are encountered, the remaining pages
-		 * are likely small as well.
+		 * Once we fail to batch pages, we expect to fail batching
+		 * for all remaining pages, so just give up.
 		 */
 		if (shift == PAGE_SHIFT)
 			break;
@@ -3627,8 +3627,8 @@ static int vmap_pages_range_batched(unsigned long addr, unsigned long end,
 
 	/* Remaining */
 	if (map_addr < end)
-		err = vmap_pages_range_noflush_walk(map_addr, end,
-				prot, pages + idx, prev_shift);
+		err = vmap_pages_range_noflush_walk(map_addr, end, prot,
+				pages + batch_start, prev_shift);
 
 out:
 	flush_cache_vmap(addr, end);