[PATCH 4/4] mm/filemap: Use a dynamically allocated pagevec in filemap_read

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

Restore Kent's optimisation for I/O sizes between 64kB and 1MB.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index f41de0759e86..2b4d8ed241bd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2427,7 +2427,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 	struct file_ra_state *ra = &filp->f_ra;
 	struct address_space *mapping = filp->f_mapping;
 	struct inode *inode = mapping->host;
-	struct pagevec pvec;
+	struct pagevec pvec_stack, *pvec = NULL;
 	int i, error = 0;
 	bool writably_mapped;
 	loff_t isize, end_offset;
@@ -2436,6 +2436,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 		return 0;
 	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
 
+	if (iter->count / PAGE_SIZE > PAGEVEC_SIZE)
+		pvec = pagevec_alloc(iter->count / PAGE_SIZE + 1, GFP_KERNEL);
+	if (!pvec)
+		pvec = &pvec_stack;
 	do {
 		cond_resched();
 
@@ -2447,7 +2451,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 		if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
 			iocb->ki_flags |= IOCB_NOWAIT;
 
-		error = filemap_get_pages(iocb, iter, &pvec);
+		error = filemap_get_pages(iocb, iter, pvec);
 		if (error < 0)
 			break;
 
@@ -2476,10 +2480,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 		 */
 		if (iocb->ki_pos >> PAGE_SHIFT !=
 		    ra->prev_pos >> PAGE_SHIFT)
-			mark_page_accessed(pvec.pages[0]);
+			mark_page_accessed(pvec->pages[0]);
 
-		for (i = 0; i < pagevec_count(&pvec); i++) {
-			struct page *page = pvec.pages[i];
+		for (i = 0; i < pagevec_count(pvec); i++) {
+			struct page *page = pvec->pages[i];
 			size_t page_size = thp_size(page);
 			size_t offset = iocb->ki_pos & (page_size - 1);
 			size_t bytes = min_t(loff_t, end_offset - iocb->ki_pos,
@@ -2514,7 +2518,7 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 			}
 		}
 put_pages:
-		pagevec_release(&pvec);
+		pagevec_release(pvec);
 	} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);
 
 	file_accessed(filp);
-- 
2.28.0