mm/filemap: remove dynamically allocated array from filemap_read

Increasing the batch size runs into diminishing returns.  It's probably
better to make, eg, three calls to filemap_get_pages() than it is to call
into kmalloc().

Link: https://lkml.kernel.org/r/20210122160140.223228-3-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2431,8 +2431,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 	struct file_ra_state *ra = &filp->f_ra;
 	struct address_space *mapping = filp->f_mapping;
 	struct inode *inode = mapping->host;
-	struct page *pages_onstack[PAGEVEC_SIZE], **pages = NULL;
-	unsigned int nr_pages = min_t(unsigned int, 512,
+	struct page *pages[PAGEVEC_SIZE];
+	unsigned int nr_pages = min_t(unsigned int, PAGEVEC_SIZE,
 			((iocb->ki_pos + iter->count + PAGE_SIZE - 1) >> PAGE_SHIFT) -
 			(iocb->ki_pos >> PAGE_SHIFT));
 	int i, pg_nr, error = 0;
@@ -2446,14 +2446,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
 
-	if (nr_pages > ARRAY_SIZE(pages_onstack))
-		pages = kmalloc_array(nr_pages, sizeof(void *), GFP_KERNEL);
-
-	if (!pages) {
-		pages = pages_onstack;
-		nr_pages = min_t(unsigned int, nr_pages, ARRAY_SIZE(pages_onstack));
-	}
-
 	do {
 		cond_resched();
 
@@ -2538,9 +2530,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 	file_accessed(filp);
 
-	if (pages != pages_onstack)
-		kfree(pages);
-
 	return written ? written : error;
 }
 EXPORT_SYMBOL_GPL(generic_file_buffered_read);