linux-kernel - [PATCH 2/7] mm: Rewrite shmem_seek_hole

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Date:   Wed, 19 Aug 2020 16:05:50 +0100
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     linux-mm@...ck.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/7] mm: Rewrite shmem_seek_hole_data

use the XArray directly instead of using the pagevec abstraction.
The code is simpler and more efficient.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 mm/shmem.c | 61 +++++++++++++++++++++---------------------------------
 1 file changed, 24 insertions(+), 37 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index a7bbc4ed9677..0f9f149f4b5e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2659,53 +2659,40 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 }
 
 /*
- * llseek SEEK_DATA or SEEK_HOLE through the page cache.
+ * llseek SEEK_DATA or SEEK_HOLE through the page cache.  We don't need
+ * to get a reference on the page because this interface is racy anyway.
+ * The page we find will have had the state at some point.
  */
 static pgoff_t shmem_seek_hole_data(struct address_space *mapping,
 				    pgoff_t index, pgoff_t end, int whence)
 {
+	XA_STATE(xas, &mapping->i_pages, index);
 	struct page *page;
-	struct pagevec pvec;
-	pgoff_t indices[PAGEVEC_SIZE];
-	bool done = false;
-	int i;
 
-	pagevec_init(&pvec);
-	pvec.nr = 1;		/* start small: we may be there already */
-	while (!done) {
-		pvec.nr = find_get_entries(mapping, index,
-					pvec.nr, pvec.pages, indices);
-		if (!pvec.nr) {
-			if (whence == SEEK_DATA)
-				index = end;
-			break;
+	rcu_read_lock();
+	if (whence == SEEK_DATA) {
+		for (;;) {
+			page = xas_find(&xas, end);
+			if (xas_retry(&xas, page))
+				continue;
+			if (!page || xa_is_value(page) || PageUptodate(page))
+				break;
 		}
-		for (i = 0; i < pvec.nr; i++, index++) {
-			if (index < indices[i]) {
-				if (whence == SEEK_HOLE) {
-					done = true;
-					break;
-				}
-				index = indices[i];
-			}
-			page = pvec.pages[i];
-			if (page && !xa_is_value(page)) {
-				if (!PageUptodate(page))
-					page = NULL;
-			}
-			if (index >= end ||
-			    (page && whence == SEEK_DATA) ||
-			    (!page && whence == SEEK_HOLE)) {
-				done = true;
+	} else /* SEEK_HOLE */ {
+		for (;;) {
+			page = xas_next(&xas);
+			if (xas_retry(&xas, page))
+				continue;
+			if (!xa_is_value(page) &&
+					(!page || !PageUptodate(page)))
+				break;
+			if (xas.xa_index >= end)
 				break;
-			}
 		}
-		pagevec_remove_exceptionals(&pvec);
-		pagevec_release(&pvec);
-		pvec.nr = PAGEVEC_SIZE;
-		cond_resched();
 	}
-	return index;
+	rcu_read_unlock();
+
+	return xas.xa_index;
 }
 
 static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
-- 
2.28.0