linux-kernel - [PATCH v2 2/2] netfs: Fix dodgy maths

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Date:   Fri, 04 Nov 2022 16:38:06 +0000
From:   David Howells <dhowells@...hat.com>
To:     willy@...radead.org
Cc:     Jeff Layton <jlayton@...nel.org>, linux-cachefs@...hat.com,
        linux-fsdevel@...r.kernel.org, dhowells@...hat.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] netfs: Fix dodgy maths

Fix the dodgy maths in netfs_rreq_unlock_folios().  start_page could be
inside the folio, in which case the calculation of pgpos will be come up
with a negative number (though for the moment rreq->start is rounded down
earlier and folios would have to get merged whilst locked)

Alter how this works to just frame the tracking in terms of absolute file
positions, rather than offsets from the start of the I/O request.  This
simplifies the maths and makes it easier to follow.

Fix the issue by using folio_pos() and folio_size() to calculate the end
position of the page.

Fixes: 3d3c95046742 ("netfs: Provide readahead and readpage netfs helpers")
Reported-by: Matthew Wilcox <willy@...radead.org>
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Jeff Layton <jlayton@...nel.org>
cc: linux-cachefs@...hat.com
cc: linux-fsdevel@...r.kernel.org
Link: https://lore.kernel.org/r/Y2SJw7w1IsIik3nb@casper.infradead.org/
---

 fs/netfs/buffered_read.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index baf668fb4315..7679a68e8193 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -17,9 +17,9 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 {
 	struct netfs_io_subrequest *subreq;
 	struct folio *folio;
-	unsigned int iopos, account = 0;
 	pgoff_t start_page = rreq->start / PAGE_SIZE;
 	pgoff_t last_page = ((rreq->start + rreq->len) / PAGE_SIZE) - 1;
+	size_t account = 0;
 	bool subreq_failed = false;
 
 	XA_STATE(xas, &rreq->mapping->i_pages, start_page);
@@ -39,23 +39,23 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 	 */
 	subreq = list_first_entry(&rreq->subrequests,
 				  struct netfs_io_subrequest, rreq_link);
-	iopos = 0;
 	subreq_failed = (subreq->error < 0);
 
 	trace_netfs_rreq(rreq, netfs_rreq_trace_unlock);
 
 	rcu_read_lock();
 	xas_for_each(&xas, folio, last_page) {
-		unsigned int pgpos, pgend;
+		loff_t pg_end;
 		bool pg_failed = false;
 
 		if (xas_retry(&xas, folio))
 			continue;
 
-		pgpos = (folio_index(folio) - start_page) * PAGE_SIZE;
-		pgend = pgpos + folio_size(folio);
+		pg_end = folio_pos(folio) + folio_size(folio) - 1;
 
 		for (;;) {
+			loff_t sreq_end;
+
 			if (!subreq) {
 				pg_failed = true;
 				break;
@@ -63,11 +63,11 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 			if (test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags))
 				folio_start_fscache(folio);
 			pg_failed |= subreq_failed;
-			if (pgend < iopos + subreq->len)
+			sreq_end = subreq->start + subreq->len - 1;
+			if (pg_end < sreq_end)
 				break;
 
 			account += subreq->transferred;
-			iopos += subreq->len;
 			if (!list_is_last(&subreq->rreq_link, &rreq->subrequests)) {
 				subreq = list_next_entry(subreq, rreq_link);
 				subreq_failed = (subreq->error < 0);
@@ -75,7 +75,8 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 				subreq = NULL;
 				subreq_failed = false;
 			}
-			if (pgend == iopos)
+
+			if (pg_end == sreq_end)
 				break;
 		}