linux-kernel - [PATCH 09/10] afs: Alter dirty range encoding in page->private

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Date:   Tue, 27 Oct 2020 13:51:01 +0000
From:   David Howells <dhowells@...hat.com>
To:     linux-afs@...ts.infradead.org
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 09/10] afs: Alter dirty range encoding in page->private

Currently, page->private on an afs page is used to store the range of
dirtied data within the page, where the range includes the lower bound, but
excludes the upper bound (e.g. 0-1 is a range covering a single byte).

This, however, requires a superfluous bit for the last-byte bound so that
on a 4KiB page, it can say 0-4096 to indicate the whole page, the idea
being that having both numbers the same would indicate an empty range.
This is unnecessary as the PG_private bit is clear if it's an empty range
(as is PG_dirty).

Alter the way the dirty range is encoded in page->private such that the
upper bound is reduced by 1 (e.g. 0-0 is then specified the same single
byte range mentioned above).

Applying this to both bounds frees up two bits, one of which can be used in
a future commit.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/afs/internal.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index acefa6bf0a4d..0ff1088a7c87 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -878,12 +878,12 @@ static inline unsigned int afs_page_dirty_from(unsigned long priv)

 static inline unsigned int afs_page_dirty_to(unsigned long priv)
 {
-	return (priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK;
+	return ((priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK) + 1;
 }

 static inline unsigned long afs_page_dirty(unsigned int from, unsigned int to)
 {
-	return ((unsigned long)to << __AFS_PAGE_PRIV_SHIFT) | from;
+	return ((unsigned long)(to - 1) << __AFS_PAGE_PRIV_SHIFT) | from;
 }

 #include <trace/events/afs.h>