From: Joanne Koong <joannelkoong@gmail.com>
Date: Tue, 11 Nov 2025 19:36:55 +0000 (-0800)
Subject: iomap: simplify when reads can be skipped for writes
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=a298febc47e0ce116b9fc8151337ba8b2137e42d;p=thirdparty%2Fkernel%2Flinux.git

iomap: simplify when reads can be skipped for writes

Currently, the logic for skipping the read range for a write is

if (!(iter->flags & IOMAP_UNSHARE) &&
    (from <= poff || from >= poff + plen) &&
    (to <= poff || to >= poff + plen))

which breaks down to skipping the read if any of these are true:
a) from <= poff && to <= poff
b) from <= poff && to >= poff + plen
c) from >= poff + plen && to <= poff
d) from >= poff + plen && to >= poff + plen

This can be simplified to
if (!(iter->flags & IOMAP_UNSHARE) && from <= poff && to >= poff + plen)

from the following reasoning:

a) from <= poff && to <= poff
This reduces to 'to <= poff' since it is guaranteed that 'from <= to'
(since to = from + len). It is not possible for 'from <= to' to be true
here because we only reach here if plen > 0 (thanks to the preceding 'if
(plen == 0)' check that would break us out of the loop). If 'to <=
poff', plen would have to be 0 since poff and plen get adjusted in
lockstep for uptodate blocks. This means we can eliminate this check.

c) from >= poff + plen && to <= poff
This is not possible since 'from <= to' and 'plen > 0'. We can eliminate
this check.

d) from >= poff + plen && to >= poff + plen
This reduces to 'from >= poff + plen' since 'from <= to'.
It is not possible for 'from >= poff + plen' to be true here. We only
reach here if plen > 0 and for writes, poff and plen will always be
block-aligned, which means poff <= from < poff + plen. We can eliminate
this check.

The only valid check is b) from <= poff && to >= poff + plen.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20251111193658.3495942-7-joannelkoong@gmail.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index c82b5b24d4b3f..17449ea134209 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -758,9 +758,12 @@ static int __iomap_write_begin(const struct iomap_iter *iter,
 		if (plen == 0)
 			break;
 
-		if (!(iter->flags & IOMAP_UNSHARE) &&
-		    (from <= poff || from >= poff + plen) &&
-		    (to <= poff || to >= poff + plen))
+		/*
+		 * If the read range will be entirely overwritten by the write,
+		 * we can skip having to zero/read it in.
+		 */
+		if (!(iter->flags & IOMAP_UNSHARE) && from <= poff &&
+		    to >= poff + plen)
 			continue;
 
 		if (iomap_block_needs_zeroing(iter, block_start)) {