linux-kernel - [PATCH 2/2] btrfs: replace 64-bit division with a shift

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Message-Id: <20250225194416.3076650-2-arnd@kernel.org>
Date: Tue, 25 Feb 2025 20:44:11 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: Chris Mason <clm@...com>,
	Josef Bacik <josef@...icpanda.com>,
	David Sterba <dsterba@...e.com>,
	Qu Wenruo <wqu@...e.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Johannes Thumshirn <johannes.thumshirn@....com>,
	Anand Jain <anand.jain@...cle.com>,
	Filipe Manana <fdmanana@...e.com>,
	Li Zetao <lizetao1@...wei.com>,
	linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] btrfs: replace 64-bit division with a shift

From: Arnd Bergmann <arnd@...db.de>

folio_size() is not a compile-time constant, so in some configurations,
the DIV_ROUND_UP() turns into a 64-bit division that is not allowed
on 32-bit architectures.

x86_64-linux-ld: fs/btrfs/extent_io.o: in function `writepage_delalloc':
extent_io.c:(.text+0x2b6e): undefined reference to `__udivdi3'

This is probably not the correct solution, but it should illustrate
the issue. A trivial fix would be DIV64_U64_ROUND_UP(), which of course
is very expensive.  Maybe there should be a DIV_ROUND_UP_FOLIO macro?

Fixes: aba063bf9336 ("btrfs: prepare extent_io.c for future larger folio support")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 fs/btrfs/extent_io.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7dc996e7e249..e4ba4fa3f48c 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1505,8 +1505,7 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
 	 * delalloc_end is already one less than the total length, so
 	 * we don't subtract one from folio_size().
 	 */
-	delalloc_to_write +=
-		DIV_ROUND_UP(delalloc_end + 1 - page_start, folio_size(folio));
+	delalloc_to_write += (delalloc_end + 1 - page_start + folio_size(folio) - 1) >> folio_order(folio);
 
 	/*
 	 * If all ranges are submitted asynchronously, we just need to account
-- 
2.39.5