linux-kernel - [PATCH] dax: Allow block size > PAGE

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Message-Id: <20241101-dax-page-size-v1-1-eedbd0c6b08f@asahilina.net>
Date: Fri, 01 Nov 2024 21:22:31 +0900
From: Asahi Lina <lina@...hilina.net>
To: Dan Williams <dan.j.williams@...el.com>, 
 Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
 Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>
Cc: Sergio Lopez Pascual <slp@...hat.com>, linux-fsdevel@...r.kernel.org, 
 nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org, asahi@...ts.linux.dev, 
 Asahi Lina <lina@...hilina.net>
Subject: [PATCH] dax: Allow block size > PAGE_SIZE

For virtio-dax, the file/FS blocksize is irrelevant. FUSE always uses
large DAX blocks (2MiB), which will work with all host page sizes. Since
we are mapping files into the DAX window on the host, the underlying
block size of the filesystem and its block device (if any) are
meaningless.

For real devices with DAX, the only requirement should be that the FS
block size is *at least* as large as PAGE_SIZE, to ensure that at least
whole pages can be mapped out of the device contiguously.

Fixes warning when using virtio-dax on a 4K guest with a 16K host,
backed by tmpfs (which sets blksz == PAGE_SIZE on the host).

Signed-off-by: Asahi Lina <lina@...hilina.net>
---
 fs/dax.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index c62acd2812f8d4981aaba82acfeaf972f555362a..406fb75bdbe9d17a6e4bf3d4cb92683e90f05910 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1032,7 +1032,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
 	int ret = 0;
 	unsigned int scanned = 0;

-	if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
+	if (WARN_ON_ONCE(inode->i_blkbits < PAGE_SHIFT))
 		return -EIO;

 	if (mapping_empty(mapping) || wbc->sync_mode != WB_SYNC_ALL)

---
base-commit: 81983758430957d9a5cb3333fe324fd70cf63e7e
change-id: 20241101-dax-page-size-83a1073b4e1b

Cheers,
~~ Lina