btrfs-progs: mkfs: make 4k sectorsize default

We have had working subpage support in Btrfs for many cycles now.
Generally, we do not want people creating filesystems by default
with non-4k sectorsizes since it creates portability problems.

As the subpage has stabilized it seems to be safe to do the switch.
This may still affect users that relying on the previous behaviour.

Issue: #604
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Signed-off-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/Subpage.rst

@@ -9,18 +9,19 @@ to the exactly same size of the block and page. On x86_64 this is typically
 pages, like 64KiB on 64bit ARM or PowerPC. This means filesystems created
 with 64KiB sector size cannot be mounted on a system with 4KiB page size.
 
-While with subpage support systems with 64KiB page size can create
-and mount filesystems with 4KiB sectorsize.  This still needs to use option "-s
-4k" option for :command:`mkfs.btrfs`.
+Since v6.7, filesystems are created with a 4KiB sectorsize by default,
+though it remains possible to create filesystems with other page sizes
+(such as 64KiB with the "-s 64k" option for :command:`mkfs.btrfs`). This
+ensures that new filesystems are compatible across other architecture
+variants using larger page sizes.
 
 Requirements, limitations
 -------------------------
 
-The initial subpage support has been added in v5.15, although it's still
-considered as experimental, most features are already working without problems.
-On a 64KiB page system filesystem with 4KiB sectorsize can be mounted and used
-as usual as long as the initial mount succeeds. There are cases a mount will be
-rejected when verifying compatible features.
+The initial subpage support has been added in v5.15. Most features are
+already working without problems. On a 64KiB page system, a filesystem with
+4KiB sectorsize can be mounted and used as long as the initial mount succeeds.
+Subpage support is used by default for systems with a non-4KiB page size since v6.7.
 
 Please refer to status page of :ref:`status-subpage-block-size` for
 compatibility.

Documentation/mkfs.btrfs.rst

@@ -122,10 +122,15 @@ OPTIONS
 -s|--sectorsize <size>
         Specify the sectorsize, the minimum data block allocation unit.
 
-        The default value is the page size and is autodetected. If the sectorsize
-        differs from the page size, the created filesystem may not be mountable by the
-        running kernel. Therefore it is not recommended to use this option unless you
-        are going to mount it on a system with the appropriate page size.
+        By default, the value is 4KiB, but it can be manually set to match the
+        system page size. However, if the sector size is different from the page
+        size, the resulting filesystem may not be mountable by the current
+        kernel, apart from the default 4KiB. Hence, using this option is not
+        advised unless you intend to mount it on a system with the suitable
+        page size.
+
+        .. note::
+                Versions prior to 6.7 set the sectorsize matching to the page size.
 
 -L|--label <string>
         Specify a label for the filesystem. The *string* should be less than 256

mkfs/main.c

@@ -1384,7 +1384,7 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 	}
 
 	if (!sectorsize)
-		sectorsize = (u32)sysconf(_SC_PAGESIZE);
+		sectorsize = (u32)SZ_4K;
 	if (btrfs_check_sectorsize(sectorsize))
 		goto error;