[PATCH v3] btrfs-progs: mkfs: Enforce 4k sectorsize by default

[Neal Gompa <neal@gompa.dev>]

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.

Signed-off-by: Neal Gompa <neal@gompa.dev>

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
 Documentation/Subpage.rst    | 17 +++++++++--------
 Documentation/mkfs.btrfs.rst | 13 +++++++++----
 mkfs/main.c                  |  2 +-
 3 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/Documentation/Subpage.rst b/Documentation/Subpage.rst
index c762b6a3..1655ae7e 100644
--- a/Documentation/Subpage.rst
+++ b/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.
diff --git a/Documentation/mkfs.btrfs.rst b/Documentation/mkfs.btrfs.rst
index 7e23b9f6..be4f49cb 100644
--- a/Documentation/mkfs.btrfs.rst
+++ b/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
diff --git a/mkfs/main.c b/mkfs/main.c
index d984c995..0570c8f8 100644
--- a/mkfs/main.c
+++ b/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;
 
-- 
2.41.0