On Sep 18, 2017, at 2:48 PM, Darrick J. Wong [off-list ref] wrote:

On Mon, Sep 18, 2017 at 05:52:24PM +0200, Lukas Czerner wrote:

quoted

Return all fallcoate modes supported by xfs file system.

Cc: linux-xfs@vger.kernel.org
Signed-off-by: Lukas Czerner <redacted>
---
fs/xfs/xfs_file.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index ebdd0bd..85e06c6 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c

@@ -746,7 +746,8 @@ xfs_file_write_iter(

#define	XFS_FALLOC_FL_SUPPORTED						\
		(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |		\
		 FALLOC_FL_COLLAPSE_RANGE | FALLOC_FL_ZERO_RANGE |	\
-		 FALLOC_FL_INSERT_RANGE | FALLOC_FL_UNSHARE_RANGE)
+		 FALLOC_FL_INSERT_RANGE | FALLOC_FL_UNSHARE_RANGE |	\
+		 FALLOC_FL_QUERY_SUPPORT | FALLOC_FL_PREALLOC_RANGE)

STATIC long
xfs_file_fallocate(

@@ -768,6 +769,9 @@ xfs_file_fallocate(

	if (mode & ~XFS_FALLOC_FL_SUPPORTED)
		return -EOPNOTSUPP;

+	if (mode & FALLOC_FL_QUERY_SUPPORT)
+		return XFS_FALLOC_FL_SUPPORTED;

Urk, manpage update describing the goals of the query interface and how
this is supposed to work is needed.

Are we required to return only the mode flags that would reasonably be
expected to work on this file, or the fs in general?  Do we return zero
if the file is immutable (I guess the fd has to be O_RDONLY?) or if the
fs is readonly?

And like hch said, why not {f,}pathconf?

The problem with pathconf() is that this is just made up by GlibC, and
doesn't actually communicate with the kernel or the filesystem at all.
For example, _PC_LINK_MAX does not return different values for ext2/ext4
filesystems based on feature flags, only what is hard-coded into GlibC
based on the filesystem magic.  This is not going to work as a per-file
source of information.

For example, even though EXT4_LINK_MAX has been 65000 for many years,
GlibC reports 32000 for an ext4 filesystem since it only uses the magic:

$ df -T /
Filesystem     Type 1024-blocks     Used Available Capacity Mounted on
/dev/mapper/vg_twoshoes-lvroot
               ext4    25991484 22475376   2205504      92% /
$ strace getconf LINK_MAX /
:
:
statfs("/", {f_type="EXT2_SUPER_MAGIC", f_bsize=4096, f_blocks=6497871,
	     f_bfree=879030, f_bavail=551379, f_files=819200, f_ffree=405130,
	     f_fsid={1950084367, 1019861045}, f_namelen=255}) = 0
write(1, "32000\n",) = 6


Cheers, Andreas