Re: [PATCH v3 1/7] iversion: update comments with info about atime updates
From: Dave Chinner <david@fromorbit.com>
Date: 2022-08-29 07:57:49
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linux-btrfs, linux-ext4, linux-fsdevel, linux-nfs, linux-xfs, lkml
On Fri, Aug 26, 2022 at 05:46:57PM -0400, Jeff Layton wrote:
quoted hunk ↗ jump to hunk
The i_version field in the kernel has had different semantics over the decades, but we're now proposing to expose it to userland via statx. This means that we need a clear, consistent definition of what it means and when it should change. Update the comments in iversion.h to describe how a conformant i_version implementation is expected to behave. This definition suits the current users of i_version (NFSv4 and IMA), but is loose enough to allow for a wide range of possible implementations. Cc: Colin Walters <redacted> Cc: NeilBrown <redacted> Cc: Trond Myklebust <redacted> Cc: Dave Chinner <david@fromorbit.com> Link: https://lore.kernel.org/linux-xfs/166086932784.5425.17134712694961326033@noble.neil.brown.name/#t (local) Signed-off-by: Jeff Layton <jlayton@kernel.org> --- include/linux/iversion.h | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)diff --git a/include/linux/iversion.h b/include/linux/iversion.h index 3bfebde5a1a6..45e93e1b4edc 100644 --- a/include/linux/iversion.h +++ b/include/linux/iversion.h@@ -9,8 +9,19 @@ * --------------------------- * The change attribute (i_version) is mandated by NFSv4 and is mostly for * knfsd, but is also used for other purposes (e.g. IMA). The i_version must - * appear different to observers if there was a change to the inode's data or - * metadata since it was last queried. + * appear different to observers if there was an explicit change to the inode's + * data or metadata since it was last queried. + * + * An explicit change is one that would ordinarily result in a change to the + * inode status change time (aka ctime). The version must appear to change, even + * if the ctime does not (since the whole point is to avoid missing updates due + * to timestamp granularity). If POSIX mandates that the ctime must change due + * to an operation, then the i_version counter must be incremented as well. + * + * A conformant implementation is allowed to increment the counter in other + * cases, but this is not optimal. NFSv4 and IMA both use this value to determine + * whether caches are up to date. Spurious increments can cause false cache + * invalidations.
"not optimal", but never-the-less allowed - that's "unspecified behaviour" if I've ever seen it. How is userspace supposed to know/deal with this? Indeed, this loophole clause doesn't exist in the man pages that define what statx.stx_ino_version means. The man pages explicitly define that stx_ino_version only ever changes when stx_ctime changes. IOWs, the behaviour userspace developers are going to expect *does not include* stx_ino_version changing it more often than ctime is changed. Hence a kernel iversion implementation that bumps the counter more often than ctime changes *is not conformant with the statx version counter specification*. IOWs, we can't export such behaviour to userspace *ever* - it is a non-conformant implementation. Hence I think anything that bumps iversion outside the bounds of the statx definition should be declared as such: "Non-conformant iversion implementations: - MUST NOT be exported by statx() to userspace - MUST be -tolerated- by kernel internal applications that use iversion for their own purposes." Cheers, Dave. -- Dave Chinner david@fromorbit.com