On Fri, Nov 13, 2020 at 09:50:50AM -0500, bfields wrote:
> On Thu, Nov 12, 2020 at 11:05:57PM +0000, Daire Byrne wrote:
> > So, I can't lay claim to identifying the exact optimisation/hack that
> > improves the retention of the re-export server's client cache when
> > re-exporting an NFSv3 server (which is then read by many clients). We
> > were working with an engineer at the time who showed an interest in
> > our use case and after we supplied a reproducer he suggested modifying
> > the nfs/inode.c
> >
> > - if (!inode_eq_iversion_raw(inode, fattr->change_attr)) {
> > + if (inode_peek_iversion_raw(inode) < fattr->change_attr)
> > {
> >
> > His reasoning at the time was:
> >
> > "Fixes inode invalidation caused by read access. The least important
> > bit is ORed with 1 and causes the inode version to differ from the one
> > seen on the NFS share. This in turn causes unnecessary re-download
> > impacting the performance significantly. This fix makes it only
> > re-fetch file content if inode version seen on the server is newer
> > than the one on the client."
> >
> > But I've always been puzzled by why this only seems to be the case
> > when using knfsd to re-export the (NFSv3) client mount. Using multiple
> > processes on a standard client mount never causes any similar
> > re-validations. And this happens with a completely read-only share
> > which is why I started to think it has something to do with atimes as
> > that could perhaps still cause a "write" modification even when
> > read-only?
>
> Ah-hah! So, it's inode_query_iversion() that's modifying a nfs inode's
> i_version. That's a special thing that only nfsd would do.
>
> I think that's totally fixable, we'll just have to think a little about
> how....
I wonder if something like this helps?--b.
commit 0add88a9ccc5
Author: J. Bruce Fields <bfields@xxxxxxxxxx>
Date: Fri Nov 13 17:03:04 2020 -0500
nfs: don't mangle i_version on NFS
The i_version on NFS has pretty much opaque to the client, so we don't
want to give the low bit any special interpretation.
Define a new FS_PRIVATE_I_VERSION flag for filesystems that manage the
i_version on their own.
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
index 29ec8b09a52d..9b8dd5b713a7 100644
--- a/fs/nfs/fs_context.c
+++ b/fs/nfs/fs_context.c
@@ -1488,7 +1488,8 @@ struct file_system_type nfs_fs_type = {
.init_fs_context = nfs_init_fs_context,
.parameters = nfs_fs_parameters,
.kill_sb = nfs_kill_super,
- .fs_flags = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
+ .fs_flags = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
+ FS_PRIVATE_I_VERSION,
};
MODULE_ALIAS_FS("nfs");
EXPORT_SYMBOL_GPL(nfs_fs_type);
@@ -1500,7 +1501,8 @@ struct file_system_type nfs4_fs_type = {
.init_fs_context = nfs_init_fs_context,
.parameters = nfs_fs_parameters,
.kill_sb = nfs_kill_super,
- .fs_flags = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
+ .fs_flags = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
+ FS_PRIVATE_I_VERSION,
};
MODULE_ALIAS_FS("nfs4");
MODULE_ALIAS("nfs4");
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 21cc971fd960..c5bb4268228b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2217,6 +2217,7 @@ struct file_system_type {
#define FS_HAS_SUBTYPE 4
#define FS_USERNS_MOUNT 8 /* Can be mounted by userns root */
#define FS_DISALLOW_NOTIFY_PERM 16 /* Disable fanotify permission events */
+#define FS_PRIVATE_I_VERSION 32 /* i_version managed by filesystem */
#define FS_THP_SUPPORT 8192 /* Remove once all fs converted */
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */
int (*init_fs_context)(struct fs_context *);
diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index 2917ef990d43..52c790a847de 100644
--- a/include/linux/iversion.h
+++ b/include/linux/iversion.h
@@ -307,6 +307,8 @@ inode_query_iversion(struct inode *inode)
u64 cur, old, new;
cur = inode_peek_iversion_raw(inode);
+ if (inode->i_sb->s_type->fs_flags & FS_PRIVATE_I_VERSION)
+ return cur;
for (;;) {
/* If flag is already set, then no need to swap */
if (cur & I_VERSION_QUERIED) {
