Re: [PATCH v4 19/19] fs: handle inode->i_version more efficiently

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2018-01-08 at 14:15 -0500, Jeff Layton wrote:
> On Mon, 2018-01-08 at 19:33 +0100, Krzysztof Kozlowski wrote:
> > On Mon, Jan 08, 2018 at 01:00:19PM -0500, Jeff Layton wrote:
> > > On Mon, 2018-01-08 at 18:29 +0100, Krzysztof Kozlowski wrote:
> > 
> > (...)
> > 
> > > > > Ok, thanks. If you're seeing hangs then that might imply that we have
> > > > > some sort of excessive looping going on in the cmpxchg loops.
> > > > > 
> > > > > Could you apply the patch below and let me know if it causes either of
> > > > > the warnings to pop? That might at least point us in the right
> > > > > direction:
> > > > 
> > > > No new warnings with attached patch (except existing already lockdep:
> > > > "INFO: trying to register non-static key.").
> > > > 
> > > 
> > > Yeah, I saw that in the original logs and it looks unrelated (and
> > > harmless).
> > > 
> > > > Systemd timeouts on mounting /home but after entering rescue shell there
> > > > is no problem running mount /home:
> > > > 	Give root password for maintenance
> > > > 	(or press Control-D to continue): 
> > > > 	root@odroidxu3:~# mount /home
> > > > 	[  220.659331] EXT4-fs (mmcblk1p2): mounted filesystem with ordered data mode. Opts: (null)
> > > > 
> > > 
> > > Ok, thanks for testing it. So I guess we can probably rule out excessive
> > > looping in those functions as the issue.
> > > 
> > > To make sure I understand the problem: When systemd tries to do the
> > > initial mount of /home (which is an ext4 filesystem), it hangs. But once
> > > it drops to the shell, it works, if you do the mount by hand.
> > > 
> > > Is that correct?
> > 
> > Yes, although it also timeouts on setting up /dev/ttySAC2 (serial
> > console).
> > 
> > > If so, then is it possible to trigger sysrq commands during the hanging
> > > mount attempt? Maybe you could use e.g. sysrq-l, sysrq-w, etc. to
> > > determine what it's blocking on?
> 
> (trimming the output)
> 
> Thanks. I don't really see anything obvious in that info,
> unfortunately. What we really need to do is find the systemd task
> performing the mount, and see what it's doing.
> 
> We do have one questionable bug in the NFS changes though. Does this
> patch help at all?
> 
> -------------------------------8<---------------------------------
> 
> SQUASH: nfs: fix i_version increment when adding a request
> 
> NFS treats this value as an opaque value with no flag, so we must
> increment it as such instead of using inode_inc_iversion.
> 
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  fs/nfs/write.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index a03fbac1f88c..48837b6250e9 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -755,7 +755,7 @@ static void nfs_inode_add_request(struct inode *inode, struct nfs_page *req)
>  	spin_lock(&mapping->private_lock);
>  	if (!nfs_have_writebacks(inode) &&
>  	    NFS_PROTO(inode)->have_delegation(inode, FMODE_WRITE))
> -		inode_inc_iversion(inode);
> +		atomic64_inc(&inode->i_version);
>  	if (likely(!PageSwapCache(req->wb_page))) {
>  		set_bit(PG_MAPPED, &req->wb_flags);
>  		SetPagePrivate(req->wb_page);

Sorry for the slow dribble of patches. I found a bug in the ext4 code
too. Might want to try this ext4 patch as well:

----------------------8<-----------------

SQUASH: ext4: use raw API for xattr inode refcounts

This could distort the values otherwise.

(Side Q: Why do we do this across the ctime and iversion like this?)

Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/ext4/xattr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index ba6fd5439aa4..63656dbafdc4 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -295,13 +295,13 @@ ext4_xattr_inode_hash(struct ext4_sb_info *sbi, const void *buffer, size_t size)
 static u64 ext4_xattr_inode_get_ref(struct inode *ea_inode)
 {
 	return ((u64)ea_inode->i_ctime.tv_sec << 32) |
-		(u32) inode_peek_iversion(ea_inode);
+		(u32) inode_peek_iversion_raw(ea_inode);
 }
 
 static void ext4_xattr_inode_set_ref(struct inode *ea_inode, u64 ref_count)
 {
 	ea_inode->i_ctime.tv_sec = (u32)(ref_count >> 32);
-	inode_set_iversion(ea_inode, ref_count & 0xffffffff);
+	inode_set_iversion_raw(ea_inode, ref_count & 0xffffffff);
 }
 
 static u32 ext4_xattr_inode_get_hash(struct inode *ea_inode)
-- 
2.14.3