Re: [PATCH V7 7/9] fs: Define I_DONTCACNE in VFS layer

[Ira Weiny]

On Tue, Apr 14, 2020 at 05:26:30PM +0200, Jan Kara wrote:
> On Sun 12-04-20 22:40:44, ira.weiny@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > DAX effective mode changes (setting of S_DAX) require inode eviction.
> > 
> > Define a flag which can be set to inform the VFS layer that inodes
> > should not be cached.  This will expedite the eviction of those nodes
> > requiring reload.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> This inode flag will have a limited impact because usually dentry will
> still hold inode reference. So until dentry is evicted, inode stays as
> well.

Agreed but at least this keeps the inode from being cached until that time.

FWIW the ext4 patches seem to have a much longer delay when issuing drop_caches
and I'm not 100% sure why.  I've sent out those patches RFC to get the
discussions started.  I feel like I have missed something there but it does
eventually flip the S_DAX flag.

> So I think we'd need something like DCACHE_DONTCACHE flag as well to
> discard a dentry whenever dentry usecount hits zero (which will be
> generally on last file close). What do you think?

I wanted to do something like this but I was not sure how to trigger the
DCACHE_DONTCACHE on the correct 'parent' dentry.  Can't their be multiple
dentries pointing to the same inode?

In which case, would you need to flag them all?

Ira

> 
> And I'd note that checking for I_DONTCACHE flag in dput() isn't
> straightforward because of locking so that's why I suggest separate dentry
> flag.
> 
> 								Honza
> 
> > ---
> >  include/linux/fs.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index a818ced22961..e2db71d150c3 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -2151,6 +2151,8 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
> >   *
> >   * I_CREATING		New object's inode in the middle of setting up.
> >   *
> > + * I_DONTCACHE		Do not cache the inode
> > + *
> >   * Q: What is the difference between I_WILL_FREE and I_FREEING?
> >   */
> >  #define I_DIRTY_SYNC		(1 << 0)
> > @@ -2173,6 +2175,7 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
> >  #define I_WB_SWITCH		(1 << 13)
> >  #define I_OVL_INUSE		(1 << 14)
> >  #define I_CREATING		(1 << 15)
> > +#define I_DONTCACHE		(1 << 16)
> >  
> >  #define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
> >  #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)
> > @@ -3042,7 +3045,8 @@ extern int inode_needs_sync(struct inode *inode);
> >  extern int generic_delete_inode(struct inode *inode);
> >  static inline int generic_drop_inode(struct inode *inode)
> >  {
> > -	return !inode->i_nlink || inode_unhashed(inode);
> > +	return !inode->i_nlink || inode_unhashed(inode) ||
> > +		(inode->i_state & I_DONTCACHE);
> >  }
> >  
> >  extern struct inode *ilookup5_nowait(struct super_block *sb,
> > -- 
> > 2.25.1
> > 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR