linux-kernel - Re: [RFC PATCH v2 14/14] dcache: Implement object migration

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Date:   Fri, 5 Apr 2019 07:29:14 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     "Tobin C. Harding" <tobin@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Alexander Viro <viro@....linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Pekka Enberg <penberg@...helsinki.fi>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Waiman Long <longman@...hat.com>,
        Tycho Andersen <tycho@...ho.ws>, Theodore Ts'o <tytso@....edu>,
        Andi Kleen <ak@...ux.intel.com>,
        David Chinner <david@...morbit.com>,
        Nick Piggin <npiggin@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 14/14] dcache: Implement object migration

On Wed, Apr 03, 2019 at 06:48:55PM +0100, Al Viro wrote:
> On Wed, Apr 03, 2019 at 06:19:21PM +0100, Al Viro wrote:
> > On Wed, Apr 03, 2019 at 06:08:11PM +0100, Al Viro wrote:
> > 
> > > Oh, *brilliant*
> > > 
> > > Let's do d_invalidate() on random dentries and hope they go away.
> > > With convoluted and brittle logics for deciding which ones to
> > > spare, which is actually wrong.  This will pick mountpoints
> > > and tear them out, to start with.
> > > 
> > > NAKed-by: Al Viro <viro@...iv.linux.org.uk>
> > > 
> > > And this is a NAK for the entire approach; if it has a positive refcount,
> > > LEAVE IT ALONE.  Period.  Don't play this kind of games, they are wrong.
> > > d_invalidate() is not something that can be done to an arbitrary dentry.
> > 
> > PS: "try to evict what can be evicted out of this set" can be done, but
> > you want something like
> > 	start with empty list
> > 	go through your array of references
> > 		grab dentry->d_lock
> > 		if dentry->d_lockref.count is not zero
> > 			unlock and continue
> > 		if dentry->d_flags & DCACHE_SHRINK_LIST
> > 			ditto, it's not for us to play with
> >                 if (dentry->d_flags & DCACHE_LRU_LIST)
> >                         d_lru_del(dentry);
> > 		d_shrink_add(dentry, &list);
> > 		unlock
> > 
> > on the collection phase and
> > 	if the list is not empty by the end of that loop
> > 		shrink_dentry_list(&list);
> > on the disposal.
> 
> Note, BTW, that your constructor is wrong - all it really needs to do
> is spin_lock_init() and setting ->d_lockref.count same as lockref_mark_dead()
> does, to match the state of dentries being torn down.

Thanks for looking at this Al.

> __d_alloc() is not holding ->d_lock, since the object is not visible to
> anybody else yet; with your changes it *is* visible.

I don't quite understand this comment.  How is the object visible?  The
constructor is only called when allocating a new page to the slab and
this is done with interrupts disabled.

>  However, if the
> assignment to ->d_lockref.count in __d_alloc() is guaranteed to be
> non-zero to non-zero, the above should be safe.

I've done as you suggest and set it to -128

Thanks for schooling me on the VFS stuff.


	Tobin