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

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Message-ID: <20190403174855.GT2217@ZenIV.linux.org.uk>
Date:   Wed, 3 Apr 2019 18:48:55 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Tobin C. Harding" <tobin@...nel.org>
Cc:     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 06:19:21PM +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.

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