linux-kernel - Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab Movable Objects

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Message-ID: <20190411023322.GD2217@ZenIV.linux.org.uk>
Date:   Thu, 11 Apr 2019 03:33:22 +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>,
        Jonathan Corbet <corbet@....net>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 14/15] dcache: Implement partial shrink via Slab
 Movable Objects

On Thu, Apr 11, 2019 at 11:34:40AM +1000, Tobin C. Harding wrote:
> +/*
> + * d_isolate() - Dentry isolation callback function.
> + * @s: The dentry cache.
> + * @v: Vector of pointers to the objects to isolate.
> + * @nr: Number of objects in @v.
> + *
> + * The slab allocator is holding off frees. We can safely examine
> + * the object without the danger of it vanishing from under us.
> + */
> +static void *d_isolate(struct kmem_cache *s, void **v, int nr)
> +{
> +	struct dentry *dentry;
> +	int i;
> +
> +	for (i = 0; i < nr; i++) {
> +		dentry = v[i];
> +		__dget(dentry);
> +	}
> +
> +	return NULL;		/* No need for private data */
> +}

Huh?  This is compeletely wrong; what you need is collecting the ones
with zero refcount (and not on shrink lists) into a private list.
*NOT* bumping the refcounts at all.  And do it in your isolate thing.

> +static void d_partial_shrink(struct kmem_cache *s, void **v, int nr,
> +		      int node, void *_unused)
> +{
> +	struct dentry *dentry;
> +	LIST_HEAD(dispose);
> +	int i;
> +
> +	for (i = 0; i < nr; i++) {
> +		dentry = v[i];
> +		spin_lock(&dentry->d_lock);
> +		dentry->d_lockref.count--;
> +
> +		if (dentry->d_lockref.count > 0 ||
> +		    dentry->d_flags & DCACHE_SHRINK_LIST) {
> +			spin_unlock(&dentry->d_lock);
> +			continue;
> +		}
> +
> +		if (dentry->d_flags & DCACHE_LRU_LIST)
> +			d_lru_del(dentry);
> +
> +		d_shrink_add(dentry, &dispose);
> +
> +		spin_unlock(&dentry->d_lock);
> +	}

Basically, that loop (sans jerking the refcount up and down) should
get moved into d_isolate().
> +
> +	if (!list_empty(&dispose))
> +		shrink_dentry_list(&dispose);
> +}

... with this left in d_partial_shrink().  And you obviously need some way
to pass the list from the former to the latter...