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Date:   Tue, 21 May 2019 00:57:47 +0000
From:   Roman Gushchin <guro@...com>
To:     "Tobin C. Harding" <tobin@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        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>,
        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-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v5 16/16] dcache: Add CONFIG_DCACHE_SMO

On Mon, May 20, 2019 at 03:40:17PM +1000, Tobin C. Harding wrote:
> In an attempt to make the SMO patchset as non-invasive as possible add a
> config option CONFIG_DCACHE_SMO (under "Memory Management options") for
> enabling SMO for the DCACHE.  Whithout this option dcache constructor is
> used but no other code is built in, with this option enabled slab
> mobility is enabled and the isolate/migrate functions are built in.
> 
> Add CONFIG_DCACHE_SMO to guard the partial shrinking of the dcache via
> Slab Movable Objects infrastructure.

Hm, isn't it better to make it a static branch? Or basically anything
that allows switching on the fly?

It seems that the cost of just building it in shouldn't be that high.
And the question if the defragmentation worth the trouble is so much
easier to answer if it's possible to turn it on and off without rebooting.

Thanks!

> 
> Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> ---
>  fs/dcache.c | 4 ++++
>  mm/Kconfig  | 7 +++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 0dfe580c2d42..96063e872366 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -3072,6 +3072,7 @@ void d_tmpfile(struct dentry *dentry, struct inode *inode)
>  }
>  EXPORT_SYMBOL(d_tmpfile);
>  
> +#ifdef CONFIG_DCACHE_SMO
>  /*
>   * d_isolate() - Dentry isolation callback function.
>   * @s: The dentry cache.
> @@ -3144,6 +3145,7 @@ static void d_partial_shrink(struct kmem_cache *s, void **_unused, int __unused,
>  
>  	kfree(private);
>  }
> +#endif	/* CONFIG_DCACHE_SMO */
>  
>  static __initdata unsigned long dhash_entries;
>  static int __init set_dhash_entries(char *str)
> @@ -3190,7 +3192,9 @@ static void __init dcache_init(void)
>  					   sizeof_field(struct dentry, d_iname),
>  					   dcache_ctor);
>  
> +#ifdef CONFIG_DCACHE_SMO
>  	kmem_cache_setup_mobility(dentry_cache, d_isolate, d_partial_shrink);
> +#endif
>  
>  	/* Hash may have been set up in dcache_init_early */
>  	if (!hashdist)
> diff --git a/mm/Kconfig b/mm/Kconfig
> index aa8d60e69a01..7dcea76e5ecc 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -265,6 +265,13 @@ config SMO_NODE
>         help
>           On NUMA systems enable moving objects to and from a specified node.
>  
> +config DCACHE_SMO
> +       bool "Enable Slab Movable Objects for the dcache"
> +       depends on SLUB
> +       help
> +         Under memory pressure we can try to free dentry slab cache objects from
> +         the partial slab list if this is enabled.
> +
>  config PHYS_ADDR_T_64BIT
>  	def_bool 64BIT
>  
> -- 
> 2.21.0
> 

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