Re: [PATCH 3/3] [RFC] tmpfs: Add FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILEhandlers

[KOSAKI Motohiro]

Hi John,

(6/1/12 2:29 PM), John Stultz wrote:
> This patch enables FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILE
> functionality for tmpfs making use of the volatile range
> management code.
> 
> Conceptually, FALLOC_FL_MARK_VOLATILE is like a delayed
> FALLOC_FL_PUNCH_HOLE.  This allows applications that have
> data caches that can be re-created to tell the kernel that
> some memory contains data that is useful in the future, but
> can be recreated if needed, so if the kernel needs, it can
> zap the memory without having to swap it out.
> 
> In use, applications use FALLOC_FL_MARK_VOLATILE to mark
> page ranges as volatile when they are not in use. Then later
> if they wants to reuse the data, they use
> FALLOC_FL_UNMARK_VOLATILE, which will return an error if the
> data has been purged.
> 
> This is very much influenced by the Android Ashmem interface by
> Robert Love so credits to him and the Android developers.
> In many cases the code&  logic come directly from the ashmem patch.
> The intent of this patch is to allow for ashmem-like behavior, but
> embeds the idea a little deeper into the VM code.
> 
> This is a reworked version of the fadvise volatile idea submitted
> earlier to the list. Thanks to Dave Chinner for suggesting to
> rework the idea in this fashion. Also thanks to Dmitry Adamushko
> for continued review and bug reporting, and Dave Hansen for
> help with the original design and mentoring me in the VM code.

I like this patch concept. This is cleaner than userland 
notification quirk. But I don't like you use shrinker. Because of,
after applying this patch, normal page reclaim path can still make
swap out. this is undesirable.


(snip)

> +static
> +int shmem_volatile_shrink(struct shrinker *ignored, struct shrink_control *sc)
> +{
> +	s64 nr_to_scan = sc->nr_to_scan;
> +	const gfp_t gfp_mask = sc->gfp_mask;
> +	struct address_space *mapping;
> +	loff_t start, end;
> +	int ret;
> +	s64 page_count;
> +
> +	if (nr_to_scan&&  !(gfp_mask&  __GFP_FS))
> +		return -1;
> +
> +	volatile_range_lock(&shmem_volatile_head);
> +	page_count = volatile_range_lru_size(&shmem_volatile_head);
> +	if (!nr_to_scan)
> +		goto out;
> +
> +	do {
> +		ret = volatile_ranges_get_last_used(&shmem_volatile_head,
> +							&mapping,&start,&end);

Why drop last used region? Not recently used region is better?
 


> +		if (ret) {
> +			shmem_truncate_range(mapping->host,
> +						start<<PAGE_CACHE_SHIFT,
> +						(end<<PAGE_CACHE_SHIFT)-1);
> +			nr_to_scan -= end-start;
> +			page_count -= end-start;
> +		};
> +	} while (ret&&  (nr_to_scan>  0));
> +
> +out:
> +	volatile_range_unlock(&shmem_volatile_head);
> +
> +	return page_count;
> +}
> +


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