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

[John Stultz]

On 06/01/2012 01:17 PM, KOSAKI Motohiro wrote:
> 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.
Any recommendations for alternative approaches? What should I be hooking
into in order to get notified that tmpfs should drop volatile pages?


>> +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?
>
Sorry, that function name isn't very good. It does return the
least-recently-used range, or more specifically: the
least-recently-marked-volatile-range.

I'll improve that function name, but if I misunderstood you and you have
a different suggestion for the purging order, let me know.

thanks
-john


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