Re: [PATCH 3/3] [RFC] tmpfs: Add FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILE handlers

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(6/1/12 5:03 PM), John Stultz wrote:
> 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?

I thought to modify shmem_write_page(). But other way is also ok to me.


>>> +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.

Ah, I misunderstood. thanks for correction.


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

No, please just rename.