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    SubjectRe: [PATCH 3/3] [RFC] tmpfs: Add FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILE handlers
    On 06/01/2012 01:17 PM, KOSAKI Motohiro wrote:
    > Hi John,
    > (6/1/12 2:29 PM), John Stultz wrote:
    >> 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

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


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