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    SubjectRe: [PATCH 2/4] send callback when swap slot is freed
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    On Fri, 2009-09-18 at 04:13 +0530, Nitin Gupta wrote:
    > Currently, we have "swap discard" mechanism which sends a discard bio request
    > when we find a free cluster during scan_swap_map(). This callback can come a
    > long time after swap slots are actually freed.
    >
    > This delay in callback is a great problem when (compressed) RAM [1] is used
    > as a swap device. So, this change adds a callback which is called as
    > soon as a swap slot becomes free. For above mentioned case of swapping
    > over compressed RAM device, this is very useful since we can immediately
    > free memory allocated for this swap page.
    >
    > This callback does not replace swap discard support. It is called with
    > swap_lock held, so it is meant to trigger action that finishes quickly.
    > However, swap discard is an I/O request and can be used for taking longer
    > actions.
    >
    > It is preferred to use this callback for ramzswap case even if discard
    > mechanism could be improved such that it can be called as often as required.
    > This is because, allocation of 'bio'(s) is undesirable since ramzswap always
    > operates under low memory conditions (its a swap device). Also, batching of
    > discard bio requests is not optimal since stale data can accumulate very
    > quickly in ramzswap devices, pushing system further into low memory state.
    >
    > Links:
    > [1] http://compcache.googlecode.com/
    >
    > Signed-off-by: Nitin Gupta <ngupta@vflare.org>
    >
    > ---
    > include/linux/swap.h | 5 +++++
    > mm/swapfile.c | 34 ++++++++++++++++++++++++++++++++++
    > 2 files changed, 39 insertions(+), 0 deletions(-)
    >
    > diff --git a/include/linux/swap.h b/include/linux/swap.h
    > index 7c15334..64796fc 100644
    > --- a/include/linux/swap.h
    > +++ b/include/linux/swap.h
    > @@ -8,6 +8,7 @@
    > #include <linux/memcontrol.h>
    > #include <linux/sched.h>
    > #include <linux/node.h>
    > +#include <linux/blkdev.h>
    >
    > #include <asm/atomic.h>
    > #include <asm/page.h>
    > @@ -20,6 +21,8 @@ struct bio;
    > #define SWAP_FLAG_PRIO_MASK 0x7fff
    > #define SWAP_FLAG_PRIO_SHIFT 0
    >
    > +typedef void (swap_free_notify_fn) (struct block_device *, unsigned long);
    > +
    > static inline int current_is_kswapd(void)
    > {
    > return current->flags & PF_KSWAPD;
    > @@ -155,6 +158,7 @@ struct swap_info_struct {
    > unsigned int max;
    > unsigned int inuse_pages;
    > unsigned int old_block_size;
    > + swap_free_notify_fn *swap_free_notify_fn;
    > };
    >
    > struct swap_list_t {
    > @@ -295,6 +299,7 @@ extern sector_t swapdev_block(int, pgoff_t);
    > extern struct swap_info_struct *get_swap_info_struct(unsigned);
    > extern int reuse_swap_page(struct page *);
    > extern int try_to_free_swap(struct page *);
    > +extern void set_swap_free_notify(struct block_device *, swap_free_notify_fn *);
    > struct backing_dev_info;
    >
    > /* linux/mm/thrash.c */
    > diff --git a/mm/swapfile.c b/mm/swapfile.c
    > index 74f1102..b165db0 100644
    > --- a/mm/swapfile.c
    > +++ b/mm/swapfile.c
    > @@ -554,6 +554,38 @@ out:
    > return NULL;
    > }
    >
    > +/*
    > + * Sets callback for event when swap_map[offset] == 0
    > + * i.e. page at this swap offset is no longer used.
    > + */
    > +void set_swap_free_notify(struct block_device *bdev,
    > + swap_free_notify_fn *notify_fn)
    > +{
    > + unsigned int i;
    > + struct swap_info_struct *sis;
    > +
    > + spin_lock(&swap_lock);
    > + for (i = 0; i <= nr_swapfiles; i++) {
    > + sis = &swap_info[i];
    > + if (!(sis->flags & SWP_USED))
    > + continue;
    > + if (sis->bdev == bdev)
    > + break;
    > + }
    > +
    > + /* swap device not found */
    > + if (i > nr_swapfiles) {
    > + spin_unlock(&swap_lock);
    > + return;
    > + }
    > +
    > + BUG_ON(!sis || sis->swap_free_notify_fn);
    > + sis->swap_free_notify_fn = notify_fn;
    > + spin_unlock(&swap_lock);
    > + return;
    > +}
    > +EXPORT_SYMBOL_GPL(set_swap_free_notify);
    > +
    > static int swap_entry_free(struct swap_info_struct *p,
    > swp_entry_t ent, int cache)
    > {
    > @@ -585,6 +617,8 @@ static int swap_entry_free(struct swap_info_struct *p,
    > swap_list.next = p - swap_info;
    > nr_swap_pages++;
    > p->inuse_pages--;
    > + if (p->swap_free_notify_fn)
    > + p->swap_free_notify_fn(p->bdev, offset);
    > }
    > if (!swap_count(count))
    > mem_cgroup_uncharge_swap(ent);

    OK, this hits core kernel code so we need to CC some more mm/swapfile.c
    people. The set_swap_free_notify() API looks strange to me. Hugh, I
    think you mentioned that you're okay with an explicit hook. Any
    suggestions how to do this cleanly?

    Pekka



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