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    SubjectRe: [PATCH 1/2] [mmotm] Add swap slot free callback to block_device_operations
    On Tue, 5 Jan 2010, Nitin Gupta wrote:

    > This callback is required when RAM based devices are
    > used as swap disks. One such device is ramzswap[1] which
    > is used as compressed in-memory swap disk. For such
    > devices, we need a callback as soon as a swap slot is no
    > longer used to allow freeing memory allocated for this
    > slot. Without this callback, stale data can quickly
    > accumulate in memory defeating the whole purpose of
    > such devices.
    >
    > Another user of this callback will be "preswap" as
    > introduced by "Transcendent Memory" patches:
    > http://lwn.net/Articles/367286/
    > (I intend to integrade preswap with ramzswap).
    >
    > [1] ramzswap: http://code.google.com/p/compcache/
    >
    > Signed-off-by: Nitin Gupta <ngupta@vflare.org>

    Yes, thanks for your patience in putting this together:
    I much prefer this version to all the other ways considered.

    (Though you should later add a comment, pointing out that this
    callback is with swap_lock and often page table lock also held.)

    We know that's unsatisfactory, but it is the nature of swap:
    cutting some corners to get the job done without allocating
    more memory. Having tried myself to do proper discard at
    swap_free time, I know that is hard: and I believe you've
    made the right decision to do it in this way for ramzswap.

    Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

    > ---
    > include/linux/blkdev.h | 1 +
    > mm/swapfile.c | 3 +++
    > 2 files changed, 4 insertions(+), 0 deletions(-)
    >
    > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
    > index 784a919..14b95a3 100644
    > --- a/include/linux/blkdev.h
    > +++ b/include/linux/blkdev.h
    > @@ -1295,6 +1295,7 @@ struct block_device_operations {
    > unsigned long long);
    > int (*revalidate_disk) (struct gendisk *);
    > int (*getgeo)(struct block_device *, struct hd_geometry *);
    > + void (*swap_slot_free_notify) (struct block_device *, unsigned long);
    > struct module *owner;
    > };
    >
    > diff --git a/mm/swapfile.c b/mm/swapfile.c
    > index 6c0585b..0c373bc 100644
    > --- a/mm/swapfile.c
    > +++ b/mm/swapfile.c
    > @@ -574,6 +574,7 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
    >
    > /* free if no reference */
    > if (!usage) {
    > + struct gendisk *disk = p->bdev->bd_disk;
    > if (offset < p->lowest_bit)
    > p->lowest_bit = offset;
    > if (offset > p->highest_bit)
    > @@ -583,6 +584,8 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
    > swap_list.next = p->type;
    > nr_swap_pages++;
    > p->inuse_pages--;
    > + if (disk->fops->swap_slot_free_notify)
    > + disk->fops->swap_slot_free_notify(p->bdev, offset);
    > }
    >
    > return usage;
    > --
    > 1.6.2.5


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