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    SubjectRe: [PATCH 0/3] ramzswap: Eliminate stale data in compressed memory
    On 05/05/2010 09:52 PM, Linus Torvalds wrote:
    > On Wed, 5 May 2010, Nitin Gupta wrote:
    >> I think such 'swap_operations' structure will be have to be part of
    >> block_device_operations, so we may access it from swap_entry_free()
    >> where a swap slot is freed. This will also get rid of all this notifier
    >> stuff.
    > Yes, I think adding it to block_device_operations would be fine. That
    > sounds like a sane layering, and would make it easy for a block device
    > driver to say "I want to know about swap events".
    > In fact, for regular block devices, a swap block free might well translate
    > into a TRIM command some day (where "some day" means when the SSD's
    > actually get their stuff together and there is real upside and not just
    > "most cases will be very slow and the upside is debatable").

    Its great if adding such a callback to block_device_operations is okay. Hugh
    suggested this approach and I'm distributing it with compcache for quite some
    time now:

    Can you please have a lot at patch above and see if its acceptable? Then I will
    post it to lkml again.

    >> The patch you nacked did something similar: it add 'swap_slot_free_callback'
    >> directly to block_device_operations. Without such change, I could not think
    >> of any way to do away with notifiers.
    > Umm. No. IIRC, the patch I NAK'ed aded it to the 'swap_info_struct', which
    > I said was the wrong level. The block device driver level would seem to be
    > the _right_ level, since that's what ramzswap is. No?
    > Also, the patch I NAK'ed also used those nasty notifier chains, making it
    > even uglier.

    Please see the original mail below (patch you nacked). Maybe, at that time, I didn't
    make it clear that ramzswap is really a *block device* :)

    -------- Original Message --------
    Subject: [nacked] mm-add-swap-slot-free-callback-to-block_device_operations.patch removed from -mm tree
    Date: Tue, 09 Mar 2010 14:30:27 -0800
    To: <snip/>

    The patch titled
    mm: add swap slot free callback to block_device_operations
    has been removed from the -mm tree. Its filename was

    This patch was dropped because it was nacked

    The current -mm tree may be found at

    Subject: mm: add swap slot free callback to block_device_operations
    From: Nitin Gupta <>

    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: (I intend
    to integrade preswap with ramzswap).

    [1] ramzswap:

    Signed-off-by: Nitin Gupta <>
    Signed-off-by: Hugh Dickins <>
    Acked-by: Hugh Dickins <>
    Cc: Greg KH <>
    Cc: Pekka Enberg <>
    Cc: Al Viro <>
    Cc: Christoph Hellwig <>
    Cc: Jens Axboe <>
    Cc: Andi Kleen <>
    Signed-off-by: Andrew Morton <>

    include/linux/blkdev.h | 2 ++
    mm/swapfile.c | 3 +++
    2 files changed, 5 insertions(+)

    diff -puN include/linux/blkdev.h~mm-add-swap-slot-free-callback-to-block_device_operations include/linux/blkdev.h
    --- a/include/linux/blkdev.h~mm-add-swap-slot-free-callback-to-block_device_operations
    +++ a/include/linux/blkdev.h
    @@ -1310,6 +1310,8 @@ struct block_device_operations {
    unsigned long long);
    int (*revalidate_disk) (struct gendisk *);
    int (*getgeo)(struct block_device *, struct hd_geometry *);
    + /* this callback is with swap_lock and sometimes page table lock held */
    + void (*swap_slot_free_notify) (struct block_device *, unsigned long);
    struct module *owner;

    diff -puN mm/swapfile.c~mm-add-swap-slot-free-callback-to-block_device_operations mm/swapfile.c
    --- a/mm/swapfile.c~mm-add-swap-slot-free-callback-to-block_device_operations
    +++ a/mm/swapfile.c
    @@ -574,6 +574,7 @@ static unsigned char swap_entry_free(str

    /* 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(str = p->type;
    + if (disk->fops->swap_slot_free_notify)
    + disk->fops->swap_slot_free_notify(p->bdev, offset);

    return usage;
    Patches currently in -mm which might be from are


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