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Message-Id: <1253256805.4959.8.camel@penberg-laptop>
Date: Fri, 18 Sep 2009 09:53:25 +0300
From: Pekka Enberg <penberg@...helsinki.fi>
To: Nitin Gupta <ngupta@...are.org>
Cc: Greg KH <greg@...ah.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Ed Tomlinson <edt@....ca>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-mm <linux-mm@...ck.org>,
linux-mm-cc <linux-mm-cc@...top.org>, hugh.dickins@...cali.co.uk,
kamezawa.hiroyu@...fujitsu.com, nishimura@....nes.nec.co.jp
Subject: Re: [PATCH 2/4] send callback when swap slot is freed
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@...are.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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