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Date: Wed, 24 Sep 2014 10:12:18 -0400
From: Dan Streetman <ddstreet@...e.org>
To: Minchan Kim <minchan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>, Hugh Dickins <hughd@...gle.com>,
Shaohua Li <shli@...nel.org>,
Jerome Marchand <jmarchan@...hat.com>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Nitin Gupta <ngupta@...are.org>,
Luigi Semenzato <semenzato@...gle.com>, juno.choi@....com
Subject: Re: [PATCH v1 3/5] mm: VM can be aware of zram fullness
On Sun, Sep 21, 2014 at 8:03 PM, Minchan Kim <minchan@...nel.org> wrote:
> VM uses nr_swap_pages to throttle amount of swap when it reclaims
> anonymous pages because the nr_swap_pages means freeable space
> of swap disk.
>
> However, it's a problem for zram because zram can limit memory
> usage by knob(ie, mem_limit) so that swap out can fail although
> VM can see lots of free space from zram disk but no more free
> space in zram by the limit. If it happens, VM should notice it
> and stop reclaimaing until zram can obtain more free space but
> we don't have a way to communicate between VM and zram.
>
> This patch adds new hint SWAP_FULL so that zram can say to VM
> "I'm full" from now on. Then VM cannot reclaim annoymous page
> any more. If VM notice swap is full, it can remove swap_info_struct
> from swap_avail_head and substract remained freeable space from
> nr_swap_pages so that VM can think swap is full until VM frees a
> swap and increase nr_swap_pages again.
>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
> include/linux/blkdev.h | 1 +
> mm/swapfile.c | 44 ++++++++++++++++++++++++++++++++++++++------
> 2 files changed, 39 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index c7220409456c..39f074e0acd7 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1611,6 +1611,7 @@ static inline bool blk_integrity_is_initialized(struct gendisk *g)
>
> enum swap_blk_hint {
> SWAP_FREE,
> + SWAP_FULL,
> };
>
> struct block_device_operations {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 209112cf8b83..71e3df0431b6 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -493,6 +493,29 @@ static unsigned long scan_swap_map(struct swap_info_struct *si,
> int latency_ration = LATENCY_LIMIT;
>
> /*
> + * If zram is full, we don't need to scan and want to stop swap.
> + * For it, we removes si from swap_avail_head and decreases
> + * nr_swap_pages to prevent further anonymous reclaim so that
> + * VM can restart swap out if zram has a free space.
> + * Look at swap_entry_free.
> + */
> + if (si->flags & SWP_BLKDEV) {
> + struct gendisk *disk = si->bdev->bd_disk;
> +
> + if (disk->fops->swap_hint && disk->fops->swap_hint(
> + si->bdev, SWAP_FULL, NULL)) {
> + spin_lock(&swap_avail_lock);
> + WARN_ON(plist_node_empty(&si->avail_list));
> + plist_del(&si->avail_list, &swap_avail_head);
> + spin_unlock(&swap_avail_lock);
> + atomic_long_sub(si->pages - si->inuse_pages,
> + &nr_swap_pages);
> + si->full = true;
> + return 0;
> + }
> + }
> +
> + /*
> * We try to cluster swap pages by allocating them sequentially
> * in swap. Once we've allocated SWAPFILE_CLUSTER pages this
> * way, however, we resort to first-free allocation, starting
> @@ -798,6 +821,14 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
> /* free if no reference */
> if (!usage) {
> bool was_full;
> + struct gendisk *virt_swap = NULL;
> +
> + /* Check virtual swap */
> + if (p->flags & SWP_BLKDEV) {
> + virt_swap = p->bdev->bd_disk;
> + if (!virt_swap->fops->swap_hint)
not a big deal, but can't you just combine these two if's to simplify this?
> + virt_swap = NULL;
> + }
>
> dec_cluster_info_page(p, p->cluster_info, offset);
> if (offset < p->lowest_bit)
> @@ -814,17 +845,18 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
> &swap_avail_head);
> spin_unlock(&swap_avail_lock);
> p->full = false;
> + if (virt_swap)
> + atomic_long_add(p->pages -
> + p->inuse_pages,
> + &nr_swap_pages);
a comment here might be good, to clarify it relies on the check at the
top of scan_swap_map previously subtracting the same number of pages.
> }
>
> atomic_long_inc(&nr_swap_pages);
> p->inuse_pages--;
> frontswap_invalidate_page(p->type, offset);
> - if (p->flags & SWP_BLKDEV) {
> - struct gendisk *disk = p->bdev->bd_disk;
> - if (disk->fops->swap_hint)
> - disk->fops->swap_hint(p->bdev,
> - SWAP_FREE, (void *)offset);
> - }
> + if (virt_swap)
> + virt_swap->fops->swap_hint(p->bdev,
> + SWAP_FREE, (void *)offset);
> }
>
> return usage;
> --
> 2.0.0
>
--
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