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Date: Thu, 25 Sep 2014 10:06:38 +0900
From: Minchan Kim <minchan@...nel.org>
To: Dan Streetman <ddstreet@...e.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 Wed, Sep 24, 2014 at 10:12:18AM -0400, Dan Streetman wrote:
> 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?
Do you want this?
if (p->flags & SWP_BLKDEV && p->bdev->bd_disk->fops->swap_hint)
virt_swap = p->bdev->bd_disk;
.
>
> > + 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.
Okay.
>
>
> > }
> >
> > 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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--
Kind regards,
Minchan Kim
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