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Date: Wed, 24 Sep 2014 21:31:20 -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 Wed, Sep 24, 2014 at 9:06 PM, Minchan Kim <minchan@...nel.org> wrote:
> 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;
ah sorry, i wasn't paying attention there, you're right accessing
virt_swap after you set it is probably better.
>
>
>
>>
>> > + 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
>> >
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@...ck.org. For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
>
> --
> Kind regards,
> Minchan Kim
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