Re: [PATCH v1 3/5] mm: VM can be aware of zram fullness

[Minchan Kim <minchan@kernel.org>]

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@kernel.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@kernel.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;


\x02
> 
> > +                               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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