Re: [RFC 2/3] mm: add swap_get_free hint for zram

[Minchan Kim]

On Sat, Sep 13, 2014 at 03:01:47PM -0400, Dan Streetman wrote:
> On Wed, Sep 3, 2014 at 9:39 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > VM uses nr_swap_pages as one of information when it does
> > anonymous reclaim so that VM is able to throttle amount of swap.
> >
> > Normally, the nr_swap_pages is equal to freeable space of swap disk
> > but for zram, it doesn't match because zram can limit memory usage
> > by knob(ie, mem_limit) so although VM can see lots of free space
> > from zram disk, zram can make fail intentionally once the allocated
> > space is over to limit. If it happens, VM should notice it and
> > stop reclaimaing until zram can obtain more free space but there
> > is a good way to do at the moment.
> >
> > This patch adds new hint SWAP_GET_FREE which zram can return how
> > many of freeable space it has. With using that, this patch adds
> > __swap_full which returns true if the zram is full and substract
> > remained freeable space of the zram-swap from nr_swap_pages.
> > IOW, VM sees there is no more swap space of zram so that it stops
> > anonymous reclaiming until swap_entry_free free a page and increase
> > nr_swap_pages again.
> >
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  include/linux/blkdev.h |  1 +
> >  mm/swapfile.c          | 45 +++++++++++++++++++++++++++++++++++++++++++--
> >  2 files changed, 44 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index 17437b2c18e4..c1199806e0f1 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_SLOT_FREE,
> > +       SWAP_GET_FREE,
> >  };
> >
> >  struct block_device_operations {
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 4bff521e649a..72737e6dd5e5 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -484,6 +484,22 @@ new_cluster:
> >         *scan_base = tmp;
> >  }
> >
> > +static bool __swap_full(struct swap_info_struct *si)
> > +{
> > +       if (si->flags & SWP_BLKDEV) {
> > +               long free;
> > +               struct gendisk *disk = si->bdev->bd_disk;
> > +
> > +               if (disk->fops->swap_hint)
> > +                       if (!disk->fops->swap_hint(si->bdev,
> > +                                               SWAP_GET_FREE,
> > +                                               &free))
> > +                               return free <= 0;
> > +       }
> > +
> > +       return si->inuse_pages == si->pages;
> > +}
> > +
> >  static unsigned long scan_swap_map(struct swap_info_struct *si,
> >                                    unsigned char usage)
> >  {
> > @@ -583,11 +599,21 @@ checks:
> >         if (offset == si->highest_bit)
> >                 si->highest_bit--;
> >         si->inuse_pages++;
> > -       if (si->inuse_pages == si->pages) {
> > +       if (__swap_full(si)) {
> 
> This check is done after an available offset has already been
> selected.  So if the variable-size blkdev is full at this point, then
> this is incorrect, as swap will try to store a page at the current
> selected offset.

So the result is just fail of a write then what happens?
Page become redirty and keep it in memory so there is no harm.

> 
> > +               struct gendisk *disk = si->bdev->bd_disk;
> > +
> >                 si->lowest_bit = si->max;
> >                 si->highest_bit = 0;
> >                 spin_lock(&swap_avail_lock);
> >                 plist_del(&si->avail_list, &swap_avail_head);
> > +               /*
> > +                * If zram is full, it decreases nr_swap_pages
> > +                * for stopping anonymous page reclaim until
> > +                * zram has free space. Look at swap_entry_free
> > +                */
> > +               if (disk->fops->swap_hint)
> 
> Simply checking for the existence of swap_hint isn't enough to know
> we're using zram...

Yes but acutally the hint have been used for only zram for several years.
If other user is coming in future, we would add more checks if we really
need it at that time.
Do you have another idea?

> 
> > +                       atomic_long_sub(si->pages - si->inuse_pages,
> > +                               &nr_swap_pages);
> >                 spin_unlock(&swap_avail_lock);
> >         }
> >         si->swap_map[offset] = usage;
> > @@ -796,6 +822,7 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
> >
> >         /* free if no reference */
> >         if (!usage) {
> > +               struct gendisk *disk = p->bdev->bd_disk;
> >                 dec_cluster_info_page(p, p->cluster_info, offset);
> >                 if (offset < p->lowest_bit)
> >                         p->lowest_bit = offset;
> > @@ -808,6 +835,21 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
> >                                 if (plist_node_empty(&p->avail_list))
> >                                         plist_add(&p->avail_list,
> >                                                   &swap_avail_head);
> > +                               if ((p->flags & SWP_BLKDEV) &&
> > +                                       disk->fops->swap_hint) {
> 
> freeing an entry from a full variable-size blkdev doesn't mean it's
> not still full.  In this case with zsmalloc, freeing one handle
> doesn't actually free any memory unless it was the only handle left in
> its containing zspage, and therefore it's possible that it is still
> full at this point.

No need to free a zspage in zsmalloc.
If we free a page in zspage, it means we have free space in zspage
so user can give a chance to user for writing out new page.

> 
> > +                                       atomic_long_add(p->pages -
> > +                                                       p->inuse_pages,
> > +                                                       &nr_swap_pages);
> > +                                       /*
> > +                                        * reset [highest|lowest]_bit to avoid
> > +                                        * scan_swap_map infinite looping if
> > +                                        * cached free cluster's index by
> > +                                        * scan_swap_map_try_ssd_cluster is
> > +                                        * above p->highest_bit.
> > +                                        */
> > +                                       p->highest_bit = p->max - 1;
> > +                                       p->lowest_bit = 1;
> 
> lowest_bit and highest_bit are likely to remain at those extremes for
> a long time, until 1 or max-1 is freed and re-allocated.
> 
> 
> By adding variable-size blkdev support to swap, I don't think
> highest_bit can be re-used as a "full" flag anymore.
> 
> Instead, I suggest that you add a "full" flag to struct
> swap_info_struct.  Then put a swap_hint GET_FREE check at the top of
> scan_swap_map(), and if full simply turn "full" on, remove the
> swap_info_struct from the avail list, reduce nr_swap_pages
> appropriately, and return failure.  Don't mess with lowest_bit or
> highest_bit at all.

Could you explain what logic in your suggestion prevent the problem
I mentioned(ie, scan_swap_map infinite looping)?

> 
> Then in swap_entry_free(), do something like:
> 
>     dec_cluster_info_page(p, p->cluster_info, offset);
>     if (offset < p->lowest_bit)
>       p->lowest_bit = offset;
> -   if (offset > p->highest_bit) {
> -     bool was_full = !p->highest_bit;
> +   if (offset > p->highest_bit)
>       p->highest_bit = offset;
> -     if (was_full && (p->flags & SWP_WRITEOK)) {
> +   if (p->full && p->flags & SWP_WRITEOK) {
> +     bool is_var_size_blkdev = is_variable_size_blkdev(p);
> +     bool blkdev_full = is_variable_size_blkdev_full(p);
> +
> +     if (!is_var_size_blkdev || !blkdev_full) {
> +       if (is_var_size_blkdev)
> +         atomic_long_add(p->pages - p->inuse_pages, &nr_swap_pages);
> +       p->full = false;
>         spin_lock(&swap_avail_lock);
>         WARN_ON(!plist_node_empty(&p->avail_list));
>         if (plist_node_empty(&p->avail_list))
>           plist_add(&p->avail_list,
>              &swap_avail_head);
>         spin_unlock(&swap_avail_lock);
> +     } else if (blkdev_full) {
> +       /* still full, so this page isn't actually
> +        * available yet to use; once non-full,
> +        * pages-inuse_pages will be the correct
> +        * number to add (above) since below will
> +        * inuse_pages--
> +        */
> +       atomic_long_dec(&nr_swap_pages);
>       }
>     }
>     atomic_long_inc(&nr_swap_pages);
> 
> 
> 
> > @@ -815,7 +857,6 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
> >                 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_SLOT_FREE,
> > --
> > 2.0.0
> >
> 
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-- 
Kind regards,
Minchan Kim
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