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Date: Mon, 15 Sep 2014 10:53:01 -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>
Subject: Re: [RFC 2/3] mm: add swap_get_free hint for zram
On Sun, Sep 14, 2014 at 8:30 PM, Minchan Kim <minchan@...nel.org> wrote:
> 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@...nel.org> 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@...nel.org>
>> > ---
>> > 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.
Happening once, it's not a big deal. But it's not as good as not
happening at all.
>
>>
>> > + 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?
Well if this hint == zram just rename it zram. Especially if it's now
going to be explicitly used to mean it == zram. But I don't think
that is necessary.
>
>>
>> > + 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.
That's not actually true, since zsmalloc has 255 different class
sizes, freeing one page means the next page to be compressed has a
1/255 chance that it will be the same size as the just-freed page
(assuming random page compressability).
>
>>
>> > + 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)?
scan_swap_map would immediately exit since the GET_FREE (or IS_FULL)
check is done at its start. And it wouldn't be called again with that
swap_info_struct until non-full since it is removed from the
avail_list.
>
>>
>> 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
>> >
>>
>> --
>> 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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