Re: [PATCH] mm: extend zero pages to same element pages for zram

From: Minchan Kim
Date: Mon Jan 09 2017 - 18:42:22 EST

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Hello,

On Fri, Jan 06, 2017 at 04:42:25PM +0800, zhouxianrong@xxxxxxxxxx wrote:
> From: zhouxianrong <zhouxianrong@xxxxxxxxxx>
>
> the idea is that without doing more calculations we extend zero pages
> to same element pages for zram. zero page is special case of
> same element page with zero element.
>
> 1. the test is done under android 7.0
> 2. startup too many applications circularly
> 3. sample the zero pages, same pages (none-zero element)
> and total pages in function page_zero_filled
>
> the result is listed as below:
>
> ZERO SAME TOTAL
> 36214 17842 598196
>
> ZERO/TOTAL SAME/TOTAL (ZERO+SAME)/TOTAL ZERO/SAME
> AVERAGE 0.060631909 0.024990816 0.085622726 2.663825038
> STDEV 0.00674612 0.005887625 0.009707034 2.115881328
> MAX 0.069698422 0.030046087 0.094975336 7.56043956
> MIN 0.03959586 0.007332205 0.056055193 1.928985507
>
> from above data, the benefit is about 2.5% and up to 3% of total
> swapout pages.

It looks great if we considers the amount of diff.
Thanks!

>
> the defect of the patch is that when we recovery a page from
> non-zero element the operations are low efficient for partial
> read.

Nice description. I love to see pros/cons. :)
If it hurts partial-read little bit, it would be not critical problem.

>
> Signed-off-by: zhouxianrong <zhouxianrong@xxxxxxxxxx>
> ---
> drivers/block/zram/zram_drv.c | 102 +++++++++++++++++++++++++++++++----------
> drivers/block/zram/zram_drv.h | 11 +++--
> 2 files changed, 84 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 15f58ab..3250a8b 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -94,6 +94,17 @@ static void zram_clear_flag(struct zram_meta *meta, u32 index,
> meta->table[index].value &= ~BIT(flag);
> }
>
> +static inline void zram_set_element(struct zram_meta *meta, u32 index,
> + unsigned long element)
> +{
> + meta->table[index].element = element;
> +}
> +
> +static inline void zram_clear_element(struct zram_meta *meta, u32 index)
> +{
> + meta->table[index].element = 0;
> +}
> +
> static size_t zram_get_obj_size(struct zram_meta *meta, u32 index)
> {
> return meta->table[index].value & (BIT(ZRAM_FLAG_SHIFT) - 1);
> @@ -158,31 +169,68 @@ static inline void update_used_max(struct zram *zram,
> } while (old_max != cur_max);
> }
>
> -static bool page_zero_filled(void *ptr)
> +static inline void zram_set_page(char *ptr, unsigned long value)

Nit:
I prefer fill to set in this case.
How about zram_fill_page?
It is a just nit so if you want to set, I'm not against.

> +{
> + int i;
> + unsigned long *page = (unsigned long *)ptr;
> +
> + for (i = PAGE_SIZE / sizeof(unsigned long) - 1; i >= 0; i--)
> + page[i] = value;
> +}
> +
> +static inline void zram_set_page_partial(char *ptr, unsigned int size,
> + unsigned long value)
> +{
> + int i;
> +
> + i = ((unsigned long)ptr) % sizeof(unsigned long);
> + if (i) {

Hmm, Is this case really happens? I guess block layer works with IO size unit
which would be aligned (unsigned long) at least, maybe SECTOR or PAGE size.

Then, I think we can merge both zram_set_page and zram_set_page_partial
like this.

static inline void zram_fill_page(char *ptr, unsigned int size, unsigned long elelment)
{
int i;
unsigned long *page = (unsigned long *)ptr;

WARN_ON_ONCE((unsigned long)ptr % sizeof(unsigned long));
for (i = size / sizeof(unsigned long) - 1; i >= 0; i--)
page[i] = element
}

> + while (i < sizeof(unsigned long)) {
> + *ptr++ = (value >> (i * 8)) & 0xff;
> + --size;
> + ++i;
> + }
> + }
> +
> + for (i = size / sizeof(unsigned long); i > 0; --i) {
> + *(unsigned long *)ptr = value;
> + ptr += sizeof(unsigned long);
> + size -= sizeof(unsigned long);
> + }
> +
> + for (i = 0; i < size; ++i)
> + *ptr++ = (value >> (i * 8)) & 0xff;
> +}
> +
> +static bool page_same_filled(void *ptr, unsigned long *element)
> {
> unsigned int pos;
> unsigned long *page;
>
> page = (unsigned long *)ptr;
>
> - for (pos = 0; pos != PAGE_SIZE / sizeof(*page); pos++) {
> - if (page[pos])
> + for (pos = PAGE_SIZE / sizeof(unsigned long) - 1; pos > 0; pos--) {
> + if (page[pos] != page[pos - 1])
> return false;
> }
>
> + if (element)

Maybe, we don't need this condition check?

> + element[0] = page[pos];
> +
> return true;
> }
>
> -static void handle_zero_page(struct bio_vec *bvec)
> +static void handle_same_page(struct bio_vec *bvec, unsigned long element)
> {
> struct page *page = bvec->bv_page;
> void *user_mem;
>
> user_mem = kmap_atomic(page);
> if (is_partial_io(bvec))
> - memset(user_mem + bvec->bv_offset, 0, bvec->bv_len);
> + zram_set_page_partial(user_mem + bvec->bv_offset, bvec->bv_len,
> + element);
> else
> - clear_page(user_mem);
> + zram_set_page(user_mem, element);
> kunmap_atomic(user_mem);
>
> flush_dcache_page(page);
> @@ -431,7 +479,7 @@ static ssize_t mm_stat_show(struct device *dev,
> mem_used << PAGE_SHIFT,
> zram->limit_pages << PAGE_SHIFT,
> max_used << PAGE_SHIFT,
> - (u64)atomic64_read(&zram->stats.zero_pages),
> + (u64)atomic64_read(&zram->stats.same_pages),

Unfortunately, we cannot replace zero pages stat with same pages's one right
now due to compatibility problem. Please add same_pages to tail of the stat
and we should warn deprecated zero_pages stat so we finally will remove it
two year later. Please reference Documentation/ABI/obsolete/sysfs-block-zram
And add zero-pages to the document.

For example,

... mm_stat_show()
{
pr_warn_once("zero pages was deprecated so it will be removed at 2019 Jan");
}

Sergey, what's your opinion?

> pool_stats.pages_compacted);
> up_read(&zram->init_lock);
>
> @@ -464,7 +512,7 @@ static ssize_t debug_stat_show(struct device *dev,
> ZRAM_ATTR_RO(failed_writes);
> ZRAM_ATTR_RO(invalid_io);
> ZRAM_ATTR_RO(notify_free);
> -ZRAM_ATTR_RO(zero_pages);
> +ZRAM_ATTR_RO(same_pages);
> ZRAM_ATTR_RO(compr_data_size);
>
> static inline bool zram_meta_get(struct zram *zram)
> @@ -538,18 +586,20 @@ static void zram_free_page(struct zram *zram, size_t index)
> struct zram_meta *meta = zram->meta;
> unsigned long handle = meta->table[index].handle;
>
> - if (unlikely(!handle)) {
> - /*
> - * No memory is allocated for zero filled pages.
> - * Simply clear zero page flag.
> - */
> - if (zram_test_flag(meta, index, ZRAM_ZERO)) {
> - zram_clear_flag(meta, index, ZRAM_ZERO);
> - atomic64_dec(&zram->stats.zero_pages);
> - }
> + /*
> + * No memory is allocated for same element filled pages.
> + * Simply clear same page flag.
> + */
> + if (zram_test_flag(meta, index, ZRAM_SAME)) {
> + zram_clear_flag(meta, index, ZRAM_SAME);
> + zram_clear_element(meta, index);
> + atomic64_dec(&zram->stats.same_pages);
> return;
> }
>
> + if (!handle)
> + return;
> +
> zs_free(meta->mem_pool, handle);
>
> atomic64_sub(zram_get_obj_size(meta, index),
> @@ -572,9 +622,9 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> handle = meta->table[index].handle;
> size = zram_get_obj_size(meta, index);
>
> - if (!handle || zram_test_flag(meta, index, ZRAM_ZERO)) {
> + if (!handle || zram_test_flag(meta, index, ZRAM_SAME)) {
> bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> - clear_page(mem);
> + zram_set_page(mem, meta->table[index].element);

> return 0;
> }
>
> @@ -610,9 +660,9 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
>
> bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> if (unlikely(!meta->table[index].handle) ||
> - zram_test_flag(meta, index, ZRAM_ZERO)) {
> + zram_test_flag(meta, index, ZRAM_SAME)) {
> bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> - handle_zero_page(bvec);
> + handle_same_page(bvec, meta->table[index].element);
> return 0;
> }
> bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> @@ -660,6 +710,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> struct zram_meta *meta = zram->meta;
> struct zcomp_strm *zstrm = NULL;
> unsigned long alloced_pages;
> + unsigned long element;
>
> page = bvec->bv_page;
> if (is_partial_io(bvec)) {
> @@ -688,16 +739,17 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> uncmem = user_mem;
> }
>
> - if (page_zero_filled(uncmem)) {
> + if (page_same_filled(uncmem, &element)) {
> if (user_mem)
> kunmap_atomic(user_mem);
> /* Free memory associated with this sector now. */
> bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> zram_free_page(zram, index);
> - zram_set_flag(meta, index, ZRAM_ZERO);
> + zram_set_flag(meta, index, ZRAM_SAME);
> + zram_set_element(meta, index, element);
> bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>
> - atomic64_inc(&zram->stats.zero_pages);
> + atomic64_inc(&zram->stats.same_pages);
> ret = 0;
> goto out;
> }
> @@ -1203,7 +1255,7 @@ static int zram_open(struct block_device *bdev, fmode_t mode)
> &dev_attr_compact.attr,
> &dev_attr_invalid_io.attr,
> &dev_attr_notify_free.attr,
> - &dev_attr_zero_pages.attr,
> + &dev_attr_same_pages.attr,
> &dev_attr_orig_data_size.attr,
> &dev_attr_compr_data_size.attr,
> &dev_attr_mem_used_total.attr,
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 74fcf10..4bb92e1 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -60,8 +60,8 @@
>
> /* Flags for zram pages (table[page_no].value) */
> enum zram_pageflags {
> - /* Page consists entirely of zeros */
> - ZRAM_ZERO = ZRAM_FLAG_SHIFT,
> + /* Page consists entirely of same elements */
> + ZRAM_SAME = ZRAM_FLAG_SHIFT,
> ZRAM_ACCESS, /* page is now accessed */
>
> __NR_ZRAM_PAGEFLAGS,
> @@ -71,7 +71,10 @@ enum zram_pageflags {
>
> /* Allocated for each disk page */
> struct zram_table_entry {
> - unsigned long handle;
> + union {
> + unsigned long handle;
> + unsigned long element;
> + };
> unsigned long value;
> };
>
> @@ -83,7 +86,7 @@ struct zram_stats {
> atomic64_t failed_writes; /* can happen when memory is too low */
> atomic64_t invalid_io; /* non-page-aligned I/O requests */
> atomic64_t notify_free; /* no. of swap slot free notifications */
> - atomic64_t zero_pages; /* no. of zero filled pages */
> + atomic64_t same_pages; /* no. of same element filled pages */
> atomic64_t pages_stored; /* no. of pages currently stored */
> atomic_long_t max_used_pages; /* no. of maximum pages stored */
> atomic64_t writestall; /* no. of write slow paths */
> --
> 1.7.9.5
>
> --
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Next in thread: Sergey Senozhatsky: "Re: [PATCH] mm: extend zero pages to same element pages for zram"
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