Re: [RFC][PATCH] zram: drop partial_io support

Minchan Kim
2015-12-16T01:00:48Z

Hello Sergey,

Sorry for the late response. I am in long vavacation now but today,
I get small time to sit down on computer. :)

On Mon, Dec 14, 2015 at 09:38:55PM +0900, Sergey Senozhatsky wrote:
> Hello,
>
> I've been thinking about this for some time, but didn't have a chance
> to properly investigate so far. My question is: why do we even bother
> with partial IO in zram?

It was done before I involved zram actively so I should spend a time
to search the reason.
Firstly, author was Jerome.
http://lists.openwall.net/linux-kernel/2011/06/10/318

And Nitin wanted to increase logical block size 64K instead of
making complex part by partial I/O.
http://lists.openwall.net/linux-kernel/2011/06/10/402

And Jeff and Martin said there is no problem to increase
logical_block_size from unsigned short if people are aware of
the implications bigger blocks have on the filesystems they put on top.

http://lists.openwall.net/linux-kernel/2011/06/14/289
http://lists.openwall.net/linux-kernel/2011/06/14/324

Jerome finally found severe problem which FAT fs are unable to
cope with 64K logical blocks at least.
http://lists.openwall.net/linux-kernel/2011/07/01/196

That's why Nitin decided to suppport partial IO in zram.
And I think it does make sense.

Thanks.

>
> We set zram->disk->queue->limits.discard_granularity to PAGE_SIZE,
> just like the rest of queue configuration, e.g.
> blk_queue_physical_block_size(zram->disk->queue, PAGE_SIZE)
> etc.
>
> And it's only blk_queue_logical_block_size() that has !PAGE_SIZE
> configuration (on systems where PAGE_SIZE != ZRAM_LOGICAL_BLOCK_SHIFT).
>
>
> The only requirement enforced on blk_queue_logical_block_size() from
> zram/zsmalloc side seems to be that it should be less than or equal
> to ZS_MAX_ALLOC_SIZE, which is:
> #define ZS_MAX_ALLOC_SIZE PAGE_SIZE
>
>
> We talk to compressing backends with PAGE_SIZE-d buffers,
> zsmalloc understands PAGE_SIZE allocations and we can simplify
> things if we will be able to drop this partial IO support thing.
> So why do we keep it?
>
> What am I missing? Sorry if there is something obvious.
>
>
> ** NOT TESTED ON SYSTEMS WITH PAGE_SIZE OTHER THAN 4K **
> Will try to do this later this week (well, if it makes sense).
>
>
> ===8<===8<===
>
> From e55739c453c1ab01ab9b6ef734b078c1d96fbde2 Mon Sep 17 00:00:00 2001
> From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Date: Mon, 14 Dec 2015 21:23:39 +0900
> Subject: [PATCH] zram: drop partial_io handling
>
> Not-yet-Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> drivers/block/zram/zram_drv.c | 99 ++++++-------------------------------------
> drivers/block/zram/zram_drv.h | 3 +-
> 2 files changed, 14 insertions(+), 88 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 47915d7..9bb5e3b 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -106,11 +106,6 @@ static void zram_set_obj_size(struct zram_meta *meta,
> meta->table[index].value = (flags << ZRAM_FLAG_SHIFT) | size;
> }
>
> -static inline bool is_partial_io(struct bio_vec *bvec)
> -{
> - return bvec->bv_len != PAGE_SIZE;
> -}
> -
> /*
> * Check if request is within bounds and aligned on zram logical blocks.
> */
> @@ -178,10 +173,7 @@ static void handle_zero_page(struct bio_vec *bvec)
> void *user_mem;
>
> user_mem = kmap_atomic(page);
> - if (is_partial_io(bvec))
> - memset(user_mem + bvec->bv_offset, 0, bvec->bv_len);
> - else
> - clear_page(user_mem);
> + clear_page(user_mem);
> kunmap_atomic(user_mem);
>
> flush_dcache_page(page);
> @@ -600,7 +592,7 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> {
> int ret;
> struct page *page;
> - unsigned char *user_mem, *uncmem = NULL;
> + unsigned char *uncmem;
> struct zram_meta *meta = zram->meta;
> page = bvec->bv_page;
>
> @@ -613,35 +605,16 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> }
> bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
>
> - if (is_partial_io(bvec))
> - /* Use a temporary buffer to decompress the page */
> - uncmem = kmalloc(PAGE_SIZE, GFP_NOIO);
> -
> - user_mem = kmap_atomic(page);
> - if (!is_partial_io(bvec))
> - uncmem = user_mem;
> -
> - if (!uncmem) {
> - pr_err("Unable to allocate temp memory\n");
> - ret = -ENOMEM;
> - goto out_cleanup;
> - }
> -
> + uncmem = kmap_atomic(page);
> ret = zram_decompress_page(zram, uncmem, index);
> /* Should NEVER happen. Return bio error if it does. */
> if (unlikely(ret))
> goto out_cleanup;
>
> - if (is_partial_io(bvec))
> - memcpy(user_mem + bvec->bv_offset, uncmem + offset,
> - bvec->bv_len);
> -
> flush_dcache_page(page);
> ret = 0;
> out_cleanup:
> - kunmap_atomic(user_mem);
> - if (is_partial_io(bvec))
> - kfree(uncmem);
> + kunmap_atomic(uncmem);
> return ret;
> }
>
> @@ -652,42 +625,17 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> size_t clen;
> unsigned long handle;
> struct page *page;
> - unsigned char *user_mem, *cmem, *src, *uncmem = NULL;
> + unsigned char *user_mem, *cmem;
> struct zram_meta *meta = zram->meta;
> struct zcomp_strm *zstrm = NULL;
> unsigned long alloced_pages;
>
> page = bvec->bv_page;
> - if (is_partial_io(bvec)) {
> - /*
> - * This is a partial IO. We need to read the full page
> - * before to write the changes.
> - */
> - uncmem = kmalloc(PAGE_SIZE, GFP_NOIO);
> - if (!uncmem) {
> - ret = -ENOMEM;
> - goto out;
> - }
> - ret = zram_decompress_page(zram, uncmem, index);
> - if (ret)
> - goto out;
> - }
> -
> zstrm = zcomp_strm_find(zram->comp);
> user_mem = kmap_atomic(page);
>
> - if (is_partial_io(bvec)) {
> - memcpy(uncmem + offset, user_mem + bvec->bv_offset,
> - bvec->bv_len);
> + if (page_zero_filled(user_mem)) {
> kunmap_atomic(user_mem);
> - user_mem = NULL;
> - } else {
> - uncmem = user_mem;
> - }
> -
> - if (page_zero_filled(uncmem)) {
> - 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);
> @@ -699,22 +647,15 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> goto out;
> }
>
> - ret = zcomp_compress(zram->comp, zstrm, uncmem, &clen);
> - if (!is_partial_io(bvec)) {
> - kunmap_atomic(user_mem);
> - user_mem = NULL;
> - uncmem = NULL;
> - }
> + ret = zcomp_compress(zram->comp, zstrm, user_mem, &clen);
> + kunmap_atomic(user_mem);
>
> if (unlikely(ret)) {
> pr_err("Compression failed! err=%d\n", ret);
> goto out;
> }
> - src = zstrm->buffer;
> if (unlikely(clen > max_zpage_size)) {
> clen = PAGE_SIZE;
> - if (is_partial_io(bvec))
> - src = uncmem;
> }
>
> handle = zs_malloc(meta->mem_pool, clen);
> @@ -736,12 +677,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>
> cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO);
>
> - if ((clen == PAGE_SIZE) && !is_partial_io(bvec)) {
> - src = kmap_atomic(page);
> + if (clen == PAGE_SIZE) {
> + unsigned char *src = kmap_atomic(page);
> copy_page(cmem, src);
> kunmap_atomic(src);
> } else {
> - memcpy(cmem, src, clen);
> + memcpy(cmem, zstrm->buffer, clen);
> }
>
> zcomp_strm_release(zram->comp, zstrm);
> @@ -765,8 +706,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> out:
> if (zstrm)
> zcomp_strm_release(zram->comp, zstrm);
> - if (is_partial_io(bvec))
> - kfree(uncmem);
> return ret;
> }
>
> @@ -1242,24 +1181,12 @@ static int zram_add(void)
> * and n*PAGE_SIZED sized I/O requests.
> */
> blk_queue_physical_block_size(zram->disk->queue, PAGE_SIZE);
> - blk_queue_logical_block_size(zram->disk->queue,
> - ZRAM_LOGICAL_BLOCK_SIZE);
> + blk_queue_logical_block_size(zram->disk->queue, PAGE_SIZE);
> blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
> blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
> zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
> blk_queue_max_discard_sectors(zram->disk->queue, UINT_MAX);
> - /*
> - * zram_bio_discard() will clear all logical blocks if logical block
> - * size is identical with physical block size(PAGE_SIZE). But if it is
> - * different, we will skip discarding some parts of logical blocks in
> - * the part of the request range which isn't aligned to physical block
> - * size. So we can't ensure that all discarded logical blocks are
> - * zeroed.
> - */
> - if (ZRAM_LOGICAL_BLOCK_SIZE == PAGE_SIZE)
> - zram->disk->queue->limits.discard_zeroes_data = 1;
> - else
> - zram->disk->queue->limits.discard_zeroes_data = 0;
> + zram->disk->queue->limits.discard_zeroes_data = 1;
> queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, zram->disk->queue);
>
> add_disk(zram->disk);
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 9b296c4..0c89c6d 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -39,12 +39,11 @@ static const size_t max_zpage_size = PAGE_SIZE / 4 * 3;
> #define SECTOR_SHIFT 9
> #define SECTORS_PER_PAGE_SHIFT (PAGE_SHIFT - SECTOR_SHIFT)
> #define SECTORS_PER_PAGE (1 << SECTORS_PER_PAGE_SHIFT)
> -#define ZRAM_LOGICAL_BLOCK_SHIFT 12
> +#define ZRAM_LOGICAL_BLOCK_SHIFT PAGE_SHIFT
> #define ZRAM_LOGICAL_BLOCK_SIZE (1 << ZRAM_LOGICAL_BLOCK_SHIFT)
> #define ZRAM_SECTOR_PER_LOGICAL_BLOCK \
> (1 << (ZRAM_LOGICAL_BLOCK_SHIFT - SECTOR_SHIFT))
>
> -
> /*
> * The lower ZRAM_FLAG_SHIFT bits of table.value is for
> * object size (excluding header), the higher bits is for
> --
> 2.6.4
>