Re: [PATCH 2/2] zram: reduce metadata overhead

[Nitin Gupta]

On 11/27/2012 08:22 AM, Jerome Marchand wrote:
> On 11/27/2012 08:26 AM, Nitin Gupta wrote:
> > For every allocated object, zram maintains the the handle, size,
> > flags and count fields. Of these, only the handle is required
> > since zsmalloc now provides the object size given the handle.
> > The flags field was needed only to mark a given page as zero-filled.
> > Instead of this field, we now use an invalid value (-1) to mark such
> Since the handle is unsigned, is this really an invalid value?

Great catch. -1 is not always invalid but +1 definitely is. I will fix this.

> > pages. Lastly, the count field was unused, so was simply removed.
> >
> > Signed-off-by: Nitin Gupta <ngupta@xxxxxxxxxx>
> > ---
> >   drivers/staging/zram/zram_drv.c |   80 ++++++++++++++-------------------------
> >   drivers/staging/zram/zram_drv.h |   18 +--------
> >   2 files changed, 31 insertions(+), 67 deletions(-)
> >
> > [...]
> > @@ -222,8 +202,8 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> >   
> >   	page = bvec->bv_page;
> >   
> > -	if (unlikely(!zram->table[index].handle) ||
> > -			zram_test_flag(zram, index, ZRAM_ZERO)) {
> > +	if (unlikely(!zram->handle[index]) ||
> > +			(zram->handle[index] == zero_page_handle)) {
> This kind of comparison appears often. To my taste, an is_zero_page()
> helper would be welcome.

Actually, I was hit by assignment-instead-of-comparison bug once
during these changes, so this helper really should be there.

> >   		handle_zero_page(bvec);
> >   		return 0;
> >   	}
> > [...]
> > @@ -573,8 +551,8 @@ int zram_init_device(struct zram *zram)
> >   	}
> >   
> >   	num_pages = zram->disksize >> PAGE_SHIFT;
> > -	zram->table = vzalloc(num_pages * sizeof(*zram->table));
> > -	if (!zram->table) {
> > +	zram->handle = vzalloc(num_pages * sizeof(*zram->handle));
> > +	if (!zram->handle) {
> >   		pr_err("Error allocating zram address table\n");
> >   		ret = -ENOMEM;
> >   		goto fail_no_table;
> Since the table goes away, for readability sake, we'd better remove all
> reference to it in messages, labels and comment and write "handle"
> instead.

Missed those messages, will fix that.

> Otherwise, the patch looks good to me.

Thanks for the review.
Nitin

> > diff --git a/drivers/staging/zram/zram_drv.h b/drivers/staging/zram/zram_drv.h
> > index df2eec4..8aa733c 100644
> > --- a/drivers/staging/zram/zram_drv.h
> > +++ b/drivers/staging/zram/zram_drv.h
> > @@ -54,24 +54,10 @@ static const size_t max_zpage_size = PAGE_SIZE / 4 * 3;
> >   #define ZRAM_SECTOR_PER_LOGICAL_BLOCK	\
> >   	(1 << (ZRAM_LOGICAL_BLOCK_SHIFT - SECTOR_SHIFT))
> >   
> > -/* Flags for zram pages (table[page_no].flags) */
> > -enum zram_pageflags {
> > -	/* Page consists entirely of zeros */
> > -	ZRAM_ZERO,
> > -
> > -	__NR_ZRAM_PAGEFLAGS,
> > -};
> > +static const unsigned long zero_page_handle = (unsigned long)(-1);
> >   
> >   /*-- Data structures */
> >   
> > -/* Allocated for each disk page */
> > -struct table {
> > -	unsigned long handle;
> > -	u16 size;	/* object size (excluding header) */
> > -	u8 count;	/* object ref count (not yet used) */
> > -	u8 flags;
> > -} __aligned(4);
> > -
> >   struct zram_stats {
> >   	u64 compr_size;		/* compressed size of pages stored */
> >   	u64 num_reads;		/* failed + successful */
> > @@ -90,7 +76,7 @@ struct zram {
> >   	struct zs_pool *mem_pool;
> >   	void *compress_workmem;
> >   	void *compress_buffer;
> > -	struct table *table;
> > +	unsigned long *handle;	/* memory handle for each disk page */
> >   	spinlock_t stat64_lock;	/* protect 64-bit stats */
> >   	struct rw_semaphore lock; /* protect compression buffers and table
> >   				   * against concurrent read and writes */

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