Re: [PATCH 1/2] zsmalloc: add function to query object size

[Nitin Gupta <ngupta@xxxxxxxxxx>]

On 11/28/2012 11:45 PM, Minchan Kim wrote:
> On Mon, Nov 26, 2012 at 11:26:40PM -0800, Nitin Gupta wrote:
> > Adds zs_get_object_size(handle) which provides the size of
> > the given object. This is useful since the user (zram etc.)
> > now do not have to maintain object sizes separately, saving
> > on some metadata size (4b per page).
> >
> > The object handle encodes <page, offset> pair which currently points
>
> Nitpick.
>
> <page, index> in descrption would be proper instead of
> <page, offset>. You are going to replace <page, idx> with <page, offset>.

I think 'offset' conveys the meaning more clearly; 'index' is after all 
just a chopped-off version of offset :)


> > to the start of the object. Now, the handle implicitly stores the size
> > information by pointing to the object's end instead. Since zsmalloc is
> > a slab based allocator, the start of the object can be easily determined
> > and the difference between the end offset encoded in the handle and the
> > start gives us the object size.
>
> It's a good idea. Look at just minor comment below.
>
> Let's talk with another concern. This patch surely helps current
> customer's memory usage who should add 4 byte for accounting the
> statistics while zsmalloc could be slow down.
> Is it really valuable?
>
> Yes. zram/zcache had a tight coupling with zsmalloc so it already
> lost modularity. :( In this POV, this patch makes sense.
> But if someone are willing to remove statistics for performance?
> Although he remove it, zsmalloc is still slow down.
>
> Statistic for user of zsmalloc should be cost of user himself, not zsmalloc
> and it accelerates dependency with customer so it makes changing allocator
> hard in future. We already had such experience(xvmalloc->zsmalloc). Of course,
> it's not good that worry future things too early without any plan.
> So I'm not strong againt you. If any reviewer don't raise an eyebrow,
> I wil rely on your decision.

Looking over the changes I do not expect any difference in performance 
-- just a bit more arithmetic, however the use of get_prev_page() which 
may dereference a few extra pointers might not be really free. Also, my 
iozone test[1] shows very little difference in performance:

With just the fix for crash (patch 1/2):
9.28user 1159.84system 21:46.54elapsed 89%CPU (0avgtext+0avgdata 
50200maxresident)k
212988544inputs+190660816outputs (1major+16706minor)pagefaults 0swaps

With the new get_object_size() code (patch 1/2 + patch 2/2):
9.20user 1112.63system 21:03.61elapsed 88%CPU (0avgtext+0avgdata 
50200maxresident)k
194636568inputs+190500424outputs (1major+16707minor)pagefaults 0swaps

I really cannot explain this ~40s speedup but anyways, I think 
optimizing zsmalloc/zram should be taken up separately, at least when 
this new code does not seem to have any significant effects.


[1] iozone test:
 - Create zram of size 1200m
 - Create ext4 fs
 - iozone -a -g 1G
 - In parallel: watch zram_stress

# zram_stress
sync
echo 1 | sudo tee /proc/sys/vm/drop_caches


Surely, not the most accurate of the tests but gives an idea if anything 
is making a significant difference.

Thanks,
Nitin


> > Signed-off-by: Nitin Gupta <ngupta@xxxxxxxxxx>
> > ---
> >   drivers/staging/zsmalloc/zsmalloc-main.c |  177 +++++++++++++++++++++---------
> >   drivers/staging/zsmalloc/zsmalloc.h      |    1 +
> >   2 files changed, 127 insertions(+), 51 deletions(-)
> >
> > diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> > index 09a9d35..65c9d3b 100644
> > --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> > +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> > @@ -112,20 +112,20 @@
> >   #define MAX_PHYSMEM_BITS 36
> >   #else /* !CONFIG_HIGHMEM64G */
> >   /*
> > - * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
> > + * If this definition of MAX_PHYSMEM_BITS is used, OFFSET_BITS will just
> >    * be PAGE_SHIFT
> >    */
> >   #define MAX_PHYSMEM_BITS BITS_PER_LONG
> >   #endif
> >   #endif
> >   #define _PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
> > -#define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS)
> > -#define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
> > +#define OFFSET_BITS	(BITS_PER_LONG - _PFN_BITS)
> > +#define OFFSET_MASK	((_AC(1, UL) << OFFSET_BITS) - 1)
> >
> >   #define MAX(a, b) ((a) >= (b) ? (a) : (b))
> >   /* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
> >   #define ZS_MIN_ALLOC_SIZE \
> > -	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
> > +	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OFFSET_BITS))
> >   #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
> >
> >   /*
> > @@ -256,6 +256,11 @@ static int is_last_page(struct page *page)
> >   	return PagePrivate2(page);
> >   }
> >
> > +static unsigned long get_page_index(struct page *page)
>
> IMO, first_obj_offset(struct page *page) would be better readable.
>
> > +{
> > +	return is_first_page(page) ? 0 : page->index;
> > +}
> > +
> >   static void get_zspage_mapping(struct page *page, unsigned int *class_idx,
> >   				enum fullness_group *fullness)
> >   {
> > @@ -433,39 +438,86 @@ static struct page *get_next_page(struct page *page)
> >   	return next;
> >   }
> >
> > -/* Encode <page, obj_idx> as a single handle value */
> > -static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
> > +static struct page *get_prev_page(struct page *page)
> >   {
> > -	unsigned long handle;
> > +	struct page *prev, *first_page;
> >
> > -	if (!page) {
> > -		BUG_ON(obj_idx);
> > -		return NULL;
> > -	}
> > +	first_page = get_first_page(page);
> > +	if (page == first_page)
> > +		prev = NULL;
> > +	else if (page == (struct page *)first_page->private)
> > +		prev = first_page;
> > +	else
> > +		prev = list_entry(page->lru.prev, struct page, lru);
> >
> > -	handle = page_to_pfn(page) << OBJ_INDEX_BITS;
> > -	handle |= (obj_idx & OBJ_INDEX_MASK);
> > +	return prev;
> >
> > -	return (void *)handle;
> >   }
> >
> > -/* Decode <page, obj_idx> pair from the given object handle */
> > -static void obj_handle_to_location(unsigned long handle, struct page **page,
> > -				unsigned long *obj_idx)
> > +static void *encode_ptr(struct page *page, unsigned long offset)
> >   {
> > -	*page = pfn_to_page(handle >> OBJ_INDEX_BITS);
> > -	*obj_idx = handle & OBJ_INDEX_MASK;
> > +	unsigned long ptr;
> > +	ptr = page_to_pfn(page) << OFFSET_BITS;
> > +	ptr |= offset & OFFSET_MASK;
> > +	return (void *)ptr;
> > +}
> > +
> > +static void decode_ptr(unsigned long ptr, struct page **page,
> > +					unsigned int *offset)
> > +{
> > +	*page = pfn_to_page(ptr >> OFFSET_BITS);
> > +	*offset = ptr & OFFSET_MASK;
> > +}
> > +
> > +static struct page *obj_handle_to_page(unsigned long handle)
> > +{
> > +	struct page *page;
> > +	unsigned int offset;
> > +
> > +	decode_ptr(handle, &page, &offset);
> > +	if (offset < get_page_index(page))
> > +		page = get_prev_page(page);
> > +
> > +	return page;
> > +}
> > +
> > +static unsigned int obj_handle_to_offset(unsigned long handle,
> > +					unsigned int class_size)
> > +{
> > +	struct page *page;
> > +	unsigned int offset;
> > +
> > +	decode_ptr(handle, &page, &offset);
> > +	if (offset < get_page_index(page))
> > +		offset = PAGE_SIZE - class_size + get_page_index(page);
>
> Althoug it's trivial, we can reduce get_page_index calling.
>
> > +	else
> > +		offset = roundup(offset, class_size) - class_size;
> > +
> > +	return offset;
> >   }
> >
> > -static unsigned long obj_idx_to_offset(struct page *page,
> > -				unsigned long obj_idx, int class_size)
> > +/* Encode <page, offset, size> as a single handle value */
>
> Need more kind comment about encoding scheme with obj's end <page, offset>
>
> > +static void *obj_location_to_handle(struct page *page, unsigned int offset,
> > +				unsigned int size, unsigned int class_size)
> >   {
> > -	unsigned long off = 0;
> > +	struct page *endpage;
> > +	unsigned int endoffset;
> >
> > -	if (!is_first_page(page))
> > -		off = page->index;
> > +	if (!page) {
> > +		BUG_ON(offset);
> > +		return NULL;
> > +	}
>
> What do you expect to catch with above check?
>
> > +	BUG_ON(offset >= PAGE_SIZE);
> > +
> > +	endpage = page;
> > +	endoffset = offset + size - 1;
> > +	if (endoffset >= PAGE_SIZE) {
> > +		endpage = get_next_page(page);
> > +		BUG_ON(!endpage);
> > +		endoffset -= PAGE_SIZE;
> > +	}
> >
> > -	return off + obj_idx * class_size;
> > +	return encode_ptr(endpage, endoffset);
> >   }
> >
> >   static void reset_page(struct page *page)
> > @@ -506,14 +558,13 @@ static void free_zspage(struct page *first_page)
> >   /* Initialize a newly allocated zspage */
> >   static void init_zspage(struct page *first_page, struct size_class *class)
> >   {
> > -	unsigned long off = 0;
> > +	unsigned long off = 0, next_off = 0;
> >   	struct page *page = first_page;
> >
> >   	BUG_ON(!is_first_page(first_page));
> >   	while (page) {
> >   		struct page *next_page;
> >   		struct link_free *link;
> > -		unsigned int i, objs_on_page;
> >
> >   		/*
> >   		 * page->index stores offset of first object starting
> > @@ -526,14 +577,12 @@ static void init_zspage(struct page *first_page, struct size_class *class)
> >
> >   		link = (struct link_free *)kmap_atomic(page) +
> >   						off / sizeof(*link);
> > -		objs_on_page = (PAGE_SIZE - off) / class->size;
> >
> > -		for (i = 1; i <= objs_on_page; i++) {
> > -			off += class->size;
> > -			if (off < PAGE_SIZE) {
> > -				link->next = obj_location_to_handle(page, i);
> > -				link += class->size / sizeof(*link);
> > -			}
> > +		next_off = off + class->size;
> > +		while (next_off < PAGE_SIZE) {
> > +			link->next = encode_ptr(page, next_off);
> > +			link += class->size / sizeof(*link);
> > +			next_off += class->size;
> >   		}
> >
> >   		/*
> > @@ -542,10 +591,11 @@ static void init_zspage(struct page *first_page, struct size_class *class)
> >   		 * page (if present)
> >   		 */
> >   		next_page = get_next_page(page);
> > -		link->next = obj_location_to_handle(next_page, 0);
> > +		next_off = next_page ? next_off - PAGE_SIZE : 0;
> > +		link->next = encode_ptr(next_page, next_off);
> >   		kunmap_atomic(link);
> >   		page = next_page;
> > -		off = (off + class->size) % PAGE_SIZE;
> > +		off = next_off;
> >   	}
> >   }
> >
> > @@ -596,7 +646,7 @@ static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
> >
> >   	init_zspage(first_page, class);
> >
> > -	first_page->freelist = obj_location_to_handle(first_page, 0);
> > +	first_page->freelist = encode_ptr(first_page, 0);
> >   	/* Maximum number of objects we can store in this zspage */
> >   	first_page->objects = class->pages_per_zspage * PAGE_SIZE / class->size;
> >
> > @@ -871,7 +921,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
> >   	struct size_class *class;
> >
> >   	struct page *first_page, *m_page;
> > -	unsigned long m_objidx, m_offset;
> > +	unsigned int m_offset;
> >
> >   	if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
> >   		return 0;
> > @@ -895,8 +945,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
> >   	}
> >
> >   	obj = (unsigned long)first_page->freelist;
> > -	obj_handle_to_location(obj, &m_page, &m_objidx);
> > -	m_offset = obj_idx_to_offset(m_page, m_objidx, class->size);
> > +	decode_ptr(obj, &m_page, &m_offset);
> >
> >   	link = (struct link_free *)kmap_atomic(m_page) +
> >   					m_offset / sizeof(*link);
> > @@ -907,6 +956,9 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
> >   	first_page->inuse++;
> >   	/* Now move the zspage to another fullness group, if required */
> >   	fix_fullness_group(pool, first_page);
> > +
> > +	obj = (unsigned long)obj_location_to_handle(m_page, m_offset,
> > +						size, class->size);
> >   	spin_unlock(&class->lock);
> >
> >   	return obj;
> > @@ -917,7 +969,7 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
> >   {
> >   	struct link_free *link;
> >   	struct page *first_page, *f_page;
> > -	unsigned long f_objidx, f_offset;
> > +	unsigned long f_offset;
> >
> >   	int class_idx;
> >   	struct size_class *class;
> > @@ -926,12 +978,12 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
> >   	if (unlikely(!obj))
> >   		return;
> >
> > -	obj_handle_to_location(obj, &f_page, &f_objidx);
> > +	f_page = obj_handle_to_page(obj);
> >   	first_page = get_first_page(f_page);
> >
> >   	get_zspage_mapping(first_page, &class_idx, &fullness);
> >   	class = &pool->size_class[class_idx];
> > -	f_offset = obj_idx_to_offset(f_page, f_objidx, class->size);
> > +	f_offset = obj_handle_to_offset(obj, class->size);
> >
> >   	spin_lock(&class->lock);
> >
> > @@ -940,7 +992,7 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
> >   							+ f_offset);
> >   	link->next = first_page->freelist;
> >   	kunmap_atomic(link);
> > -	first_page->freelist = (void *)obj;
> > +	first_page->freelist = encode_ptr(f_page, f_offset);
> >
> >   	first_page->inuse--;
> >   	fullness = fix_fullness_group(pool, first_page);
> > @@ -970,10 +1022,10 @@ EXPORT_SYMBOL_GPL(zs_free);
> >    * This function returns with preemption and page faults disabled.
> >   */
> >   void *zs_map_object(struct zs_pool *pool, unsigned long handle,
> > -			enum zs_mapmode mm)
> > +					enum zs_mapmode mm)
> >   {
> >   	struct page *page;
> > -	unsigned long obj_idx, off;
> > +	unsigned long off;
> >
> >   	unsigned int class_idx;
> >   	enum fullness_group fg;
> > @@ -990,10 +1042,10 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
> >   	 */
> >   	BUG_ON(in_interrupt());
> >
> > -	obj_handle_to_location(handle, &page, &obj_idx);
> > +	page = obj_handle_to_page(handle);
> >   	get_zspage_mapping(get_first_page(page), &class_idx, &fg);
> >   	class = &pool->size_class[class_idx];
> > -	off = obj_idx_to_offset(page, obj_idx, class->size);
> > +	off = obj_handle_to_offset(handle, class->size);
> >
> >   	area = &get_cpu_var(zs_map_area);
> >   	area->vm_mm = mm;
> > @@ -1015,7 +1067,7 @@ EXPORT_SYMBOL_GPL(zs_map_object);
> >   void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
> >   {
> >   	struct page *page;
> > -	unsigned long obj_idx, off;
> > +	unsigned long off;
> >
> >   	unsigned int class_idx;
> >   	enum fullness_group fg;
> > @@ -1024,10 +1076,10 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
> >
> >   	BUG_ON(!handle);
> >
> > -	obj_handle_to_location(handle, &page, &obj_idx);
> > +	page = obj_handle_to_page(handle);
> >   	get_zspage_mapping(get_first_page(page), &class_idx, &fg);
> >   	class = &pool->size_class[class_idx];
> > -	off = obj_idx_to_offset(page, obj_idx, class->size);
> > +	off = obj_handle_to_offset(handle, class->size);
> >
> >   	area = &__get_cpu_var(zs_map_area);
> >   	if (off + class->size <= PAGE_SIZE)
> > @@ -1045,6 +1097,29 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
> >   }
> >   EXPORT_SYMBOL_GPL(zs_unmap_object);
> >
> > +size_t zs_get_object_size(struct zs_pool *pool, unsigned long handle)
> > +{
> > +	struct page *endpage;
> > +	unsigned int endoffset, size;
> > +
> > +	unsigned int class_idx;
> > +	enum fullness_group fg;
> > +	struct size_class *class;
> > +
> > +	decode_ptr(handle, &endpage, &endoffset);
> > +	get_zspage_mapping(endpage, &class_idx, &fg);
> > +	class = &pool->size_class[class_idx];
> > +
> > +	size = endoffset + 1;
> > +	if (endoffset < get_page_index(endpage))
> > +		size += class->size - get_page_index(endpage);
> > +	else
> > +		size -= rounddown(endoffset, class->size);
> > +
> > +	return size;
> > +}
> > +EXPORT_SYMBOL_GPL(zs_get_object_size);
> > +
> >   u64 zs_get_total_size_bytes(struct zs_pool *pool)
> >   {
> >   	int i;
> > diff --git a/drivers/staging/zsmalloc/zsmalloc.h b/drivers/staging/zsmalloc/zsmalloc.h
> > index de2e8bf..2830fdf 100644
> > --- a/drivers/staging/zsmalloc/zsmalloc.h
> > +++ b/drivers/staging/zsmalloc/zsmalloc.h
> > @@ -38,6 +38,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
> >   			enum zs_mapmode mm);
> >   void zs_unmap_object(struct zs_pool *pool, unsigned long handle);
> >
> > +size_t zs_get_object_size(struct zs_pool *pool, unsigned long handle);
> >   u64 zs_get_total_size_bytes(struct zs_pool *pool);
> >
> >   #endif
> > --
> > 1.7.10.4
> >
> > --
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