[PATCH v2 1/2] zsmalloc: add function to query object size

[Nitin Gupta]

On Nov 30, 2012, at 5:54 AM, Minchan Kim <minchan.kernel.2 at gmail.com> wrote:

> On Thu, Nov 29, 2012 at 10:54:48PM -0800, Nitin Gupta wrote:
>> Changelog v2 vs v1:
>> - None
>> 
>> 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
>> 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.
>> 
>> Signed-off-by: Nitin Gupta <ngupta at vflare.org>
> Acked-by: Minchan Kim <minchan at kernel.org>
> 
> I already had a few comment in your previous versoin.
> I'm OK although you ignore them because I can make follow up patch about
> my nitpick but could you answer below my question?
> 
>> ---
>> 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)
>> +{
>> +    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);
>> +    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 */
>> +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?
> 
> 

This would catch cases where, say, user passes handle to a zero page to this function. In general, just a sanity check since pfn 0 and any non-zero offset is invalid.

Thanks,
Nitin