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

[Nitin Gupta]

On Sun, Dec 2, 2012 at 11:52 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Sun, Dec 02, 2012 at 11:20:42PM -0800, Nitin Gupta wrote:
>>
>>
>> On Nov 30, 2012, at 5:54 AM, Minchan Kim <minchan.kernel.2@xxxxxxxxx> 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@xxxxxxxxxx>
>> > Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
>> >
>> > 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.
>
> You mean zero_page is always pfn 0?
> No, we can't assume it.
>

Sorry, missed this point.    Though it's not correct to embed the
assumption that the zero-page handle will have pfn of 0, but in
general, PFN 0 is always invalid. However, for 0 size requests
zsmalloc returns NULL, so user should be also to pass <PFN=0,offset=0>
pair to this function. However, <PFN=0, offset!=0> is invalid and this
is what this assert checks.

Thanks,
Nitin
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