Message-ID: <20121211075012.GG22698@blaptop>
Date:	Tue, 11 Dec 2012 16:50:12 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Greg KH <greg@...ah.com>, Jerome Marchand <jmarchan@...hat.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Sam Hansen <solid.se7en@...il.com>,
	Linux Driver Project <devel@...uxdriverproject.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: feel Re: [PATCH v2 1/2] zsmalloc: add function to query object
 size

On Mon, Dec 10, 2012 at 11:19:49PM -0800, Nitin Gupta wrote:
> On 12/10/2012 07:59 PM, Minchan Kim wrote:
> > On Fri, Dec 07, 2012 at 04:45:53PM -0800, Nitin Gupta wrote:
> >> On Sun, Dec 2, 2012 at 11:52 PM, Minchan Kim <minchan@...nel.org> 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@...il.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@...are.org>
> >>>>> Acked-by: Minchan Kim <minchan@...nel.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.
> >>>
> >>> 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
> > 
> > Not true. PFN 0 is valid. Why do you think pfn 0 is invalid?
> > 
> 
> I thought PFN 0 must be reserved or already used during bootup, so no
> chance of it getting swapped out and reaching zram.

Please. You can't assume it. :(

> 
> 
> >> 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
> > 
> > I don't get it. If zs_malloc return 0, it means FAILED. 
> > How can user pass it in obj_location_to_handle?
> > 
> 
> zsmalloc also returns 0 as a result of zero sized request. In general,
> malloc is supposed to treat zero-sized request as valid and is supposed
> to return either 0 or a valid pointer in that case. So, in that user
> should be able to pass on 0 to any zs_* API. Anyways, I don't consider

Look at zs_map_object.

        BUG_ON(!handle);

Am I seeing Unix zram?

> wither removing or keeping the assert as-is or just do BUG_ON(!page). so
> any future cleanups could cleanup this BUG_ON if you feel like.
> 
> >> is what this assert checks.
> > 
> > Although you are worry that someone might use the function in future,
> > Just enough with below.
> > 
> >         BUG_ON(!page);
> > 
> > The assertion isn't effecitve so we don't need that.
> > 
> 
> Ok, then we can clean this up in a future patch.

Nitin, Could you resend your patch without the assertion?
Apparently, it is wrong. Why should another person send follow-up patch to fix it
although we already know the problem during review process?
I don't want to make unnecessary git log churning.

> 
> 
> Thanks,
> Nitin
> 
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-- 
Kind regards,
Minchan Kim
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