Re: [RFC PATCH] mm/zsmalloc: remove unnecessary check

[Minchan Kim <minchan@kernel.org>]

Hello Ganesh,

On Fri, Nov 21, 2014 at 10:56:10PM +0800, Ganesh Mahendran wrote:
> Hello Minchan
> 
> 2014-11-21 18:32 GMT+08:00 Minchan Kim <minchan@kernel.org>:
> > On Fri, Nov 21, 2014 at 06:48:49AM +0000, Minchan Kim wrote:
> >> On Fri, Nov 21, 2014 at 01:33:26PM +0800, Ganesh Mahendran wrote:
> >> > Hello
> >> >
> >> > 2014-11-21 11:54 GMT+08:00 Minchan Kim <minchan@kernel.org>:
> >> > > On Thu, Nov 20, 2014 at 09:21:56PM +0800, Mahendran Ganesh wrote:
> >> > >> ZS_SIZE_CLASSES is calc by:
> >> > >>   ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1)
> >> > >>
> >> > >> So when i is in [0, ZS_SIZE_CLASSES - 1), the size:
> >> > >>   size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA
> >> > >> will not be greater than ZS_MAX_ALLOC_SIZE
> >> > >>
> >> > >> This patch removes the unnecessary check.
> >> > >
> >> > > It depends on ZS_MIN_ALLOC_SIZE.
> >> > > For example, we would change min to 8 but MAX is still 4096.
> >> > > ZS_SIZE_CLASSES is (4096 - 8) / 16 + 1 = 256 so 8 + 255 * 16 = 4088,
> >> > > which exceeds the max.
> >> > Here, 4088 is less than MAX(4096).
> >> >
> >> > ZS_SIZE_CLASSES = (MAX - MIN) / Delta + 1
> >> > So, I think the value of
> >> >     MIN + (ZS_SIZE_CLASSES - 1) * Delta =
> >> >     MIN + ((MAX - MIN) / Delta) * Delta =
> >> >     MAX
> >> > will not exceed the MAX
> >>
> >> You're right. It was complext math for me.
> >> I should go back to elementary school.
> >>
> >> Thanks!
> >>
> >> Acked-by: Minchan Kim <minchan@kernel.org>
> >
> > I catch a nasty cold but above my poor math makes me think more.
> > ZS_SIZE_CLASSES is broken. In above my example, current code cannot
> > allocate 4096 size class so we should correct ZS_SIZE_CLASSES
> > at first.
> >
> > zs_size_classes = zs_max - zs_min / delta + 1;
> > if ((zs_max - zs_min) % delta)
> >         zs_size_classes += 1;
> Yes, you are right.
> When the zs_min is less than delta, we can not allocate PAGE_SIZE size class.
> 
> >
> > Then, we need to code piece you removed.
> > As well, we need to fix below.
> >
> > - area->vm_buf = (char *)__get_free_page(GFP_KERNEL);
> > + area->vm_buf = kmalloc(ZS_MAX_ALLOC_SIZE);
> If our purpose is to allocate the max obj size as len of PAGE_SIZE, we
> do not need to
> change this line. Since the ZS_MAX_ALLOC_SIZE will always be PAGE_SIZE

No, please don't assume ZS_MAX_ALLOC_SIZE is PAGE_SIZE forever.
Some reason could make buffer larger than PAGE_SIZE.
For example, we might put allocator's metadata into each object's head/tail.

-- 
Kind regards,
Minchan Kim

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