linux-kernel - Re: [PATCH] staging: zsmalloc: Ensure handle is never 0 on success

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Date:	Wed, 6 Nov 2013 15:46:19 -0800
From:	Nitin Gupta <ngupta@...are.org>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Olav Haugan <ohaugan@...eaurora.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan@...nel.org>, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] staging: zsmalloc: Ensure handle is never 0 on success

On Wed, Nov 6, 2013 at 2:10 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Wed, Nov 06, 2013 at 01:09:59PM -0800, Nitin Gupta wrote:
>> On Tue, Nov 5, 2013 at 5:56 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> > On Tue, Nov 05, 2013 at 04:54:12PM -0800, Olav Haugan wrote:
>> >> zsmalloc encodes a handle using the page pfn and an object
>> >> index. On some hardware platforms the pfn could be 0 and this
>> >> causes the encoded handle to be 0 which is interpreted as an
>> >> allocation failure.
>> >
>> > What platforms specifically have this issue?
>> >
>> >>
>> >> To prevent this false error we ensure that the encoded handle
>> >> will not be 0 when allocation succeeds.
>> >>
>> >> Change-Id: Ifff930dcf254915b497aec5cb36f152a5e5365d6
>> >
>> > What is this?  What can anyone do with it?
>> >
>> >> Signed-off-by: Olav Haugan <ohaugan@...eaurora.org>
>> >> ---
>> >>  drivers/staging/zsmalloc/zsmalloc-main.c | 4 ++--
>> >>  1 file changed, 2 insertions(+), 2 deletions(-)
>> >>
>> >> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
>> >> index 523b937..0e32c0f 100644
>> >> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>> >> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>> >> @@ -441,7 +441,7 @@ static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
>> >>       }
>> >>
>> >>       handle = page_to_pfn(page) << OBJ_INDEX_BITS;
>> >> -     handle |= (obj_idx & OBJ_INDEX_MASK);
>> >> +     handle |= ((obj_idx + 1) & OBJ_INDEX_MASK);
>> >>
>> >>       return (void *)handle;
>> >>  }
>> >> @@ -451,7 +451,7 @@ static void obj_handle_to_location(unsigned long handle, struct page **page,
>> >>                               unsigned long *obj_idx)
>> >>  {
>> >>       *page = pfn_to_page(handle >> OBJ_INDEX_BITS);
>> >> -     *obj_idx = handle & OBJ_INDEX_MASK;
>> >> +     *obj_idx = (handle & OBJ_INDEX_MASK) - 1;
>> >>  }
>> >
>> > I need someone who knows how to test this code to ack it before I can
>> > take it...
>> >
>> > And I thought we were deleting zsmalloc anyway, why are you using this
>> > code?  Isn't it no longer needed anymore?
>> >
>>
>> zsmalloc is used by zram. Other zstuff has switched to zbud since they
>> need to do shrinking which is much easier to implement with simpler
>> design of zbud. For zram, which is a block device, we don't do such
>> active shrinking, so uses zsmalloc which provides much better density.
>
> Ok, so what's the plan of getting these other things out of staging?

Other zstuff: zswap and zcache

1) zswap (along with zbud allocator, frontcache, cleancache) is
already out of staging into mm/ (by Seth Jennings)
2) zcache seems to have been completely removed (not sure if Dan ever
wants to reintroduce it)

> I'm getting really tired of them hanging around in here for many years
> now...
>

Minchan has tried many times to promote zram out of staging. This was
his most recent attempt:

https://lkml.org/lkml/2013/8/21/54

There he provided arguments for zram inclusion, how it can help in
situations where zswap can't and why generalizing /dev/ramX would
not be a great idea. So, cannot say why it wasn't picked up
for inclusion at that time.

> Should I just remove them if no one is working on getting them merged
> "properly"?
>

Please refer the mail thread (link above) and see Minchan's
justifications for zram.
If they don't sound convincing enough then please remove zram+zsmalloc
from staging.

Nitin
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