Re: [PATCH] Revert "mm/vmalloc: interchage the implementation of vmalloc_to_{pfn,page}"

[Vladimir Murzin <murzin.v@gmail.com>]

Hi Andrew

On Fri, Jan 24, 2014 at 2:49 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Thu, 23 Jan 2014 20:27:29 +0400 (MSK) malc <av1474@comtv.ru> wrote:
>
>> Sep 17 00:00:00 2001
>> From: Vladimir Murzin <murzin.v@gmail.com>
>> Date: Thu, 23 Jan 2014 14:54:20 +0400
>> Subject: [PATCH] Revert "mm/vmalloc: interchage the implementation of
>>  vmalloc_to_{pfn,page}"
>>
>> This reverts commit ece86e222db48d04bda218a2be70e384518bb08c.
>>
>> Despite being claimed that patch doesn't introduce any functional
>> changes in fact it does.
>>
>> The "no page" path behaves different now. Originally, vmalloc_to_page
>> might return NULL under some conditions, with new implementation it returns
>> pfn_to_page(0) which is not the same as NULL.
>>
>> Simple test shows the difference.
>>
>> test.c
>>
>> #include <linux/kernel.h>
>> #include <linux/module.h>
>> #include <linux/vmalloc.h>
>> #include <linux/mm.h>
>>
>> int __init myi(void)
>> {
>>       struct page *p;
>>       void *v;
>>
>>       v = vmalloc(PAGE_SIZE);
>>       /* trigger the "no page" path in vmalloc_to_page*/
>>       vfree(v);
>>
>>       p = vmalloc_to_page(v);
>>
>>       pr_err("expected val = NULL, returned val = %p", p);
>>
>>       return -EBUSY;
>> }
>>
>> void __exit mye(void)
>> {
>>
>> }
>> module_init(myi)
>> module_exit(mye)
>>
>> Before interchange:
>> expected val = NULL, returned val =   (null)
>>
>> After interchange:
>> expected val = NULL, returned val = c7ebe000
>>
>
> hm, yes, I suppose that's bad.
>
> Rather than reverting the patch we could fix up vmalloc_to_pfn() and/or
> vmalloc_to_page() to handle this situation.  Did you try that?
>

Personally, I didn't try; I leaved this responsibility to the author
of the patch
as a review feedback. Unfortunately, there was no any response.

Being said that original patch makes vmalloc_to_* "slightly more efficient",
I'm in doubt that with additional handling it'd still improve something. I'd be
very glad if someone point me at the benefit of the patch - just to have an
idea why we need to put extra effort here.

Thanks
Vladimir

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