linux-kernel - Re: [RFC] Tagging of vmalloc pages for supporting the pmalloc allocator

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Date:   Thu, 3 Aug 2017 17:41:24 +0300
From:   Igor Stoppa <igor.stoppa@...wei.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Jerome Glisse <jglisse@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [RFC] Tagging of vmalloc pages for supporting the pmalloc
 allocator



On 03/08/17 16:55, Michal Hocko wrote:
> On Thu 03-08-17 15:20:31, Igor Stoppa wrote:
>> On 03/08/17 14:48, Michal Hocko wrote:
>>> On Thu 03-08-17 13:11:45, Igor Stoppa wrote:

[...]

>>>> But, to reply more specifically to your advice, yes, I think I could add
>>>> a flag to vm_struct and then retrieve its value, for the address being
>>>> processed, by passing through find_vm_area().
>>>
>>> ... and you can store vm_struct pointer to the struct page there 
>>
>> "there" as in the new field of the union?
>> btw, what would be a meaningful name, since "private" is already taken?
>>
>> For simplicity, I'll use, for now, "private2"
> 
> why not explicit vm_area?

ok :-)

>>> and you won't need to do the slow find_vm_area. I haven't checked
>> very closely
>>> but this should be possible in principle. I guess other callers might
>>> benefit from this as well.
>>
>> I am confused about this: if "private2" is a pointer, but when I get an
>> address, I do not even know if the address represents a valid pmalloc
>> page, how can i know when it's ok to dereference "private2"?
> 
> because you can make all pages which back vmalloc mappings have vm_area
> pointer set.

Ah, now I see, I had missed that the field would be set for *all* pages
backed by vmalloc.

So, given a pointer, I still have to figure out if it refers to a
vmalloc area or not.

However, that is something I need to do anyway, to get the reference to
the corresponding page struct, in case it is indeed a vmalloc address.

--
thanks, igor