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

From: Igor Stoppa
Date: Thu Aug 03 2017 - 10:43:04 EST

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