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

[Igor Stoppa <igor.stoppa@huawei.com>]

On 07/08/17 16:31, Jerome Glisse wrote:
> On Mon, Aug 07, 2017 at 02:26:21PM +0300, Igor Stoppa wrote:

[...]

>> I'll add a vm_area field as you advised.
>>
>> Is this something I could send as standalone patch?
> 
> Note that vmalloc() is not the only thing that use vmalloc address
> space. There is also vmap() and i know one set of drivers that use
> vmap() and also use the mapping field of struct page namely GPU
> drivers.

Ah, yes, you mentioned this.

> So like i said previously i would store a flag inside vm_struct to
> know if page you are looking at are pmalloc or not.

And I was planning to follow your advice, using one of the flags.
But ...

> Again do you
> need to store something per page ? Would storing it per vm_struct
> not be enough ?

... there was this further comment, about speeding up the access to
vm_area, which seemed good from performance perspective.

---8<--------------8<--------------8<--------------8<--------------8<---
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 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.
---8<--------------8<--------------8<--------------8<--------------8<---

I do not strictly need to modify the page struct, but it seems it might
harm performance, if it is added on the path of hardened usercopy.

I have an updated version of the old proposal:

* put a magic number in the private field, during initialization of
pmalloc pages

* during hardened usercopy verification, when I have to assess if a page
is of pmalloc type, compare the private field against the magic number

* if and only if the private field matches the magic number, then invoke
find_vm_area(), so that the slowness affects only a possibly limited
amount of false positives.


--
igor

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