On 07/08/17 16:31, Jerome Glisse wrote:

On Mon, Aug 07, 2017 at 02:26:21PM +0300, Igor Stoppa wrote:

[...]

quoted

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.

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On 03/08/17 14:48, Michal Hocko wrote:

On Thu 03-08-17 13:11:45, Igor Stoppa wrote:

[...]

quoted

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.

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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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