Re: [PATCH 3/6] struct page: add field for vm_struct

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

On 02/02/18 20:43, Christopher Lameter wrote:
> On Thu, 1 Feb 2018, Igor Stoppa wrote:
> 
>>> Would it not be better to use compound page allocations here?

[...]

> Ok its compound_head(). See also the use in the SLAB and SLUB allocator.
> 
>> During hardened user copy permission check, I need to confirm if the
>> memory range that would be exposed to userspace is a legitimate
>> sub-range of a pmalloc allocation.
> 
> If you save the size in the head page struct then you could do that pretty
> fast.

Ok, now I get what you mean.
But it doesn't seem to fit the intended use case, for other reasons
(maybe the same, from 2 different POV):

- compound pages are aggregates of regular pages, in numbers that are
powers of 2, while the amount of pages to allocate is not known upfront.
One *could* give a hint to pmalloc about how many pages to allocate
every time there is a need to grow the pool.
Iow it would be the size of a chunk. But I'm afraid the granularity
would still be pretty low, so maybe it would be 2-4 times less.

- the property of the compound page will affect the property of all the
pages in the compound, so when one is write protected, it can generate a
lot of wasted memory, if there is too much slack (because of the order)
With vmalloc, I can allocate any number of pages, minimizing the waste.


Finally, there was a discussion about optimization:
http://www.openwall.com/lists/kernel-hardening/2017/08/07/2

The patch I sent does indeed take advantage of the new information, not
just for pmalloc use.

I have not measured if/where/what there is gain, but it does look like
the extra info can be exploited also elsewhere.

--
igor

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