Re: [PATCH, RFC] x86/mm/pat: Restore large pages after fragmentation

[Kirill A. Shutemov]

On Thu, Apr 16, 2020 at 03:03:12PM -0700, Dave Hansen wrote:
> On 4/16/20 2:32 PM, Kirill A. Shutemov wrote:
> > With current code it's one way road: kernel tries to avoid splitting
> > large pages, but it doesn't restore them back even if page attributes
> > got compatible again.
> 
> Looks pretty sane to me, and sounds like something we've needed for a
> long time.
> 
> I'd having doubts in my ability to find nasty corner cases in this code,
> though.  Could you rig up some tests to poke at this thing further?  Maybe:
> 
> Record what the direct map looks like (even from userspace).  Then,
> allocate some memory, including odd-sized and aligned ranges.  Try to do
> things >4M like the hugetlbfs code does.  Make the allocation (or a
> piece of it) not-present (or whatever), which usually fractures some
> large pages.  Then put it back the way it was.  All the large pages
> should come back.
> 
> If it survives that for an hour or two, it should be pretty good to go.
>  Basically, fuzz it.

We already have it in kernel: CONFIG_CPA_DEBUG. It messes up with the
mapping every 30 seconds. It is pretty good for the change too. It
produces a lot of 2M/1G pages to be restored. I run it over night in my
setup and it survives.

-- 
 Kirill A. Shutemov