linux-kernel - Re: Pinning ZONE

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Date:   Mon, 23 Nov 2020 10:31:16 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, sthemmin@...rosoft.com
Subject: Re: Pinning ZONE_MOVABLE pages

> > I've spoken with Stephen Hemminger, and he said that DPDK is moving in
> > the direction of using transparent huge pages instead of HugeTLBs,
> > which means that we need to allow at least anonymous, and anonymous
> > transparent huge pages to come from non-movable zones on demand.
> >
>
> I'd like to know more about this use case, ZONE_MOVABLE is typically a
> great way to optimize for thp availability because, absent memory pinning,
> this memory can always be defragmented.  So the idea is that DPDK will now
> allocate all of its thp from ZONE_NORMAL or only a small subset?  Seems
> like an invitation for oom kill if the sizing of ZONE_NORMAL is
> insufficient.

The idea is to allocate only those THP and anon pages that are long
term pinned from ZONE_NORMAL, the rest can still be allocated from
ZONE_MOVABLE.

>
> > Here is what I am proposing:
> > 1. Add a new flag that is passed through pin_user_pages_* down to
> > fault handlers, and allow the fault handler to allocate from a
> > non-movable zone.
> >
> > Sample function stacks through which this info needs to be passed is this:
> >
> > pin_user_pages_remote(gup_flags)
> >  __get_user_pages_remote(gup_flags)
> >   __gup_longterm_locked(gup_flags)
> >    __get_user_pages_locked(gup_flags)
> >     __get_user_pages(gup_flags)
> >      faultin_page(gup_flags)
> >       Convert gup_flags into fault_flags
> >       handle_mm_fault(fault_flags)
> >
> > From handle_mm_fault(), the stack diverges into various faults,
> > examples include:
> >
> > Transparent Huge Page
> > handle_mm_fault(fault_flags)
> > __handle_mm_fault(fault_flags)
> > Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask
> > create_huge_pmd(vmf);
> > do_huge_pmd_anonymous_page(vmf);
> > mm_get_huge_zero_page(vma->vm_mm); -> flag is lost, so flag from
> > vmf.gfp_mask should be passed as well.
> >
> > There are several other similar paths in a transparent huge page, also
> > there is a named path where allocation is based on filesystems, and
> > the flag should be honored there as well, but it does not have to be
> > added at the same time.
> >
> > Regular Pages
> > handle_mm_fault(fault_flags)
> > __handle_mm_fault(fault_flags)
> > Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask
> > handle_pte_fault(vmf)
> > do_anonymous_page(vmf);
> > page = alloc_zeroed_user_highpage_movable(vma, vmf->address); ->
> > replace change this call according to gfp_mask.
> >
>
> This would likely be useful for AMD SEV as well, which requires guest
> pages to be pinned because the encryption algorithm depends on the host
> physical address.  This ensures that plaintext memory for two pages don't
> result in the same ciphertext.