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Re: [Qemu-devel] Dual userfaultfd behavior

From: Andrea Arcangeli
Subject: Re: [Qemu-devel] Dual userfaultfd behavior
Date: Mon, 13 Mar 2017 22:46:36 +0100
User-agent: Mutt/1.8.0 (2017-02-23)


On Mon, Mar 13, 2017 at 10:53:39AM +0000, Dr. David Alan Gilbert wrote:
> * Alexey Perevalov (address@hidden) wrote:
> > Hi, David, Andrea and Mike
> Hi Alexey,
> > The problem I want to discuss it's 1G hugepage based VM and post copy live
> > migration.
> > 
> > I would like to know your opinion on following approach of avoiding such
> > problem:
> > Once we have mmap'ed area through 1G hugetlbfs, remap physical pages
> > with /dev/mem. It will be 2 types of vmas mapped to the same PFN.
> > Register userfaultfd for newly obtained virtual
> > addresses, it could reduce granularity of pages and reduce downtime per
> > one 1G page. So registering userfaultfd for 2Mb, when the real hugepage
> > was 1G, I think, could help.
> > 
> > Current postcopy implementation in QEMU allows to make live migration
> > from 1G based hugepage VM to 2Mb based hugepages VM (sanity checks prevent
> > it).
> > 
> > Also I checked, it's possible to remap through /dev/mem and get PFN
> > based vmas, register userfaultfd (with allowance in vma_can_userfault)
> > and finally make UFFDIO_COPY with allowing PFN based vmas in __mcopy_atomic.
> > 
> > But there are a lot of drawback of such approach:
> > First of all it's /dev/mem interface. Need to provide full access
> > (kernel w/o CONFIG_STRICT_DEVMEM) and need to disable PAT.
> > The second drawback, maybe I just didn't find possibility to remap
> > hugepages again, but mmap of /dev/mem character driver maps 4Kb pages.
> > I don't know how THP could help here, but madvise with MADV_HUGEPAGE
> > didn't. So 4Kb is not exactly what needed, due to overhead of
> > encapsulation summary downtime is worse than in other cases.
> > It would be great to have interface to obtain new virtual address based
> > on existing PFN, but for hugepages.
> Yes, and I think as well on some architectures there can be cache problems
> from mapping the same page in two addresses unless we're careful.
> I think to do this we'd basically need the kernel to set up something
> similar to what you're saying, but without the mess of having to
> go via /dev/mem.  Ideally it would all happen magically when I mark

/dev/mem is problematic also if the hugetlbfs page is being migrated
for example (the virtual2physical mapping that may change and be
directed to a different physical page with no notification to userland).

> a hugetlb page as userfault and start UFFDIO_COPYing in 4kb pages;
> but I can imagine perhaps some more syscalls needed to tell it to do it.

I think what is needed is an extension to UFFDIO_COPY so that it will
not fail if used with lower granularity than the hugetlbfs page
size. Then a special feature flag should be set in the
uffdio_api.features to tell userland it can use a lower granularity
than the hugetlbfs page size.

UFFDIO_COPY will still allocate a 1GB page, but it will copy only part
of and then map only the copied part with a pte of 4k granularity (or
2MB granularity if the uffdio_copy.dst/len parameters allows for
hugepmds). This way if there's a missing fault in the part that is not
copied yet, it'll still trigger a page fault and we'll call
handle_userfault from hugetlb_no_page, which will trigger a new
UFFDIO_COPY and map another fragment of the 1GB page.

The same code than should allow us to map a 2MB hugetlbfs page with 4k
granularity in UFFDIO_COPY so then userland can choose if to do
postcopy live migration on hugetlbfs with 2MB or 4kb granularity (on
very slow network 4kb would be preferable for example for the latency).

Mapping a 1GB page without a hugepud or mapping a 2MB page without a
hugepmd breaks all sort of invariants into the hugetlbfs code, page
migration wouldn't be able to cope with it either. Solving the fallout
from such breakage is what is required to implement this basically.

Ideally this could be a UFFDIO_COPY-only feature, simply every other
part of hugetlbfs should be able to cope with whatever UFFDIO_COPY has
generated in terms of pagetables mapping those hugepages.

The only thing that will change from hugetlbfs point of view is that a
pte or hugepmd could be mapping a 1GB page, that couldn't be the case
before it.

Once the entirety of the hugepage has been mapped or when the
UFFDIO_UNREGISTER is called on the 1GB range, all those hugepmds
and/or ptes should be dropped and the hugepud would then point
directly to the 1GB page restoring the current behavior (which is
required to get a 1GB large TLB and top performance, i.e. immediately
after the live migration completed).

This is very different concept than THP that in fact never requires to
allocate hugepages. THP nowadays also allows to map a THP page using
ptes, instead of hugepmds, just for a different reason (i.e. to allow
split_huge_page to fail and be deferred, so get/put_page are
simplified, while still guaranteeing that split_huge_page_pmd cannot
fail). So we're already doing something like the above with THP but we
only cover ptes vs hugepmd. The codebase however is different,
hugetlbfs doesn't interact much with the main VM, so there's likely
nothing to share with THP, but the concept of mapping an hugepage with
ptes is somewhat similar.

THP fundamentally always allows fallback, in the UFFDIO_COPY on
hugetlbfs instead the idea would be to never allow fallback on the
physical side, but to allow partial mapping so that the UFFDIO_COPY
granularity can be reduced, despite the physical granularity remains
native. This also means the collapse after live migration will be
zerocopy: the collapse would be of the virtual kind for hugetlbfs
(khugepaged as opposed has to copy the actual data because the
physical pages were not huge to begin with, to allow the fallback).

When UFFDIO_COPY is called again on the same 1GB range, it would need
to notice that the pud is not null but is not huge (new case
introduced), and then keep going in the pagetables lookup until it
finds the copied range is not mapped, skip the 1GB allocation in such
case and copy into the pre-existing 1GB page. If instead it finds the
dst range is already mapped by a pte or a hugepmd, it'll instead
return -EEXIST as usual.


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