[PATCH] mm: fix hugetlb page unmap count balance issue

[Kasireddy, Vivek]

Hi Gerd,

> 
> On Mon, May 15, 2023 at 10:04:42AM -0700, Mike Kravetz wrote:
> > On 05/12/23 16:29, Mike Kravetz wrote:
> > > On 05/12/23 14:26, James Houghton wrote:
> > > > On Fri, May 12, 2023 at 12:20 AM Junxiao Chang
> <junxiao.chang at intel.com> wrote:
> > > >
> > > > This alone doesn't fix mapcounting for PTE-mapped HugeTLB pages.
> You
> > > > need something like [1]. I can resend it if that's what we should be
> > > > doing, but this mapcounting scheme doesn't work when the page
> structs
> > > > have been freed.
> > > >
> > > > It seems like it was a mistake to include support for hugetlb memfds in
> udmabuf.
> > >
> > > IIUC, it was added with commit 16c243e99d33 udmabuf: Add support for
> mapping
> > > hugepages (v4).  Looks like it was never sent to linux-mm?  That is
> unfortunate
> > > as hugetlb vmemmap freeing went in at about the same time.  And, as
> you have
> > > noted udmabuf will not work if hugetlb vmemmap freeing is enabled.
> > >
> > > Sigh!
> > >
> > > Trying to think of a way forward.
> > > --
> > > Mike Kravetz
> > >
> > > >
> > > > [1]: https://lore.kernel.org/linux-mm/20230306230004.1387007-2-
> jthoughton at google.com/
> > > >
> > > > - James
> >
> > Adding people and list on Cc: involved with commit 16c243e99d33.
> >
> > There are several issues with trying to map tail pages of hugetllb pages
> > not taken into account with udmabuf.  James spent quite a bit of time
> trying
> > to understand and address all the issues with the HGM code.  While using
> > the scheme proposed by James, may be an approach to the mapcount
> issue there
> > are also other issues that need attention.  For example, I do not see how
> > the fault code checks the state of the hugetlb page (such as poison) as none
> > of that state is carried in tail pages.
> >
> > The more I think about it, the more I think udmabuf should treat hugetlb
> > pages as hugetlb pages.  They should be mapped at the appropriate level
> > in the page table.  Of course, this would impose new restrictions on the
> > API (mmap and ioctl) that may break existing users.  I have no idea how
> > extensively udmabuf is being used with hugetlb mappings.
> 
> User of this is qemu.  It can use the udmabuf driver to create host
> dma-bufs for guest resources (virtio-gpu buffers), to avoid copying
> data when showing the guest display in a host window.
> 
> hugetlb support is needed in case qemu guest memory is backed by
> hugetlbfs.  That does not imply the virtio-gpu buffers are hugepage
> aligned though, udmabuf would still need to operate on smaller chunks
> of memory.  So with additional restrictions this will not work any
> more for qemu.  I'd suggest to just revert hugetlb support instead
> and go back to the drawing board.
> 
> Also not sure why hugetlbfs is used for guest memory in the first place.
> It used to be a thing years ago, but with the arrival of transparent
> hugepages there is as far I know little reason to still use hugetlbfs.
The main reason why we are interested in using hugetlbfs for guest memory
is because we observed non-trivial performance improvement while running
certain 3D heavy workloads in the guest. And, we noticed this by only
switching the Guest memory backend to include hugepages (i.e, hugetlb=on)
and with no other changes.

To address the current situation, I am readying a patch for udmabuf driver that
would add back support for mapping hugepages but without making use of
the subpages directly.

Thanks,
Vivek

> 
> Vivek? Dongwon?
> 
> take care,
>   Gerd