[PATCH] mm: fix hugetlb page unmap count balance issue
Andrew Morton
akpm at linux-foundation.org
Wed Jun 7 19:03:12 UTC 2023
On Tue, 16 May 2023 15:34:40 -0700 Mike Kravetz <mike.kravetz at oracle.com> wrote:
> On 05/15/23 10:04, 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@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.
>
> Verified that using udmabug on a hugetlb mapping with vmemmap optimization will
> BUG as:
BUGs aren't good. Can we please find a way to push this along?
Have we heard anything from any udmabuf people?
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