linux-kernel - Re: [PATCH] mm: fix hugetlb page unmap count balance issue

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Date:   Wed, 7 Jun 2023 12:03:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>,
        Junxiao Chang <junxiao.chang@...el.com>,
        kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        jmarchan@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, muchun.song@...ux.dev,
        Vivek Kasireddy <vivek.kasireddy@...el.com>,
        Dongwon Kim <dongwon.kim@...el.com>,
        James Houghton <jthoughton@...gle.com>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] mm: fix hugetlb page unmap count balance issue

On Tue, 16 May 2023 15:34:40 -0700 Mike Kravetz <mike.kravetz@...cle.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@...el.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?