On Wed, Mar 10, 2021 at 01:18:42PM +0530, Naresh Kamboju wrote:
> Hi Peter,
Hi, Naresh,
>
> On Fri, 19 Feb 2021 at 04:43, Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > Huge pmd sharing could bring problem to userfaultfd. The thing is that
> > userfaultfd is running its logic based on the special bits on page table
> > entries, however the huge pmd sharing could potentially share page table
> > entries for different address ranges. That could cause issues on either:
> >
> > - When sharing huge pmd page tables for an uffd write protected range, the
> > newly mapped huge pmd range will also be write protected unexpectedly, or,
> >
> > - When we try to write protect a range of huge pmd shared range, we'll first
> > do huge_pmd_unshare() in hugetlb_change_protection(), however that also
> > means the UFFDIO_WRITEPROTECT could be silently skipped for the shared
> > region, which could lead to data loss.
> >
> > Since at it, a few other things are done altogether:
> >
> > - Move want_pmd_share() from mm/hugetlb.c into linux/hugetlb.h, because
> > that's definitely something that arch code would like to use too
> >
> > - ARM64 currently directly check against CONFIG_ARCH_WANT_HUGE_PMD_SHARE when
> > trying to share huge pmd. Switch to the want_pmd_share() helper.
> >
> > Since at it, move vma_shareable() from huge_pmd_share() into want_pmd_share().
> >
> > Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Reviewed-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> > arch/arm64/mm/hugetlbpage.c | 3 +--
> > include/linux/hugetlb.h | 2 ++
> > include/linux/userfaultfd_k.h | 9 +++++++++
> > mm/hugetlb.c | 20 ++++++++++++++------
> > 4 files changed, 26 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> > index 6e3bcffe2837..58987a98e179 100644
> > --- a/arch/arm64/mm/hugetlbpage.c
> > +++ b/arch/arm64/mm/hugetlbpage.c
> > @@ -284,8 +284,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
> > */
> > ptep = pte_alloc_map(mm, pmdp, addr);
> > } else if (sz == PMD_SIZE) {
> > - if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
> > - pud_none(READ_ONCE(*pudp)))
> > + if (want_pmd_share(vma, addr) && pud_none(READ_ONCE(*pudp)))
>
> While building Linux next 20210310 tag for arm64 architecture with
>
> - CONFIG_ARM64_64K_PAGES=y
>
> enabled the build failed due to below errors / warnings
>
> make --silent --keep-going --jobs=8
> O=/home/tuxbuild/.cache/tuxmake/builds/1/tmp ARCH=arm64
> CROSS_COMPILE=aarch64-linux-gnu- 'CC=sccache aarch64-linux-gnu-gcc'
> 'HOSTCC=sccache gcc'
> aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
> aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
> aarch64-linux-gnu-ld: arch/arm64/mm/hugetlbpage.o: in function `huge_pte_alloc':
> hugetlbpage.c:(.text+0x7d8): undefined reference to `want_pmd_share'
>
> Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Sorry for the issue & thanks for the report. Would you please check whether
the patch attached could fix the issue?
--
Peter Xu
>From 4072042b415d1f78ac1ab017671e345b414ca6ab Mon Sep 17 00:00:00 2001
From: Peter Xu <peterx@xxxxxxxxxx>
Date: Wed, 10 Mar 2021 11:48:56 -0500
Subject: [PATCH] mm/hugetlb: Fix build with !ARCH_WANT_HUGE_PMD_SHARE
want_pmd_share() is undefined with !ARCH_WANT_HUGE_PMD_SHARE since it's put
by accident into a "#ifdef ARCH_WANT_HUGE_PMD_SHARE" block. Moving it out
won't work either since vma_shareable() is only defined within the block.
Define it for !ARCH_WANT_HUGE_PMD_SHARE instead.
Fixes: 5b109cc1cdcc ("hugetlb/userfaultfd: forbid huge pmd sharing when uffd enabled")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
mm/hugetlb.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d58f1456fe27..8dda7e034477 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5469,9 +5469,6 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr)
{
-#ifndef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
- return false;
-#endif
#ifdef CONFIG_USERFAULTFD
if (uffd_disable_huge_pmd_share(vma))
return false;
@@ -5616,6 +5613,11 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
unsigned long *start, unsigned long *end)
{
}
+
+bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr)
+{
+ return false;
+}
#endif /* CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
#ifdef CONFIG_ARCH_WANT_GENERAL_HUGETLB
--
2.26.2
