Messages in this thread | | | Date | Wed, 7 Dec 2022 15:19:55 -0800 | Subject | Re: [PATCH v2 05/10] mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare | From | John Hubbard <> |
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On 12/7/22 12:30, Peter Xu wrote: > We can take the hugetlb walker lock, here taking vma lock directly. > > Reviewed-by: David Hildenbrand <david@redhat.com> > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com> > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > fs/userfaultfd.c | 18 ++++++++++++++---- > 1 file changed, 14 insertions(+), 4 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 07c81ab3fd4d..a602f008dde5 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -376,7 +376,8 @@ static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags) > */ > vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) > { > - struct mm_struct *mm = vmf->vma->vm_mm; > + struct vm_area_struct *vma = vmf->vma; > + struct mm_struct *mm = vma->vm_mm; > struct userfaultfd_ctx *ctx; > struct userfaultfd_wait_queue uwq; > vm_fault_t ret = VM_FAULT_SIGBUS; > @@ -403,7 +404,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) > */ > mmap_assert_locked(mm); > > - ctx = vmf->vma->vm_userfaultfd_ctx.ctx; > + ctx = vma->vm_userfaultfd_ctx.ctx; > if (!ctx) > goto out; > > @@ -493,6 +494,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) > > blocking_state = userfaultfd_get_blocking_state(vmf->flags); > > + /* > + * This stablizes pgtable for hugetlb on e.g. pmd unsharing. Need > + * to be before setting current state. > + */
Looking at this code, I am not able to come up with a reason for why the vma lock/unlock placement is exactly where it is. It looks quite arbitrary.
Why not, for example, take and drop the vma lock within userfaultfd_huge_must_wait()? That makes more sense to me, but I'm not familiar with userfaultfd so of course I'm missing something.
But the comment above certainly doesn't supply that something.
thanks, -- John Hubbard NVIDIA
> + if (is_vm_hugetlb_page(vma)) > + hugetlb_vma_lock_read(vma); > + > spin_lock_irq(&ctx->fault_pending_wqh.lock); > /* > * After the __add_wait_queue the uwq is visible to userland > @@ -507,13 +515,15 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) > set_current_state(blocking_state); > spin_unlock_irq(&ctx->fault_pending_wqh.lock); > > - if (!is_vm_hugetlb_page(vmf->vma)) > + if (!is_vm_hugetlb_page(vma)) > must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags, > reason); > else > - must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma, > + must_wait = userfaultfd_huge_must_wait(ctx, vma, > vmf->address, > vmf->flags, reason); > + if (is_vm_hugetlb_page(vma)) > + hugetlb_vma_unlock_read(vma); > mmap_read_unlock(mm); > > if (likely(must_wait && !READ_ONCE(ctx->released))) {
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