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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id w3-20020ac87183000000b0039cd4d87aacsm13882213qto.15.2022.12.07.15.44.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Dec 2022 15:44:32 -0800 (PST) Date: Wed, 7 Dec 2022 18:44:31 -0500 From: Peter Xu To: John Hubbard Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Muchun Song , Andrea Arcangeli , James Houghton , Jann Horn , Rik van Riel , Miaohe Lin , Andrew Morton , Mike Kravetz , David Hildenbrand , Nadav Amit Subject: Re: [PATCH v2 05/10] mm/hugetlb: Make userfaultfd_huge_must_wait() safe to pmd unshare Message-ID: References: <20221207203034.650899-1-peterx@redhat.com> <20221207203034.650899-6-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 07, 2022 at 03:19:55PM -0800, John Hubbard wrote: > 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 > > Reviewed-by: Mike Kravetz > > Signed-off-by: Peter Xu > > --- > > 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. The part that matters in the comment is "need to be before setting current state". blocking_state = userfaultfd_get_blocking_state(vmf->flags); if (is_vm_hugetlb_page(vma)) hugetlb_vma_lock_read(vma); set_current_state(blocking_state); down_read() can sleep and also modify the task state, we cannot take the lock after that point because otherwise the task state will be messed up. -- Peter Xu