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[86.156.84.164]) by smtp.gmail.com with ESMTPSA id z8-20020adfe548000000b00306281cfa59sm3321041wrm.47.2023.05.17.10.23.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 May 2023 10:23:19 -0700 (PDT) Date: Wed, 17 May 2023 18:23:18 +0100 From: Lorenzo Stoakes To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , "Liam R . Howlett" , Mark Rutland , Andrea Arcangeli , Mike Rapoport , Alexander Viro , linux-stable Subject: Re: [PATCH 2/2] mm/uffd: Allow vma to merge as much as possible Message-ID: References: <20230517150408.3411044-1-peterx@redhat.com> <20230517150408.3411044-3-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230517150408.3411044-3-peterx@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 17, 2023 at 11:04:08AM -0400, Peter Xu wrote: > We used to not pass in the pgoff correctly when register/unregister uffd > regions, it caused incorrect behavior on vma merging and can cause > mergeable vmas being separate after ioctls return. > > For example, when we have: > > vma1(range 0-9, with uffd), vma2(range 10-19, no uffd) > > Then someone unregisters uffd on range (5-9), it should logically become: > > vma1(range 0-4, with uffd), vma2(range 5-19, no uffd) > > But with current code we'll have: > > vma1(range 0-4, with uffd), vma3(range 5-9, no uffd), vma2(range 10-19, no uffd) > > This patch allows such merge to happen correctly before ioctl returns. > > This behavior seems to have existed since the 1st day of uffd. Since pgoff > for vma_merge() is only used to identify the possibility of vma merging, > meanwhile here what we did was always passing in a pgoff smaller than what > we should, so there should have no other side effect besides not merging > it. Let's still tentatively copy stable for this, even though I don't see > anything will go wrong besides vma being split (which is mostly not user > visible). > Maybe a Reported-by me since I discovered the fragmentation was already happening via the repro? :) > Cc: Andrea Arcangeli > Cc: Mike Rapoport (IBM) > Cc: linux-stable > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization") > Signed-off-by: Peter Xu > --- > fs/userfaultfd.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 17c8c345dac4..4e800bb7d2ab 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -1332,6 +1332,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, > bool basic_ioctls; > unsigned long start, end, vma_end; > struct vma_iterator vmi; > + pgoff_t pgoff; > > user_uffdio_register = (struct uffdio_register __user *) arg; > > @@ -1484,8 +1485,9 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, > vma_end = min(end, vma->vm_end); > > new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags; > + pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT); > prev = vma_merge(&vmi, mm, prev, start, vma_end, new_flags, > - vma->anon_vma, vma->vm_file, vma->vm_pgoff, > + vma->anon_vma, vma->vm_file, pgoff, > vma_policy(vma), > ((struct vm_userfaultfd_ctx){ ctx }), > anon_vma_name(vma)); > @@ -1565,6 +1567,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, > unsigned long start, end, vma_end; > const void __user *buf = (void __user *)arg; > struct vma_iterator vmi; > + pgoff_t pgoff; > > ret = -EFAULT; > if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister))) > @@ -1667,8 +1670,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, > uffd_wp_range(vma, start, vma_end - start, false); > > new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS; > + pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT); > prev = vma_merge(&vmi, mm, prev, start, vma_end, new_flags, > - vma->anon_vma, vma->vm_file, vma->vm_pgoff, > + vma->anon_vma, vma->vm_file, pgoff, > vma_policy(vma), > NULL_VM_UFFD_CTX, anon_vma_name(vma)); > if (prev) { > -- > 2.39.1 > Acked-by: Lorenzo Stoakes