Re: [PATCH 2/2] mm/uffd: Allow vma to merge as much as possible

[Lorenzo Stoakes]

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 <aarcange@xxxxxxxxxx>
> Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  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 <lstoakes@xxxxxxxxx>