Hi, Mike,
On Tue, Jan 27, 2026 at 09:29:23PM +0200, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <[email protected]>
>
> Split the code that finds, locks and verifies VMA from mfill_atomic()
> into a helper function.
>
> This function will be used later during refactoring of
> mfill_atomic_pte_copy().
>
> Add a counterpart mfill_put_vma() helper that unlocks the VMA and
> releases map_changing_lock.
>
> Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
> ---
> mm/userfaultfd.c | 124 ++++++++++++++++++++++++++++-------------------
> 1 file changed, 73 insertions(+), 51 deletions(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 9dd285b13f3b..45d8f04aaf4f 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -157,6 +157,73 @@ static void uffd_mfill_unlock(struct vm_area_struct *vma)
> }
> #endif
>
> +static void mfill_put_vma(struct mfill_state *state)
> +{
> + up_read(&state->ctx->map_changing_lock);
> + uffd_mfill_unlock(state->vma);
> + state->vma = NULL;
> +}
> +
> +static int mfill_get_vma(struct mfill_state *state)
> +{
> + struct userfaultfd_ctx *ctx = state->ctx;
> + uffd_flags_t flags = state->flags;
> + struct vm_area_struct *dst_vma;
> + int err;
> +
> + /*
> + * Make sure the vma is not shared, that the dst range is
> + * both valid and fully within a single existing vma.
> + */
> + dst_vma = uffd_mfill_lock(ctx->mm, state->dst_start, state->len);
> + if (IS_ERR(dst_vma))
> + return PTR_ERR(dst_vma);
> +
> + /*
> + * If memory mappings are changing because of non-cooperative
> + * operation (e.g. mremap) running in parallel, bail out and
> + * request the user to retry later
> + */
> + down_read(&ctx->map_changing_lock);
> + err = -EAGAIN;
> + if (atomic_read(&ctx->mmap_changing))
> + goto out_unlock;
> +
> + err = -EINVAL;
> +
> + /*
> + * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
> + * it will overwrite vm_ops, so vma_is_anonymous must return false.
> + */
> + if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
> + dst_vma->vm_flags & VM_SHARED))
> + goto out_unlock;
> +
> + /*
> + * validate 'mode' now that we know the dst_vma: don't allow
> + * a wrprotect copy if the userfaultfd didn't register as WP.
> + */
> + if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
> + goto out_unlock;
> +
> + if (is_vm_hugetlb_page(dst_vma))
> + goto out;
> +
> + if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> + goto out_unlock;
> + if (!vma_is_shmem(dst_vma) &&
> + uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> + goto out_unlock;
IMHO it's a bit weird to check for vma permissions in a get_vma() function.
Also, in the follow up patch it'll be also reused in
mfill_copy_folio_retry() which doesn't need to check vma permission.
Maybe we can introduce mfill_vma_check() for these two checks? Then we can
also drop the slightly weird is_vm_hugetlb_page() check (and "out" label)
above.
> +
> +out:
> + state->vma = dst_vma;
> + return 0;
> +
> +out_unlock:
> + mfill_put_vma(state);
> + return err;
> +}
> +
> static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
> {
> pgd_t *pgd;
> @@ -768,8 +835,6 @@ static __always_inline ssize_t mfill_atomic(struct
> userfaultfd_ctx *ctx,
> .src_addr = src_start,
> .dst_addr = dst_start,
> };
> - struct mm_struct *dst_mm = ctx->mm;
> - struct vm_area_struct *dst_vma;
> long copied = 0;
> ssize_t err;
>
> @@ -784,57 +849,17 @@ static __always_inline ssize_t mfill_atomic(struct
> userfaultfd_ctx *ctx,
> VM_WARN_ON_ONCE(dst_start + len <= dst_start);
>
> retry:
> - /*
> - * Make sure the vma is not shared, that the dst range is
> - * both valid and fully within a single existing vma.
> - */
> - dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
> - if (IS_ERR(dst_vma)) {
> - err = PTR_ERR(dst_vma);
> + err = mfill_get_vma(&state);
> + if (err)
> goto out;
> - }
> -
> - /*
> - * If memory mappings are changing because of non-cooperative
> - * operation (e.g. mremap) running in parallel, bail out and
> - * request the user to retry later
> - */
> - down_read(&ctx->map_changing_lock);
> - err = -EAGAIN;
> - if (atomic_read(&ctx->mmap_changing))
> - goto out_unlock;
> -
> - err = -EINVAL;
> - /*
> - * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
> - * it will overwrite vm_ops, so vma_is_anonymous must return false.
> - */
> - if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
> - dst_vma->vm_flags & VM_SHARED))
> - goto out_unlock;
> -
> - /*
> - * validate 'mode' now that we know the dst_vma: don't allow
> - * a wrprotect copy if the userfaultfd didn't register as WP.
> - */
> - if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
> - goto out_unlock;
>
> /*
> * If this is a HUGETLB vma, pass off to appropriate routine
> */
> - if (is_vm_hugetlb_page(dst_vma))
> - return mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
> + if (is_vm_hugetlb_page(state.vma))
> + return mfill_atomic_hugetlb(ctx, state.vma, dst_start,
> src_start, len, flags);
>
> - if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
> - goto out_unlock;
> - if (!vma_is_shmem(dst_vma) &&
> - uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> - goto out_unlock;
> -
> - state.vma = dst_vma;
> -
> while (state.src_addr < src_start + len) {
> VM_WARN_ON_ONCE(state.dst_addr >= dst_start + len);
>
> @@ -853,8 +878,7 @@ static __always_inline ssize_t mfill_atomic(struct
> userfaultfd_ctx *ctx,
> if (unlikely(err == -ENOENT)) {
> void *kaddr;
>
> - up_read(&ctx->map_changing_lock);
> - uffd_mfill_unlock(state.vma);
> + mfill_put_vma(&state);
> VM_WARN_ON_ONCE(!state.folio);
>
> kaddr = kmap_local_folio(state.folio, 0);
> @@ -883,9 +907,7 @@ static __always_inline ssize_t mfill_atomic(struct
> userfaultfd_ctx *ctx,
> break;
> }
>
> -out_unlock:
> - up_read(&ctx->map_changing_lock);
> - uffd_mfill_unlock(state.vma);
> + mfill_put_vma(&state);
> out:
> if (state.folio)
> folio_put(state.folio);
> --
> 2.51.0
>
--
Peter Xu
