Re: [PATCH v4 3/3] userfaultfd: use per-vma locks in userfaultfd operations
From: Liam R. Howlett
Date: Thu Feb 08 2024 - 22:07:43 EST
* Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240208 16:22]:
> All userfaultfd operations, except write-protect, opportunistically use
> per-vma locks to lock vmas. On failure, attempt again inside mmap_lock
> critical section.
>
> Write-protect operation requires mmap_lock as it iterates over multiple
> vmas.
>
> Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> ---
> fs/userfaultfd.c | 13 +-
> include/linux/userfaultfd_k.h | 5 +-
> mm/userfaultfd.c | 356 ++++++++++++++++++++++++++--------
> 3 files changed, 275 insertions(+), 99 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index c00a021bcce4..60dcfafdc11a 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -2005,17 +2005,8 @@ static int userfaultfd_move(struct userfaultfd_ctx *ctx,
> return -EINVAL;
>
> if (mmget_not_zero(mm)) {
> - mmap_read_lock(mm);
> -
> - /* Re-check after taking map_changing_lock */
> - down_read(&ctx->map_changing_lock);
> - if (likely(!atomic_read(&ctx->mmap_changing)))
> - ret = move_pages(ctx, mm, uffdio_move.dst, uffdio_move.src,
> - uffdio_move.len, uffdio_move.mode);
> - else
> - ret = -EAGAIN;
> - up_read(&ctx->map_changing_lock);
> - mmap_read_unlock(mm);
> + ret = move_pages(ctx, uffdio_move.dst, uffdio_move.src,
> + uffdio_move.len, uffdio_move.mode);
> mmput(mm);
> } else {
> return -ESRCH;
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 3210c3552976..05d59f74fc88 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -138,9 +138,8 @@ extern long uffd_wp_range(struct vm_area_struct *vma,
> /* move_pages */
> void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
> void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
> -ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
> - unsigned long dst_start, unsigned long src_start,
> - unsigned long len, __u64 flags);
> +ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> + unsigned long src_start, unsigned long len, __u64 flags);
> int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pmd_t dst_pmdval,
> struct vm_area_struct *dst_vma,
> struct vm_area_struct *src_vma,
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 74aad0831e40..1e25768b2136 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -19,20 +19,12 @@
> #include <asm/tlb.h>
> #include "internal.h"
>
> -static __always_inline
> -struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
> - unsigned long dst_start,
> - unsigned long len)
You could probably leave the __always_inline for this.
> +static bool validate_dst_vma(struct vm_area_struct *dst_vma,
> + unsigned long dst_end)
> {
> - /*
> - * Make sure that the dst range is both valid and fully within a
> - * single existing vma.
> - */
> - struct vm_area_struct *dst_vma;
> -
> - dst_vma = find_vma(dst_mm, dst_start);
> - if (!range_in_vma(dst_vma, dst_start, dst_start + len))
> - return NULL;
> + /* Make sure that the dst range is fully within dst_vma. */
> + if (dst_end > dst_vma->vm_end)
> + return false;
>
> /*
> * Check the vma is registered in uffd, this is required to
> @@ -40,11 +32,125 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
> * time.
> */
> if (!dst_vma->vm_userfaultfd_ctx.ctx)
> - return NULL;
> + return false;
> +
> + return true;
> +}
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +/*
> + * lock_vma() - Lookup and lock vma corresponding to @address.
> + * @mm: mm to search vma in.
> + * @address: address that the vma should contain.
> + * @prepare_anon: If true, then prepare the vma (if private) with anon_vma.
> + *
> + * Should be called without holding mmap_lock. vma should be unlocked after use
> + * with unlock_vma().
> + *
> + * Return: A locked vma containing @address, NULL if no vma is found, or
> + * -ENOMEM if anon_vma couldn't be allocated.
> + */
> +static struct vm_area_struct *lock_vma(struct mm_struct *mm,
> + unsigned long address,
> + bool prepare_anon)
> +{
> + struct vm_area_struct *vma;
> +
> + vma = lock_vma_under_rcu(mm, address);
> + if (vma) {
> + /*
> + * lock_vma_under_rcu() only checks anon_vma for private
> + * anonymous mappings. But we need to ensure it is assigned in
> + * private file-backed vmas as well.
> + */
> + if (prepare_anon && !(vma->vm_flags & VM_SHARED) &&
> + !vma->anon_vma)
> + vma_end_read(vma);
> + else
> + return vma;
> + }
> +
> + mmap_read_lock(mm);
> + vma = vma_lookup(mm, address);
> + if (vma) {
> + if (prepare_anon && !(vma->vm_flags & VM_SHARED) &&
> + anon_vma_prepare(vma)) {
> + vma = ERR_PTR(-ENOMEM);
> + } else {
> + /*
> + * We cannot use vma_start_read() as it may fail due to
> + * false locked (see comment in vma_start_read()). We
> + * can avoid that by directly locking vm_lock under
> + * mmap_lock, which guarantees that nobody can lock the
> + * vma for write (vma_start_write()) under us.
> + */
> + down_read(&vma->vm_lock->lock);
> + }
> + }
> +
> + mmap_read_unlock(mm);
> + return vma;
> +}
> +
> +static void unlock_vma(struct vm_area_struct *vma)
> +{
> + vma_end_read(vma);
> +}
> +
> +static struct vm_area_struct *find_and_lock_dst_vma(struct mm_struct *dst_mm,
> + unsigned long dst_start,
> + unsigned long len)
> +{
> + struct vm_area_struct *dst_vma;
> +
> + /* Ensure anon_vma is assigned for private vmas */
> + dst_vma = lock_vma(dst_mm, dst_start, true);
> +
> + if (!dst_vma)
> + return ERR_PTR(-ENOENT);
> +
> + if (PTR_ERR(dst_vma) == -ENOMEM)
> + return dst_vma;
> +
> + if (!validate_dst_vma(dst_vma, dst_start + len))
> + goto out_unlock;
>
> return dst_vma;
> +out_unlock:
> + unlock_vma(dst_vma);
> + return ERR_PTR(-ENOENT);
> }
>
> +#else
> +
> +static struct vm_area_struct *lock_mm_and_find_dst_vma(struct mm_struct *dst_mm,
> + unsigned long dst_start,
> + unsigned long len)
> +{
> + struct vm_area_struct *dst_vma;
> + int err = -ENOENT;
> +
> + mmap_read_lock(dst_mm);
> + dst_vma = vma_lookup(dst_mm, dst_start);
> + if (!dst_vma)
> + goto out_unlock;
> +
> + /* Ensure anon_vma is assigned for private vmas */
> + if (!(dst_vma->vm_flags & VM_SHARED) && anon_vma_prepare(dst_vma)) {
> + err = -ENOMEM;
> + goto out_unlock;
> + }
> +
> + if (!validate_dst_vma(dst_vma, dst_start + len))
> + goto out_unlock;
> +
> + return dst_vma;
> +out_unlock:
> + mmap_read_unlock(dst_mm);
> + return ERR_PTR(err);
> +}
> +#endif
> +
> /* Check if dst_addr is outside of file's size. Must be called with ptl held. */
> static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
> unsigned long dst_addr)
> @@ -350,7 +456,8 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
> #ifdef CONFIG_HUGETLB_PAGE
> /*
> * mfill_atomic processing for HUGETLB vmas. Note that this routine is
> - * called with mmap_lock held, it will release mmap_lock before returning.
> + * called with either vma-lock or mmap_lock held, it will release the lock
> + * before returning.
> */
> static __always_inline ssize_t mfill_atomic_hugetlb(
> struct userfaultfd_ctx *ctx,
> @@ -361,7 +468,6 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> uffd_flags_t flags)
> {
> struct mm_struct *dst_mm = dst_vma->vm_mm;
> - int vm_shared = dst_vma->vm_flags & VM_SHARED;
> ssize_t err;
> pte_t *dst_pte;
> unsigned long src_addr, dst_addr;
> @@ -380,7 +486,11 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> */
> if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
> up_read(&ctx->map_changing_lock);
> +#ifdef CONFIG_PER_VMA_LOCK
> + unlock_vma(dst_vma);
> +#else
> mmap_read_unlock(dst_mm);
> +#endif
> return -EINVAL;
> }
>
> @@ -403,24 +513,32 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> * retry, dst_vma will be set to NULL and we must lookup again.
> */
> if (!dst_vma) {
> +#ifdef CONFIG_PER_VMA_LOCK
> + dst_vma = find_and_lock_dst_vma(dst_mm, dst_start, len);
> +#else
> + dst_vma = lock_mm_and_find_dst_vma(dst_mm, dst_start, len);
> +#endif
> + if (IS_ERR(dst_vma)) {
> + err = PTR_ERR(dst_vma);
> + goto out;
> + }
> +
> err = -ENOENT;
> - dst_vma = find_dst_vma(dst_mm, dst_start, len);
> - if (!dst_vma || !is_vm_hugetlb_page(dst_vma))
> - goto out_unlock;
> + if (!is_vm_hugetlb_page(dst_vma))
> + goto out_unlock_vma;
>
> err = -EINVAL;
> if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
> - goto out_unlock;
> -
> - vm_shared = dst_vma->vm_flags & VM_SHARED;
> - }
> + goto out_unlock_vma;
>
> - /*
> - * If not shared, ensure the dst_vma has a anon_vma.
> - */
> - err = -ENOMEM;
> - if (!vm_shared) {
> - if (unlikely(anon_vma_prepare(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;
> }
>
> @@ -465,7 +583,11 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>
> if (unlikely(err == -ENOENT)) {
> up_read(&ctx->map_changing_lock);
> +#ifdef CONFIG_PER_VMA_LOCK
> + unlock_vma(dst_vma);
> +#else
> mmap_read_unlock(dst_mm);
> +#endif
> BUG_ON(!folio);
>
> err = copy_folio_from_user(folio,
> @@ -474,17 +596,6 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
> err = -EFAULT;
> goto out;
> }
> - mmap_read_lock(dst_mm);
> - down_read(&ctx->map_changing_lock);
> - /*
> - * 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
> - */
> - if (atomic_read(&ctx->mmap_changing)) {
> - err = -EAGAIN;
> - break;
> - }
>
> dst_vma = NULL;
> goto retry;
> @@ -505,7 +616,12 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>
> out_unlock:
> up_read(&ctx->map_changing_lock);
> +out_unlock_vma:
> +#ifdef CONFIG_PER_VMA_LOCK
> + unlock_vma(dst_vma);
> +#else
> mmap_read_unlock(dst_mm);
> +#endif
> out:
> if (folio)
> folio_put(folio);
> @@ -597,7 +713,19 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> copied = 0;
> folio = NULL;
> retry:
> - mmap_read_lock(dst_mm);
> + /*
> + * Make sure the vma is not shared, that the dst range is
> + * both valid and fully within a single existing vma.
> + */
> +#ifdef CONFIG_PER_VMA_LOCK
> + dst_vma = find_and_lock_dst_vma(dst_mm, dst_start, len);
> +#else
> + dst_vma = lock_mm_and_find_dst_vma(dst_mm, dst_start, len);
> +#endif
> + if (IS_ERR(dst_vma)) {
> + err = PTR_ERR(dst_vma);
> + goto out;
> + }
>
> /*
> * If memory mappings are changing because of non-cooperative
> @@ -609,15 +737,6 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> if (atomic_read(&ctx->mmap_changing))
> goto out_unlock;
>
> - /*
> - * Make sure the vma is not shared, that the dst range is
> - * both valid and fully within a single existing vma.
> - */
> - err = -ENOENT;
> - dst_vma = find_dst_vma(dst_mm, dst_start, len);
> - if (!dst_vma)
> - goto out_unlock;
> -
> err = -EINVAL;
> /*
> * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
> @@ -647,16 +766,6 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
> goto out_unlock;
>
> - /*
> - * Ensure the dst_vma has a anon_vma or this page
> - * would get a NULL anon_vma when moved in the
> - * dst_vma.
> - */
> - err = -ENOMEM;
> - if (!(dst_vma->vm_flags & VM_SHARED) &&
> - unlikely(anon_vma_prepare(dst_vma)))
> - goto out_unlock;
> -
> while (src_addr < src_start + len) {
> pmd_t dst_pmdval;
>
> @@ -699,7 +808,11 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
> void *kaddr;
>
> up_read(&ctx->map_changing_lock);
> +#ifdef CONFIG_PER_VMA_LOCK
> + unlock_vma(dst_vma);
> +#else
> mmap_read_unlock(dst_mm);
> +#endif
> BUG_ON(!folio);
>
> kaddr = kmap_local_folio(folio, 0);
> @@ -730,7 +843,11 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>
> out_unlock:
> up_read(&ctx->map_changing_lock);
> +#ifdef CONFIG_PER_VMA_LOCK
> + unlock_vma(dst_vma);
> +#else
> mmap_read_unlock(dst_mm);
> +#endif
> out:
> if (folio)
> folio_put(folio);
> @@ -1267,16 +1384,67 @@ static int validate_move_areas(struct userfaultfd_ctx *ctx,
> if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
> return -EINVAL;
>
> - /*
> - * Ensure the dst_vma has a anon_vma or this page
> - * would get a NULL anon_vma when moved in the
> - * dst_vma.
> - */
> - if (unlikely(anon_vma_prepare(dst_vma)))
> - return -ENOMEM;
> + return 0;
> +}
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +static int find_and_lock_vmas(struct mm_struct *mm,
> + unsigned long dst_start,
> + unsigned long src_start,
> + struct vm_area_struct **dst_vmap,
> + struct vm_area_struct **src_vmap)
> +{
> + int err;
> +
> + /* There is no need to prepare anon_vma for src_vma */
> + *src_vmap = lock_vma(mm, src_start, false);
> + if (!*src_vmap)
> + return -ENOENT;
> +
> + /* Ensure anon_vma is assigned for anonymous vma */
> + *dst_vmap = lock_vma(mm, dst_start, true);
> + err = -ENOENT;
> + if (!*dst_vmap)
> + goto out_unlock;
> +
> + err = -ENOMEM;
> + if (PTR_ERR(*dst_vmap) == -ENOMEM)
> + goto out_unlock;
If you change lock_vma() to return the vma or ERR_PTR(-ENOENT) /
ERR_PTR(-ENOMEM), then you could change this to check IS_ERR() and
return the PTR_ERR().
You could also use IS_ERR_OR_NULL here, but the first suggestion will
simplify your life for find_and_lock_dst_vma() and the error type to
return.
What you have here will work though.
>
> return 0;
> +out_unlock:
> + unlock_vma(*src_vmap);
> + return err;
> }
> +#else
> +static int lock_mm_and_find_vmas(struct mm_struct *mm,
> + unsigned long dst_start,
> + unsigned long src_start,
> + struct vm_area_struct **dst_vmap,
> + struct vm_area_struct **src_vmap)
> +{
> + int err = -ENOENT;
Nit: new line after declarations.
> + mmap_read_lock(mm);
> +
> + *src_vmap = vma_lookup(mm, src_start);
> + if (!*src_vmap)
> + goto out_unlock;
> +
> + *dst_vmap = vma_lookup(mm, dst_start);
> + if (!*dst_vmap)
> + goto out_unlock;
> +
> + /* Ensure anon_vma is assigned */
> + err = -ENOMEM;
> + if (vma_is_anonymous(*dst_vmap) && anon_vma_prepare(*dst_vmap))
> + goto out_unlock;
> +
> + return 0;
> +out_unlock:
> + mmap_read_unlock(mm);
> + return err;
> +}
> +#endif
>
> /**
> * move_pages - move arbitrary anonymous pages of an existing vma
> @@ -1287,8 +1455,6 @@ static int validate_move_areas(struct userfaultfd_ctx *ctx,
> * @len: length of the virtual memory range
> * @mode: flags from uffdio_move.mode
> *
> - * Must be called with mmap_lock held for read.
> - *
Will either use the mmap_lock in read mode or per-vma locking ?
> * move_pages() remaps arbitrary anonymous pages atomically in zero
> * copy. It only works on non shared anonymous pages because those can
> * be relocated without generating non linear anon_vmas in the rmap
> @@ -1355,10 +1521,10 @@ static int validate_move_areas(struct userfaultfd_ctx *ctx,
> * could be obtained. This is the only additional complexity added to
> * the rmap code to provide this anonymous page remapping functionality.
> */
> -ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
> - unsigned long dst_start, unsigned long src_start,
> - unsigned long len, __u64 mode)
> +ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> + unsigned long src_start, unsigned long len, __u64 mode)
> {
> + struct mm_struct *mm = ctx->mm;
You dropped the argument, but left the comment for the argument.
> struct vm_area_struct *src_vma, *dst_vma;
> unsigned long src_addr, dst_addr;
> pmd_t *src_pmd, *dst_pmd;
> @@ -1376,28 +1542,40 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
> WARN_ON_ONCE(dst_start + len <= dst_start))
> goto out;
>
> +#ifdef CONFIG_PER_VMA_LOCK
> + err = find_and_lock_vmas(mm, dst_start, src_start,
> + &dst_vma, &src_vma);
> +#else
> + err = lock_mm_and_find_vmas(mm, dst_start, src_start,
> + &dst_vma, &src_vma);
> +#endif
I was hoping you could hide this completely, but it's probably better to
show what's going on and the function names document it well.
> + if (err)
> + goto out;
> +
> + /* Re-check after taking map_changing_lock */
> + down_read(&ctx->map_changing_lock);
> + if (likely(atomic_read(&ctx->mmap_changing))) {
> + err = -EAGAIN;
> + goto out_unlock;
> + }
> /*
> * Make sure the vma is not shared, that the src and dst remap
> * ranges are both valid and fully within a single existing
> * vma.
> */
> - src_vma = find_vma(mm, src_start);
> - if (!src_vma || (src_vma->vm_flags & VM_SHARED))
> - goto out;
> - if (src_start < src_vma->vm_start ||
> - src_start + len > src_vma->vm_end)
> - goto out;
> + if (src_vma->vm_flags & VM_SHARED)
> + goto out_unlock;
> + if (src_start + len > src_vma->vm_end)
> + goto out_unlock;
>
> - dst_vma = find_vma(mm, dst_start);
> - if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
> - goto out;
> - if (dst_start < dst_vma->vm_start ||
> - dst_start + len > dst_vma->vm_end)
> - goto out;
> + if (dst_vma->vm_flags & VM_SHARED)
> + goto out_unlock;
> + if (dst_start + len > dst_vma->vm_end)
> + goto out_unlock;
>
> err = validate_move_areas(ctx, src_vma, dst_vma);
> if (err)
> - goto out;
> + goto out_unlock;
>
> for (src_addr = src_start, dst_addr = dst_start;
> src_addr < src_start + len;) {
> @@ -1514,6 +1692,14 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
> moved += step_size;
> }
>
> +out_unlock:
> + up_read(&ctx->map_changing_lock);
> +#ifdef CONFIG_PER_VMA_LOCK
> + unlock_vma(dst_vma);
> + unlock_vma(src_vma);
> +#else
> + mmap_read_unlock(mm);
> +#endif
> out:
> VM_WARN_ON(moved < 0);
> VM_WARN_ON(err > 0);
> --
> 2.43.0.687.g38aa6559b0-goog
>