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Message-ID: <855ee176-e628-489f-a3ee-cf0573cf5506@arm.com>
Date: Wed, 14 Feb 2024 22:12:24 +0000
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v6 3/3] userfaultfd: use per-vma locks in userfaultfd
 operations
To: Lokesh Gidra <lokeshgidra@google.com>, akpm@linux-foundation.org
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, selinux@vger.kernel.org, surenb@google.com,
 kernel-team@android.com, aarcange@redhat.com, peterx@redhat.com,
 david@redhat.com, axelrasmussen@google.com, bgeffon@google.com,
 willy@infradead.org, jannh@google.com, kaleshsingh@google.com,
 ngeoffray@google.com, timmurray@google.com, rppt@kernel.org,
 Liam.Howlett@oracle.com
References: <20240213215741.3816570-1-lokeshgidra@google.com>
 <20240213215741.3816570-4-lokeshgidra@google.com>
Content-Language: en-GB
From: Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <20240213215741.3816570-4-lokeshgidra@google.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

On 13/02/2024 21:57, Lokesh Gidra wrote:
> 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.

Hi,

I'm seeing the below OOPS when running on arm64 against mm-unstable. It can be reliably reproduced by running the `uffd-unit-tests` mm selftest. Bisecting mm-unstable leads to this patch:

# bad: [649936c3db47f8f75b9b927e4edf5922a0f240a6] mm: add swappiness= arg to memory.reclaim
git bisect bad 649936c3db47f8f75b9b927e4edf5922a0f240a6
# good: [54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478] Linux 6.8-rc3
git bisect good 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478
# good: [10794cd18bb46c91c75fac44e551201cfe006baf] mm: zswap: warn when referencing a dead entry
git bisect good 10794cd18bb46c91c75fac44e551201cfe006baf
# good: [cb769b427edc7f46c7c764daa3421725bffdf315] mm-memcg-use-larger-batches-for-proactive-reclaim-v4
git bisect good cb769b427edc7f46c7c764daa3421725bffdf315
# good: [cfc5c1be4010c9972bc3f3d991235e8ea6928672] mm/z3fold: remove unneeded spinlock
git bisect good cfc5c1be4010c9972bc3f3d991235e8ea6928672
# good: [31094bce101651acb4747ab25d614bc893d65c89] kasan/test: avoid gcc warning for intentional overflow
git bisect good 31094bce101651acb4747ab25d614bc893d65c89
# bad: [55be0b2cd1fbf00e036e2e48ee0999599135af66] zram: do not allocate physically contiguous strm buffers
git bisect bad 55be0b2cd1fbf00e036e2e48ee0999599135af66
# good: [b11ca4a0a13024c0175dde56f9bd848803eddcd2] mm/mglru: improve swappiness handling
git bisect good b11ca4a0a13024c0175dde56f9bd848803eddcd2
# good: [22e7ccd57a1220afcd7c4da1f3005fd04d70014e] userfaultfd: move userfaultfd_ctx struct to header file
git bisect good 22e7ccd57a1220afcd7c4da1f3005fd04d70014e
# bad: [0a0d05338f13e64c9fb7ccd8f8d1793aaf33ec7d] userfaultfd: use per-vma locks in userfaultfd operations
git bisect bad 0a0d05338f13e64c9fb7ccd8f8d1793aaf33ec7d
# good: [8459e1c7acbe4442c6c0eef59825da1339e0a3cf] userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx
git bisect good 8459e1c7acbe4442c6c0eef59825da1339e0a3cf
# first bad commit: [0a0d05338f13e64c9fb7ccd8f8d1793aaf33ec7d] userfaultfd: use per-vma locks in userfaultfd operations

This is the oops:

[   21.280142] mm ffff049340ed0000 task_size 281474976710656
[   21.280142] get_unmapped_area ffffb3dbdc725e48
[   21.280142] mmap_base 281474842492928 mmap_legacy_base 0
[   21.280142] pgd ffff04935829a000 mm_users 3 mm_count 4 pgtables_bytes 98304 map_count 21
[   21.280142] hiwater_rss 2167 hiwater_vm 8234 total_vm 4a44 locked_vm 0
[   21.280142] pinned_vm 0 data_vm 4835 exec_vm 1c6 stack_vm 21
[   21.280142] start_code aaaaaaaa0000 end_code aaaaaaab1b28 start_data aaaaaaac2a60 end_data aaaaaaac3410
[   21.280142] start_brk aaaaaaac5000 brk aaaaaaae6000 start_stack fffffffff6b0
[   21.280142] arg_start fffffffff8c7 arg_end fffffffff8e2 env_start fffffffff8e2 env_end ffffffffffdd
[   21.280142] binfmt ffffb3dbdf2f8cf8 flags 82008d
[   21.280142] ioctx_table 0000000000000000
[   21.280142] owner ffff049311f02280 exe_file ffff049355314c00
[   21.280142] notifier_subscriptions 0000000000000000
[   21.280142] numa_next_scan 4294897864 numa_scan_offset 0 numa_scan_seq 0
[   21.280142] tlb_flush_pending 0
[   21.280142] def_flags: 0x0()
[   21.283302] kernel BUG at include/linux/mmap_lock.h:66!
[   21.283481] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
[   21.283720] Modules linked in:
[   21.283867] CPU: 3 PID: 1226 Comm: uffd-unit-tests Not tainted 6.8.0-rc3-00297-g0a0d05338f13 #19
[   21.284306] Hardware name: linux,dummy-virt (DT)
[   21.284495] pstate: 61400005 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[   21.284833] pc : move_pages_huge_pmd+0x4d0/0x8a8
[   21.285072] lr : move_pages_huge_pmd+0x4d0/0x8a8
[   21.285289] sp : ffff800088573b30
[   21.285439] x29: ffff800088573b30 x28: ffff049358292d78 x27: fffffc0000000000
[   21.285770] x26: ffff049358292d78 x25: 0000fffff5e00000 x24: ffff049359db3730
[   21.286097] x23: 0000fffff6000000 x22: ffff049340ed0000 x21: ffff049359db3678
[   21.286422] x20: 0000000000000002 x19: fffffc124d60a480 x18: 0000000000000006
[   21.286747] x17: 626c740a30207165 x16: 735f6e6163735f61 x15: 6d756e2030207465
[   21.287084] x14: 7366666f5f6e6163 x13: 2928307830203a73 x12: 67616c665f666564
[   21.287417] x11: 0a3020676e69646e x10: ffffb3dbdf28bff8 x9 : ffffb3dbdc620f80
[   21.287745] x8 : 00000000ffffefff x7 : ffffb3dbdf28bff8 x6 : 80000000fffff000
[   21.288082] x5 : ffff04933ffdcd08 x4 : 0000000000000000 x3 : 0000000000000000
[   21.288364] x2 : 0000000000000000 x1 : ffff049340106780 x0 : 0000000000000328
[   21.288712] Call trace:
[   21.288832]  move_pages_huge_pmd+0x4d0/0x8a8
[   21.289058]  move_pages+0x8b8/0x13d8
[   21.289205]  userfaultfd_ioctl+0x11e0/0x1e90
[   21.289411]  __arm64_sys_ioctl+0xb4/0x100
[   21.289579]  invoke_syscall+0x50/0x128
[   21.289738]  el0_svc_common.constprop.0+0x48/0xf0
[   21.289942]  do_el0_svc+0x24/0x38
[   21.290078]  el0_svc+0x34/0xb8
[   21.290219]  el0t_64_sync_handler+0x100/0x130
[   21.290387]  el0t_64_sync+0x190/0x198
[   21.290535] Code: 17ffff7e a90363f7 a9046bf9 97fde2a7 (d4210000) 
[   21.290776] ---[ end trace 0000000000000000 ]---
[   21.290966] note: uffd-unit-tests[1226] exited with irqs disabled
[   21.291254] note: uffd-unit-tests[1226] exited with preempt_count 2
[   21.292864] ------------[ cut here ]------------
[   21.293020] WARNING: CPU: 3 PID: 0 at kernel/context_tracking.c:128 ct_kernel_exit.constprop.0+0x108/0x120
[   21.293711] Modules linked in:
[   21.294070] CPU: 3 PID: 0 Comm: swapper/3 Tainted: G      D            6.8.0-rc3-00297-g0a0d05338f13 #19
[   21.295447] Hardware name: linux,dummy-virt (DT)
[   21.295850] pstate: 214003c5 (nzCv DAIF +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[   21.296152] pc : ct_kernel_exit.constprop.0+0x108/0x120
[   21.296685] lr : ct_idle_enter+0x10/0x20
[   21.296848] sp : ffff8000801b3dc0
[   21.296983] x29: ffff8000801b3dc0 x28: 0000000000000000 x27: 0000000000000000
[   21.297255] x26: 0000000000000000 x25: ffff049301496780 x24: 0000000000000000
[   21.297533] x23: 0000000000000000 x22: ffff049301496780 x21: ffffb3dbdf209ae0
[   21.297844] x20: ffffb3dbdf209a20 x19: ffff04933ffeace0 x18: ffff800088573648
[   21.298152] x17: ffffb3dbdf65def0 x16: 00000000d72145a7 x15: 00000000f1a17b35
[   21.298439] x14: 0000000000000004 x13: ffffb3dbdf22c3a8 x12: 0000000000000000
[   21.298738] x11: ffff0493541472a8 x10: 2b58349b7608bfc4 x9 : ffffb3dbdc57f43c
[   21.299031] x8 : ffff049301497858 x7 : 00000000000001c1 x6 : 000000001eeeb49c
[   21.299329] x5 : 4000000000000002 x4 : ffff50b7618d7000 x3 : ffff8000801b3dc0
[   21.299615] x2 : ffffb3dbde713ce0 x1 : 4000000000000000 x0 : ffffb3dbde713ce0
[   21.299900] Call trace:
[   21.299998]  ct_kernel_exit.constprop.0+0x108/0x120
[   21.300460]  ct_idle_enter+0x10/0x20
[   21.300690]  default_idle_call+0x3c/0x170
[   21.300861]  do_idle+0x218/0x278
[   21.300992]  cpu_startup_entry+0x3c/0x50
[   21.301148]  secondary_start_kernel+0x130/0x158
[   21.301329]  __secondary_switched+0xb8/0xc0
[   21.301641] ---[ end trace 0000000000000000 ]---


Can this series be removed from mm-unstable until fixed, please?

Thanks,
Ryan


> 
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> ---
>  fs/userfaultfd.c              |  13 +-
>  include/linux/userfaultfd_k.h |   5 +-
>  mm/userfaultfd.c              | 380 ++++++++++++++++++++++++++--------
>  3 files changed, 296 insertions(+), 102 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..4744d6a96f96 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -20,19 +20,11 @@
>  #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)
> +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 +32,122 @@ 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;
> +}
> +
> +static __always_inline
> +struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
> +						 unsigned long addr)
> +{
> +	struct vm_area_struct *vma;
> +
> +	mmap_assert_locked(mm);
> +	vma = vma_lookup(mm, addr);
> +	if (!vma)
> +		vma = ERR_PTR(-ENOENT);
> +	else if (!(vma->vm_flags & VM_SHARED) &&
> +		 unlikely(anon_vma_prepare(vma)))
> +		vma = ERR_PTR(-ENOMEM);
> +
> +	return vma;
> +}
> +
> +#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.
> + *
> + * Should be called without holding mmap_lock. vma should be unlocked after use
> + * with unlock_vma().
> + *
> + * Return: A locked vma containing @address, -ENOENT 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)
> +{
> +	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 (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma))
> +			vma_end_read(vma);
> +		else
> +			return vma;
> +	}
> +
> +	mmap_read_lock(mm);
> +	vma = find_vma_and_prepare_anon(mm, address);
> +	if (!IS_ERR(vma)) {
> +		/*
> +		 * 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 struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
> +					      unsigned long dst_start,
> +					      unsigned long len)
> +{
> +	struct vm_area_struct *dst_vma;
>  
> +	dst_vma = lock_vma(dst_mm, dst_start);
> +	if (IS_ERR(dst_vma) || validate_dst_vma(dst_vma, dst_start + len))
> +		return dst_vma;
> +
> +	vma_end_read(dst_vma);
> +	return ERR_PTR(-ENOENT);
> +}
> +
> +static void uffd_mfill_unlock(struct vm_area_struct *vma)
> +{
> +	vma_end_read(vma);
> +}
> +
> +#else
> +
> +static struct vm_area_struct *uffd_mfill_lock(struct mm_struct *dst_mm,
> +					      unsigned long dst_start,
> +					      unsigned long len)
> +{
> +	struct vm_area_struct *dst_vma;
> +
> +	mmap_read_lock(dst_mm);
> +	dst_vma = find_vma_and_prepare_anon(dst_mm, dst_start);
> +	if (IS_ERR(dst_vma))
> +		goto out_unlock;
> +
> +	if (validate_dst_vma(dst_vma, dst_start + len))
> +		return dst_vma;
> +
> +	dst_vma = ERR_PTR(-ENOENT);
> +out_unlock:
> +	mmap_read_unlock(dst_mm);
>  	return dst_vma;
>  }
>  
> +static void uffd_mfill_unlock(struct vm_area_struct *vma)
> +{
> +	mmap_read_unlock(vma->vm_mm);
> +}
> +#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 +453,8 @@ 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 +465,6 @@ 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 +483,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	 */
>  	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_ZEROPAGE)) {
>  		up_read(&ctx->map_changing_lock);
> -		mmap_read_unlock(dst_mm);
> +		uffd_mfill_unlock(dst_vma);
>  		return -EINVAL;
>  	}
>  
> @@ -403,24 +506,28 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  	 * retry, dst_vma will be set to NULL and we must lookup again.
>  	 */
>  	if (!dst_vma) {
> +		dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
> +		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 +572,7 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  
>  		if (unlikely(err == -ENOENT)) {
>  			up_read(&ctx->map_changing_lock);
> -			mmap_read_unlock(dst_mm);
> +			uffd_mfill_unlock(dst_vma);
>  			BUG_ON(!folio);
>  
>  			err = copy_folio_from_user(folio,
> @@ -474,17 +581,6 @@ 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 +601,8 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>  
>  out_unlock:
>  	up_read(&ctx->map_changing_lock);
> -	mmap_read_unlock(dst_mm);
> +out_unlock_vma:
> +	uffd_mfill_unlock(dst_vma);
>  out:
>  	if (folio)
>  		folio_put(folio);
> @@ -597,7 +694,15 @@ 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.
> +	 */
> +	dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
> +	if (IS_ERR(dst_vma)) {
> +		err = PTR_ERR(dst_vma);
> +		goto out;
> +	}
>  
>  	/*
>  	 * If memory mappings are changing because of non-cooperative
> @@ -609,15 +714,6 @@ 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 +743,6 @@ 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 +785,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  			void *kaddr;
>  
>  			up_read(&ctx->map_changing_lock);
> -			mmap_read_unlock(dst_mm);
> +			uffd_mfill_unlock(dst_vma);
>  			BUG_ON(!folio);
>  
>  			kaddr = kmap_local_folio(folio, 0);
> @@ -730,7 +816,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>  
>  out_unlock:
>  	up_read(&ctx->map_changing_lock);
> -	mmap_read_unlock(dst_mm);
> +	uffd_mfill_unlock(dst_vma);
>  out:
>  	if (folio)
>  		folio_put(folio);
> @@ -1267,27 +1353,136 @@ static int validate_move_areas(struct userfaultfd_ctx *ctx,
>  	if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
>  		return -EINVAL;
>  
> +	return 0;
> +}
> +
> +static __always_inline
> +int find_vmas_mm_locked(struct mm_struct *mm,
> +			unsigned long dst_start,
> +			unsigned long src_start,
> +			struct vm_area_struct **dst_vmap,
> +			struct vm_area_struct **src_vmap)
> +{
> +	struct vm_area_struct *vma;
> +
> +	mmap_assert_locked(mm);
> +	vma = find_vma_and_prepare_anon(mm, dst_start);
> +	if (IS_ERR(vma))
> +		return PTR_ERR(vma);
> +
> +	*dst_vmap = vma;
> +	/* Skip finding src_vma if src_start is in dst_vma */
> +	if (src_start >= vma->vm_start && src_start < vma->vm_end)
> +		goto out_success;
> +
> +	vma = vma_lookup(mm, src_start);
> +	if (!vma)
> +		return -ENOENT;
> +out_success:
> +	*src_vmap = vma;
> +	return 0;
> +}
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +static int uffd_move_lock(struct mm_struct *mm,
> +			  unsigned long dst_start,
> +			  unsigned long src_start,
> +			  struct vm_area_struct **dst_vmap,
> +			  struct vm_area_struct **src_vmap)
> +{
> +	struct vm_area_struct *vma;
> +	int err;
> +
> +	vma = lock_vma(mm, dst_start);
> +	if (IS_ERR(vma))
> +		return PTR_ERR(vma);
> +
> +	*dst_vmap = vma;
>  	/*
> -	 * Ensure the dst_vma has a anon_vma or this page
> -	 * would get a NULL anon_vma when moved in the
> -	 * dst_vma.
> +	 * Skip finding src_vma if src_start is in dst_vma. This also ensures
> +	 * that we don't lock the same vma twice.
>  	 */
> -	if (unlikely(anon_vma_prepare(dst_vma)))
> -		return -ENOMEM;
> +	if (src_start >= vma->vm_start && src_start < vma->vm_end) {
> +		*src_vmap = vma;
> +		return 0;
> +	}
>  
> -	return 0;
> +	/*
> +	 * Using lock_vma() to get src_vma can lead to following deadlock:
> +	 *
> +	 * Thread1				Thread2
> +	 * -------				-------
> +	 * vma_start_read(dst_vma)
> +	 *					mmap_write_lock(mm)
> +	 *					vma_start_write(src_vma)
> +	 * vma_start_read(src_vma)
> +	 * mmap_read_lock(mm)
> +	 *					vma_start_write(dst_vma)
> +	 */
> +	*src_vmap = lock_vma_under_rcu(mm, src_start);
> +	if (likely(*src_vmap))
> +		return 0;
> +
> +	/* Undo any locking and retry in mmap_lock critical section */
> +	vma_end_read(*dst_vmap);
> +
> +	mmap_read_lock(mm);
> +	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
> +	if (!err) {
> +		/*
> +		 * See comment in lock_vma() as to why not using
> +		 * vma_start_read() here.
> +		 */
> +		down_read(&(*dst_vmap)->vm_lock->lock);
> +		if (*dst_vmap != *src_vmap)
> +			down_read(&(*src_vmap)->vm_lock->lock);
> +	}
> +	mmap_read_unlock(mm);
> +	return err;
> +}
> +
> +static void uffd_move_unlock(struct vm_area_struct *dst_vma,
> +			     struct vm_area_struct *src_vma)
> +{
> +	vma_end_read(src_vma);
> +	if (src_vma != dst_vma)
> +		vma_end_read(dst_vma);
>  }
>  
> +#else
> +
> +static int uffd_move_lock(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;
> +
> +	mmap_read_lock(mm);
> +	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
> +	if (err)
> +		mmap_read_unlock(mm);
> +	return err;
> +}
> +
> +static void uffd_move_unlock(struct vm_area_struct *dst_vma,
> +			     struct vm_area_struct *src_vma)
> +{
> +	mmap_assert_locked(src_vma->vm_mm);
> +	mmap_read_unlock(dst_vma->vm_mm);
> +}
> +#endif
> +
>  /**
>   * move_pages - move arbitrary anonymous pages of an existing vma
>   * @ctx: pointer to the userfaultfd context
> - * @mm: the address space to move pages
>   * @dst_start: start of the destination virtual memory range
>   * @src_start: start of the source virtual memory range
>   * @len: length of the virtual memory range
>   * @mode: flags from uffdio_move.mode
>   *
> - * Must be called with mmap_lock held for read.
> + * It will either use the mmap_lock in read mode or per-vma locks
>   *
>   * move_pages() remaps arbitrary anonymous pages atomically in zero
>   * copy. It only works on non shared anonymous pages because those can
> @@ -1355,10 +1550,10 @@ 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;
>  	struct vm_area_struct *src_vma, *dst_vma;
>  	unsigned long src_addr, dst_addr;
>  	pmd_t *src_pmd, *dst_pmd;
> @@ -1376,28 +1571,34 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
>  	    WARN_ON_ONCE(dst_start + len <= dst_start))
>  		goto out;
>  
> +	err = uffd_move_lock(mm, dst_start, src_start, &dst_vma, &src_vma);
> +	if (err)
> +		goto out;
> +
> +	/* Re-check after taking map_changing_lock */
> +	err = -EAGAIN;
> +	down_read(&ctx->map_changing_lock);
> +	if (likely(atomic_read(&ctx->mmap_changing)))
> +		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;
> +	err = -EINVAL;
> +	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 +1715,9 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
>  		moved += step_size;
>  	}
>  
> +out_unlock:
> +	up_read(&ctx->map_changing_lock);
> +	uffd_move_unlock(dst_vma, src_vma);
>  out:
>  	VM_WARN_ON(moved < 0);
>  	VM_WARN_ON(err > 0);