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Date:   Sun, 22 Oct 2023 13:01:58 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
        brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com,
        lokeshgidra@google.com, david@redhat.com, hughd@google.com,
        mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org,
        willy@infradead.org, Liam.Howlett@oracle.com, jannh@google.com,
        zhangpeng362@huawei.com, bgeffon@google.com,
        kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        kernel-team@android.com
Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI
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On Thu, Oct 19, 2023 at 02:24:06PM -0700, Suren Baghdasaryan wrote:
> On Thu, Oct 12, 2023 at 3:00 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Sun, Oct 08, 2023 at 11:42:27PM -0700, Suren Baghdasaryan wrote:
> > > From: Andrea Arcangeli <aarcange@redhat.com>
> > >
> > > Implement the uABI of UFFDIO_MOVE ioctl.
> > > UFFDIO_COPY performs ~20% better than UFFDIO_MOVE when the application
> > > needs pages to be allocated [1]. However, with UFFDIO_MOVE, if pages are
> > > available (in userspace) for recycling, as is usually the case in heap
> > > compaction algorithms, then we can avoid the page allocation and memcpy
> > > (done by UFFDIO_COPY). Also, since the pages are recycled in the
> > > userspace, we avoid the need to release (via madvise) the pages back to
> > > the kernel [2].
> > > We see over 40% reduction (on a Google pixel 6 device) in the compacting
> > > thread’s completion time by using UFFDIO_MOVE vs. UFFDIO_COPY. This was
> > > measured using a benchmark that emulates a heap compaction implementation
> > > using userfaultfd (to allow concurrent accesses by application threads).
> > > More details of the usecase are explained in [2].
> > > Furthermore, UFFDIO_MOVE enables moving swapped-out pages without
> > > touching them within the same vma. Today, it can only be done by mremap,
> > > however it forces splitting the vma.
> > >
> > > [1] https://lore.kernel.org/all/1425575884-2574-1-git-send-email-aarcange@redhat.com/
> > > [2] https://lore.kernel.org/linux-mm/CA+EESO4uO84SSnBhArH4HvLNhaUQ5nZKNKXqxRCyjniNVjp0Aw@mail.gmail.com/
> > >
> > > Update for the ioctl_userfaultfd(2)  manpage:
> > >
> > >    UFFDIO_MOVE
> > >        (Since Linux xxx)  Move a continuous memory chunk into the
> > >        userfault registered range and optionally wake up the blocked
> > >        thread. The source and destination addresses and the number of
> > >        bytes to move are specified by the src, dst, and len fields of
> > >        the uffdio_move structure pointed to by argp:
> > >
> > >            struct uffdio_move {
> > >                __u64 dst;    /* Destination of move */
> > >                __u64 src;    /* Source of move */
> > >                __u64 len;    /* Number of bytes to move */
> > >                __u64 mode;   /* Flags controlling behavior of move */
> > >                __s64 move;   /* Number of bytes moved, or negated error */
> > >            };
> > >
> > >        The following value may be bitwise ORed in mode to change the
> > >        behavior of the UFFDIO_MOVE operation:
> > >
> > >        UFFDIO_MOVE_MODE_DONTWAKE
> > >               Do not wake up the thread that waits for page-fault
> > >               resolution
> > >
> > >        UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES
> > >               Allow holes in the source virtual range that is being moved.
> > >               When not specified, the holes will result in ENOENT error.
> > >               When specified, the holes will be accounted as successfully
> > >               moved memory. This is mostly useful to move hugepage aligned
> > >               virtual regions without knowing if there are transparent
> > >               hugepages in the regions or not, but preventing the risk of
> > >               having to split the hugepage during the operation.
> > >
> > >        The move field is used by the kernel to return the number of
> > >        bytes that was actually moved, or an error (a negated errno-
> > >        style value).  If the value returned in move doesn't match the
> > >        value that was specified in len, the operation fails with the
> > >        error EAGAIN.  The move field is output-only; it is not read by
> > >        the UFFDIO_MOVE operation.
> > >
> > >        The operation may fail for various reasons. Usually, remapping of
> > >        pages that are not exclusive to the given process fail; once KSM
> > >        might deduplicate pages or fork() COW-shares pages during fork()
> > >        with child processes, they are no longer exclusive. Further, the
> > >        kernel might only perform lightweight checks for detecting whether
> > >        the pages are exclusive, and return -EBUSY in case that check fails.
> > >        To make the operation more likely to succeed, KSM should be
> > >        disabled, fork() should be avoided or MADV_DONTFORK should be
> > >        configured for the source VMA before fork().
> > >
> > >        This ioctl(2) operation returns 0 on success.  In this case, the
> > >        entire area was moved.  On error, -1 is returned and errno is
> > >        set to indicate the error.  Possible errors include:
> > >
> > >        EAGAIN The number of bytes moved (i.e., the value returned in
> > >               the move field) does not equal the value that was
> > >               specified in the len field.
> > >
> > >        EINVAL Either dst or len was not a multiple of the system page
> > >               size, or the range specified by src and len or dst and len
> > >               was invalid.
> > >
> > >        EINVAL An invalid bit was specified in the mode field.
> > >
> > >        ENOENT
> > >               The source virtual memory range has unmapped holes and
> > >               UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is not set.
> > >
> > >        EEXIST
> > >               The destination virtual memory range is fully or partially
> > >               mapped.
> > >
> > >        EBUSY
> > >               The pages in the source virtual memory range are not
> > >               exclusive to the process. The kernel might only perform
> > >               lightweight checks for detecting whether the pages are
> > >               exclusive. To make the operation more likely to succeed,
> > >               KSM should be disabled, fork() should be avoided or
> > >               MADV_DONTFORK should be configured for the source virtual
> > >               memory area before fork().
> > >
> > >        ENOMEM Allocating memory needed for the operation failed.
> > >
> > >        ESRCH
> > >               The faulting process has exited at the time of a
> >
> > Nit pick comment for future man page: there's no faulting process in this
> > context. Perhaps "target process"?
> 
> Ack.
> 
> >
> > >               UFFDIO_MOVE operation.
> > >
> > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  Documentation/admin-guide/mm/userfaultfd.rst |   3 +
> > >  fs/userfaultfd.c                             |  63 ++
> > >  include/linux/rmap.h                         |   5 +
> > >  include/linux/userfaultfd_k.h                |  12 +
> > >  include/uapi/linux/userfaultfd.h             |  29 +-
> > >  mm/huge_memory.c                             | 138 +++++
> > >  mm/khugepaged.c                              |   3 +
> > >  mm/rmap.c                                    |   6 +
> > >  mm/userfaultfd.c                             | 602 +++++++++++++++++++
> > >  9 files changed, 860 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
> > > index 203e26da5f92..e5cc8848dcb3 100644
> > > --- a/Documentation/admin-guide/mm/userfaultfd.rst
> > > +++ b/Documentation/admin-guide/mm/userfaultfd.rst
> > > @@ -113,6 +113,9 @@ events, except page fault notifications, may be generated:
> > >    areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
> > >    support for shmem virtual memory areas.
> > >
> > > +- ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an
> > > +  existing page contents from userspace.
> > > +
> > >  The userland application should set the feature flags it intends to use
> > >  when invoking the ``UFFDIO_API`` ioctl, to request that those features be
> > >  enabled if supported.
> > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > index a7c6ef764e63..ac52e0f99a69 100644
> > > --- a/fs/userfaultfd.c
> > > +++ b/fs/userfaultfd.c
> > > @@ -2039,6 +2039,66 @@ static inline unsigned int uffd_ctx_features(__u64 user_features)
> > >       return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
> > >  }
> > >
> > > +static int userfaultfd_remap(struct userfaultfd_ctx *ctx,
> > > +                          unsigned long arg)
> >
> > If we do want to rename from REMAP to MOVE, we'd better rename the
> > functions too, as "remap" still exists all over the place..
> 
> Ok. I thought that since the current implementation only remaps and
> never copies it would be correct to keep "remap" in these internal
> names and change that later if we support copying. But I'm fine with
> renaming them now to avoid confusion. Will do.

"move", not "copy", btw.

Not a big deal, take your preference at each place; "remap" sometimes can
read better, maybe.  Fundamentally, I think it's because both "remap" and
"move" work in 99% cases.  That's also why I think either name would work
here.

> 
> 
> >
> > > +{
> > > +     __s64 ret;
> > > +     struct uffdio_move uffdio_move;
> > > +     struct uffdio_move __user *user_uffdio_move;
> > > +     struct userfaultfd_wake_range range;
> > > +
> > > +     user_uffdio_move = (struct uffdio_move __user *) arg;
> > > +
> > > +     ret = -EAGAIN;
> > > +     if (atomic_read(&ctx->mmap_changing))
> > > +             goto out;
> >
> > I didn't notice this before, but I think we need to re-check this after
> > taking target mm's mmap read lock..
> 
> Ack.
> 
> >
> > maybe we'd want to pass in ctx* into remap_pages(), more reasoning below.
> 
> Makes sense.
> 
> >
> > > +
> > > +     ret = -EFAULT;
> > > +     if (copy_from_user(&uffdio_move, user_uffdio_move,
> > > +                        /* don't copy "remap" last field */
> >
> > s/remap/move/
> 
> Ack.
> 
> >
> > > +                        sizeof(uffdio_move)-sizeof(__s64)))
> > > +             goto out;
> > > +
> > > +     ret = validate_range(ctx->mm, uffdio_move.dst, uffdio_move.len);
> > > +     if (ret)
> > > +             goto out;
> > > +
> > > +     ret = validate_range(current->mm, uffdio_move.src, uffdio_move.len);
> > > +     if (ret)
> > > +             goto out;
> > > +
> > > +     ret = -EINVAL;
> > > +     if (uffdio_move.mode & ~(UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES|
> > > +                               UFFDIO_MOVE_MODE_DONTWAKE))
> > > +             goto out;
> > > +
> > > +     if (mmget_not_zero(ctx->mm)) {
> > > +             ret = remap_pages(ctx->mm, current->mm,
> > > +                               uffdio_move.dst, uffdio_move.src,
> > > +                               uffdio_move.len, uffdio_move.mode);
> > > +             mmput(ctx->mm);
> > > +     } else {
> > > +             return -ESRCH;
> > > +     }
> > > +
> > > +     if (unlikely(put_user(ret, &user_uffdio_move->move)))
> > > +             return -EFAULT;
> > > +     if (ret < 0)
> > > +             goto out;
> > > +
> > > +     /* len == 0 would wake all */
> > > +     BUG_ON(!ret);
> > > +     range.len = ret;
> > > +     if (!(uffdio_move.mode & UFFDIO_MOVE_MODE_DONTWAKE)) {
> > > +             range.start = uffdio_move.dst;
> > > +             wake_userfault(ctx, &range);
> > > +     }
> > > +     ret = range.len == uffdio_move.len ? 0 : -EAGAIN;
> > > +
> > > +out:
> > > +     return ret;
> > > +}
> > > +
> > >  /*
> > >   * userland asks for a certain API version and we return which bits
> > >   * and ioctl commands are implemented in this kernel for such API
> > > @@ -2131,6 +2191,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
> > >       case UFFDIO_ZEROPAGE:
> > >               ret = userfaultfd_zeropage(ctx, arg);
> > >               break;
> > > +     case UFFDIO_MOVE:
> > > +             ret = userfaultfd_remap(ctx, arg);
> > > +             break;
> > >       case UFFDIO_WRITEPROTECT:
> > >               ret = userfaultfd_writeprotect(ctx, arg);
> > >               break;
> > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > index b26fe858fd44..8034eda972e5 100644
> > > --- a/include/linux/rmap.h
> > > +++ b/include/linux/rmap.h
> > > @@ -121,6 +121,11 @@ static inline void anon_vma_lock_write(struct anon_vma *anon_vma)
> > >       down_write(&anon_vma->root->rwsem);
> > >  }
> > >
> > > +static inline int anon_vma_trylock_write(struct anon_vma *anon_vma)
> > > +{
> > > +     return down_write_trylock(&anon_vma->root->rwsem);
> > > +}
> > > +
> > >  static inline void anon_vma_unlock_write(struct anon_vma *anon_vma)
> > >  {
> > >       up_write(&anon_vma->root->rwsem);
> > > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > > index f2dc19f40d05..ce8d20b57e8c 100644
> > > --- a/include/linux/userfaultfd_k.h
> > > +++ b/include/linux/userfaultfd_k.h
> > > @@ -93,6 +93,18 @@ extern int mwriteprotect_range(struct mm_struct *dst_mm,
> > >  extern long uffd_wp_range(struct vm_area_struct *vma,
> > >                         unsigned long start, unsigned long len, bool enable_wp);
> > >
> > > +/* remap_pages */
> > > +void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
> > > +void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
> > > +ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > +                 unsigned long dst_start, unsigned long src_start,
> > > +                 unsigned long len, __u64 flags);
> > > +int remap_pages_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_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,
> > > +                      unsigned long dst_addr, unsigned long src_addr);
> > > +
> > >  /* mm helpers */
> > >  static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
> > >                                       struct vm_userfaultfd_ctx vm_ctx)
> > > diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> > > index 0dbc81015018..2841e4ea8f2c 100644
> > > --- a/include/uapi/linux/userfaultfd.h
> > > +++ b/include/uapi/linux/userfaultfd.h
> > > @@ -41,7 +41,8 @@
> > >                          UFFD_FEATURE_WP_HUGETLBFS_SHMEM |    \
> > >                          UFFD_FEATURE_WP_UNPOPULATED |        \
> > >                          UFFD_FEATURE_POISON |                \
> > > -                        UFFD_FEATURE_WP_ASYNC)
> > > +                        UFFD_FEATURE_WP_ASYNC |              \
> > > +                        UFFD_FEATURE_MOVE)
> > >  #define UFFD_API_IOCTLS                              \
> > >       ((__u64)1 << _UFFDIO_REGISTER |         \
> > >        (__u64)1 << _UFFDIO_UNREGISTER |       \
> > > @@ -50,6 +51,7 @@
> > >       ((__u64)1 << _UFFDIO_WAKE |             \
> > >        (__u64)1 << _UFFDIO_COPY |             \
> > >        (__u64)1 << _UFFDIO_ZEROPAGE |         \
> > > +      (__u64)1 << _UFFDIO_MOVE |             \
> > >        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> > >        (__u64)1 << _UFFDIO_CONTINUE |         \
> > >        (__u64)1 << _UFFDIO_POISON)
> > > @@ -73,6 +75,7 @@
> > >  #define _UFFDIO_WAKE                 (0x02)
> > >  #define _UFFDIO_COPY                 (0x03)
> > >  #define _UFFDIO_ZEROPAGE             (0x04)
> > > +#define _UFFDIO_MOVE                 (0x05)
> > >  #define _UFFDIO_WRITEPROTECT         (0x06)
> > >  #define _UFFDIO_CONTINUE             (0x07)
> > >  #define _UFFDIO_POISON                       (0x08)
> > > @@ -92,6 +95,8 @@
> > >                                     struct uffdio_copy)
> > >  #define UFFDIO_ZEROPAGE              _IOWR(UFFDIO, _UFFDIO_ZEROPAGE, \
> > >                                     struct uffdio_zeropage)
> > > +#define UFFDIO_MOVE          _IOWR(UFFDIO, _UFFDIO_MOVE,     \
> > > +                                   struct uffdio_move)
> > >  #define UFFDIO_WRITEPROTECT  _IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
> > >                                     struct uffdio_writeprotect)
> > >  #define UFFDIO_CONTINUE              _IOWR(UFFDIO, _UFFDIO_CONTINUE, \
> > > @@ -222,6 +227,9 @@ struct uffdio_api {
> > >        * asynchronous mode is supported in which the write fault is
> > >        * automatically resolved and write-protection is un-set.
> > >        * It implies UFFD_FEATURE_WP_UNPOPULATED.
> > > +      *
> > > +      * UFFD_FEATURE_MOVE indicates that the kernel supports moving an
> > > +      * existing page contents from userspace.
> > >        */
> > >  #define UFFD_FEATURE_PAGEFAULT_FLAG_WP               (1<<0)
> > >  #define UFFD_FEATURE_EVENT_FORK                      (1<<1)
> > > @@ -239,6 +247,7 @@ struct uffdio_api {
> > >  #define UFFD_FEATURE_WP_UNPOPULATED          (1<<13)
> > >  #define UFFD_FEATURE_POISON                  (1<<14)
> > >  #define UFFD_FEATURE_WP_ASYNC                        (1<<15)
> > > +#define UFFD_FEATURE_MOVE                    (1<<16)
> > >       __u64 features;
> > >
> > >       __u64 ioctls;
> > > @@ -347,6 +356,24 @@ struct uffdio_poison {
> > >       __s64 updated;
> > >  };
> > >
> > > +struct uffdio_move {
> > > +     __u64 dst;
> > > +     __u64 src;
> > > +     __u64 len;
> > > +     /*
> > > +      * Especially if used to atomically remove memory from the
> > > +      * address space the wake on the dst range is not needed.
> > > +      */
> > > +#define UFFDIO_MOVE_MODE_DONTWAKE            ((__u64)1<<0)
> > > +#define UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES     ((__u64)1<<1)
> > > +     __u64 mode;
> > > +     /*
> > > +      * "move" is written by the ioctl and must be at the end: the
> > > +      * copy_from_user will not read the last 8 bytes.
> > > +      */
> > > +     __s64 move;
> > > +};
> > > +
> > >  /*
> > >   * Flags for the userfaultfd(2) system call itself.
> > >   */
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 9656be95a542..6fac5c3d66e6 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -2086,6 +2086,144 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > >       return ret;
> > >  }
> > >
> > > +#ifdef CONFIG_USERFAULTFD
> > > +/*
> > > + * The PT lock for src_pmd and the mmap_lock for reading are held by
> > > + * the caller, but it must return after releasing the
> > > + * page_table_lock. Just move the page from src_pmd to dst_pmd if possible.
> > > + * Return zero if succeeded in moving the page, -EAGAIN if it needs to be
> > > + * repeated by the caller, or other errors in case of failure.
> > > + */
> > > +int remap_pages_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_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,
> > > +                      unsigned long dst_addr, unsigned long src_addr)
> > > +{
> > > +     pmd_t _dst_pmd, src_pmdval;
> > > +     struct page *src_page;
> > > +     struct folio *src_folio;
> > > +     struct anon_vma *src_anon_vma;
> > > +     spinlock_t *src_ptl, *dst_ptl;
> > > +     pgtable_t src_pgtable, dst_pgtable;
> > > +     struct mmu_notifier_range range;
> > > +     int err = 0;
> > > +
> > > +     src_pmdval = *src_pmd;
> > > +     src_ptl = pmd_lockptr(src_mm, src_pmd);
> > > +
> > > +     lockdep_assert_held(src_ptl);
> > > +     mmap_assert_locked(src_mm);
> > > +     mmap_assert_locked(dst_mm);
> > > +
> > > +     BUG_ON(!pmd_none(dst_pmdval));
> > > +     BUG_ON(src_addr & ~HPAGE_PMD_MASK);
> > > +     BUG_ON(dst_addr & ~HPAGE_PMD_MASK);
> > > +
> > > +     if (!pmd_trans_huge(src_pmdval)) {
> > > +             spin_unlock(src_ptl);
> > > +             if (is_pmd_migration_entry(src_pmdval)) {
> > > +                     pmd_migration_entry_wait(src_mm, &src_pmdval);
> > > +                     return -EAGAIN;
> > > +             }
> > > +             return -ENOENT;
> > > +     }
> > > +
> > > +     src_page = pmd_page(src_pmdval);
> > > +     if (unlikely(!PageAnonExclusive(src_page))) {
> > > +             spin_unlock(src_ptl);
> > > +             return -EBUSY;
> > > +     }
> > > +
> > > +     src_folio = page_folio(src_page);
> > > +     folio_get(src_folio);
> > > +     spin_unlock(src_ptl);
> > > +
> > > +     /* preallocate dst_pgtable if needed */
> > > +     if (dst_mm != src_mm) {
> > > +             dst_pgtable = pte_alloc_one(dst_mm);
> > > +             if (unlikely(!dst_pgtable)) {
> > > +                     err = -ENOMEM;
> > > +                     goto put_folio;
> > > +             }
> > > +     } else {
> > > +             dst_pgtable = NULL;
> > > +     }
> > > +
> >
> > IIUC Lokesh's comment applies here, we probably need the
> > flush_cache_range(), not for x86 but for the other ones..
> >
> > cachetlb.rst:
> >
> >         Next, we have the cache flushing interfaces.  In general, when Linux
> >         is changing an existing virtual-->physical mapping to a new value,
> >         the sequence will be in one of the following forms::
> >
> >         1) flush_cache_mm(mm);
> >            change_all_page_tables_of(mm);
> >            flush_tlb_mm(mm);
> >
> >         2) flush_cache_range(vma, start, end);
> >            change_range_of_page_tables(mm, start, end);
> >            flush_tlb_range(vma, start, end);
> >
> >         3) flush_cache_page(vma, addr, pfn);
> >            set_pte(pte_pointer, new_pte_val);
> >            flush_tlb_page(vma, addr);
> 
> Thanks for the reference. I guess that's to support VIVT caches?

I'm not 100% sure VIVT the only case, but.. yeah flush anything to ram as
long as things cached in va form would be required.

> 
> >
> > > +     mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, src_mm, src_addr,
> > > +                             src_addr + HPAGE_PMD_SIZE);
> > > +     mmu_notifier_invalidate_range_start(&range);
> > > +
> > > +     folio_lock(src_folio);
> > > +
> > > +     /*
> > > +      * split_huge_page walks the anon_vma chain without the page
> > > +      * lock. Serialize against it with the anon_vma lock, the page
> > > +      * lock is not enough.
> > > +      */
> > > +     src_anon_vma = folio_get_anon_vma(src_folio);
> > > +     if (!src_anon_vma) {
> > > +             err = -EAGAIN;
> > > +             goto unlock_folio;
> > > +     }
> > > +     anon_vma_lock_write(src_anon_vma);
> > > +
> > > +     dst_ptl = pmd_lockptr(dst_mm, dst_pmd);
> > > +     double_pt_lock(src_ptl, dst_ptl);
> > > +     if (unlikely(!pmd_same(*src_pmd, src_pmdval) ||
> > > +                  !pmd_same(*dst_pmd, dst_pmdval))) {
> > > +             double_pt_unlock(src_ptl, dst_ptl);
> > > +             err = -EAGAIN;
> > > +             goto put_anon_vma;
> > > +     }
> > > +     if (!PageAnonExclusive(&src_folio->page)) {
> > > +             double_pt_unlock(src_ptl, dst_ptl);
> > > +             err = -EBUSY;
> > > +             goto put_anon_vma;
> > > +     }
> > > +
> > > +     BUG_ON(!folio_test_head(src_folio));
> > > +     BUG_ON(!folio_test_anon(src_folio));
> > > +
> > > +     folio_move_anon_rmap(src_folio, dst_vma);
> > > +     WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> > > +
> > > +     src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
> > > +     _dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
> > > +     _dst_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
> >
> > Last time the conclusion is we leverage can_change_pmd_writable(), no?
> 
> After your explanation that this works correctly for soft-dirty and
> UFFD_WP I thought the only thing left to handle was the check for
> VM_WRITE in both src_vma and dst_vma (which I added into
> validate_remap_areas()). Maybe I misunderstood and if so, I can
> replace the above PageAnonExclusive() with can_change_pmd_writable()
> (note that we err out on VM_SHARED VMAs, so PageAnonExclusive() will
> be included in that check).

I think we still need PageAnonExclusive() because that's the first guard to
decide whether the page can be moved over at all.

What I meant is something like keeping that, then:

        if (pmd_soft_dirty(src_pmdval))
                _dst_pmd = pmd_swp_mksoft_dirty(_dst_pmd);
        if (pmd_uffd_wp(src_pmdval))
                _dst_pmd = pte_swp_mkuffd_wp(swp_pte);
        if (can_change_pmd_writable(_dst_vma, addr, _dst_pmd))
		_dst_pmd = pmd_mkwrite(_dst_pmd, dst_vma);

But I'm not really sure anyone can leverage that, especially after I just
saw move_soft_dirty_pte(): mremap() treat everything dirty after movement.
I don't think there's a clear definition of how we treat memory dirty after
remap.

Maybe we should follow what it does with mremap()?  Then your current code
is fine.  Maybe that's the better start.

> 
> >
> > > +     set_pmd_at(dst_mm, dst_addr, dst_pmd, _dst_pmd);
> > > +
> > > +     src_pgtable = pgtable_trans_huge_withdraw(src_mm, src_pmd);
> > > +     if (dst_pgtable) {
> > > +             pgtable_trans_huge_deposit(dst_mm, dst_pmd, dst_pgtable);
> > > +             pte_free(src_mm, src_pgtable);
> > > +             dst_pgtable = NULL;
> > > +
> > > +             mm_inc_nr_ptes(dst_mm);
> > > +             mm_dec_nr_ptes(src_mm);
> > > +             add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
> > > +             add_mm_counter(src_mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > > +     } else {
> > > +             pgtable_trans_huge_deposit(dst_mm, dst_pmd, src_pgtable);
> > > +     }
> > > +     double_pt_unlock(src_ptl, dst_ptl);
> > > +
> > > +put_anon_vma:
> > > +     anon_vma_unlock_write(src_anon_vma);
> > > +     put_anon_vma(src_anon_vma);
> > > +unlock_folio:
> > > +     /* unblock rmap walks */
> > > +     folio_unlock(src_folio);
> > > +     mmu_notifier_invalidate_range_end(&range);
> > > +     if (dst_pgtable)
> > > +             pte_free(dst_mm, dst_pgtable);
> > > +put_folio:
> > > +     folio_put(src_folio);
> > > +
> > > +     return err;
> > > +}
> > > +#endif /* CONFIG_USERFAULTFD */
> > > +
> > >  /*
> > >   * Returns page table lock pointer if a given pmd maps a thp, NULL otherwise.
> > >   *
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 2b5c0321d96b..0c1ee7172852 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1136,6 +1136,9 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > >        * Prevent all access to pagetables with the exception of
> > >        * gup_fast later handled by the ptep_clear_flush and the VM
> > >        * handled by the anon_vma lock + PG_lock.
> > > +      *
> > > +      * UFFDIO_MOVE is prevented to race as well thanks to the
> > > +      * mmap_lock.
> > >        */
> > >       mmap_write_lock(mm);
> > >       result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index f9ddc50269d2..a5919cac9a08 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -490,6 +490,12 @@ void __init anon_vma_init(void)
> > >   * page_remove_rmap() that the anon_vma pointer from page->mapping is valid
> > >   * if there is a mapcount, we can dereference the anon_vma after observing
> > >   * those.
> > > + *
> > > + * NOTE: the caller should normally hold folio lock when calling this.  If
> > > + * not, the caller needs to double check the anon_vma didn't change after
> > > + * taking the anon_vma lock for either read or write (UFFDIO_MOVE can modify it
> > > + * concurrently without folio lock protection). See folio_lock_anon_vma_read()
> > > + * which has already covered that, and comment above remap_pages().
> > >   */
> > >  struct anon_vma *folio_get_anon_vma(struct folio *folio)
> > >  {
> > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > index 96d9eae5c7cc..45ce1a8b8ab9 100644
> > > --- a/mm/userfaultfd.c
> > > +++ b/mm/userfaultfd.c
> > > @@ -842,3 +842,605 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> > >       mmap_read_unlock(dst_mm);
> > >       return err;
> > >  }
> > > +
> > > +
> > > +void double_pt_lock(spinlock_t *ptl1,
> > > +                 spinlock_t *ptl2)
> > > +     __acquires(ptl1)
> > > +     __acquires(ptl2)
> > > +{
> > > +     spinlock_t *ptl_tmp;
> > > +
> > > +     if (ptl1 > ptl2) {
> > > +             /* exchange ptl1 and ptl2 */
> > > +             ptl_tmp = ptl1;
> > > +             ptl1 = ptl2;
> > > +             ptl2 = ptl_tmp;
> > > +     }
> > > +     /* lock in virtual address order to avoid lock inversion */
> > > +     spin_lock(ptl1);
> > > +     if (ptl1 != ptl2)
> > > +             spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
> > > +     else
> > > +             __acquire(ptl2);
> > > +}
> > > +
> > > +void double_pt_unlock(spinlock_t *ptl1,
> > > +                   spinlock_t *ptl2)
> > > +     __releases(ptl1)
> > > +     __releases(ptl2)
> > > +{
> > > +     spin_unlock(ptl1);
> > > +     if (ptl1 != ptl2)
> > > +             spin_unlock(ptl2);
> > > +     else
> > > +             __release(ptl2);
> > > +}
> > > +
> > > +
> > > +static int remap_present_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > +                          struct vm_area_struct *dst_vma,
> > > +                          struct vm_area_struct *src_vma,
> > > +                          unsigned long dst_addr, unsigned long src_addr,
> > > +                          pte_t *dst_pte, pte_t *src_pte,
> > > +                          pte_t orig_dst_pte, pte_t orig_src_pte,
> > > +                          spinlock_t *dst_ptl, spinlock_t *src_ptl,
> > > +                          struct folio *src_folio)
> > > +{
> > > +     double_pt_lock(dst_ptl, src_ptl);
> > > +
> > > +     if (!pte_same(*src_pte, orig_src_pte) ||
> > > +         !pte_same(*dst_pte, orig_dst_pte)) {
> > > +             double_pt_unlock(dst_ptl, src_ptl);
> > > +             return -EAGAIN;
> > > +     }
> > > +     if (folio_test_large(src_folio) ||
> > > +         !PageAnonExclusive(&src_folio->page)) {
> > > +             double_pt_unlock(dst_ptl, src_ptl);
> > > +             return -EBUSY;
> > > +     }
> > > +
> > > +     BUG_ON(!folio_test_anon(src_folio));
> > > +
> > > +     folio_move_anon_rmap(src_folio, dst_vma);
> > > +     WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> > > +
> > > +     orig_src_pte = ptep_clear_flush(src_vma, src_addr, src_pte);
> > > +     orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
> > > +     orig_dst_pte = maybe_mkwrite(pte_mkdirty(orig_dst_pte), dst_vma);
> >
> > can_change_pte_writable()?
> 
> Same as my previous comment. If that's still needed I'll replace the
> above PageAnonExclusive() check with can_change_pte_writable().

If no one else sees any problem, let's keep your current code, per my above
observations.. to match mremap(), also keep it simple.

One more thing I just remembered on memcg: only uncharge+charge may not
work, I think the lruvec needs to be maintained as well, or memcg shrink
can try to swap some irrelevant page at least, and memcg accounting can
also go wrong.

AFAICT, that means something like another pair of:

        folio_isolate_lru() + folio_putback_lru()

Besides the charge/uncharge.

Yu Zhao should be familiar with that code, maybe you can double check with
him before sending the new version.

I think this will belong to the separate patch to add cross-mm support, but
please also double check even just in case there can be implication of
single-mm that I missed.

Please also don't feel stressed over cross-mm support: at some point if you
see that separate patch grows we can stop from there, listing all the
cross-mm todos/investigations in the cover letter and start with single-mm.

Thanks,

-- 
Peter Xu