Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

[David Hildenbrand]

On 09.10.23 18:21, Suren Baghdasaryan wrote:
> On Mon, Oct 9, 2023 at 7:38 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 09.10.23 08:42, Suren Baghdasaryan wrote:
> > > From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx/
> > > [2] https://lore.kernel.org/linux-mm/CA+EESO4uO84SSnBhArH4HvLNhaUQ5nZKNKXqxRCyjniNVjp0Aw@xxxxxxxxxxxxxx/
> > >
> > > 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
> > >                 UFFDIO_MOVE operation.
> >
> > A general comment simply because I realized that just now: does anything
> > speak against limiting the operations now to a single MM?
> >
> > The use cases I heard so far don't need it. If ever required, we could
> > consider extending it.
> >
> > Let's reduce complexity and KIS unless really required.
>
> Let me check if there are use cases that require moves between MMs.
> Andrea seems to have put considerable effort to make it work between
> MMs and it would be a pity to lose that. I can send a follow-up patch
> to recover that functionality and even if it does not get merged, it
> can be used in the future as a reference. But first let me check if we
> can drop it.

Yes, that sounds reasonable. Unless the big important use cases requires 
moving pages between processes, let's leave that as future work for now.

--
Cheers,

David / dhildenb