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

[David Hildenbrand <david@redhat.com>]

On 09.10.23 18:21, Suren Baghdasaryan wrote:
> On Mon, Oct 9, 2023 at 7:38 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 09.10.23 08:42, 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
>>>                 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