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

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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




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