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

[David Hildenbrand <david@redhat.com>]

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.


Further: see "22) Do not crash the kernel" in coding-style.rst. All 
these BUG_ON need to go. Ideally, use WARN_ON_ONCE() or just VM_WARN_ON().

-- 
Cheers,

David / dhildenb