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

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