On Mon, Oct 9, 2023 at 5:24 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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. For the compaction use case that we have it's fine to limit it to single MM. However, for general use I think Peter will have a better idea. > > 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 >
