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David Hildenbrand b2b63008b3 libvhost-user: Use most of mmap_offset as fd_offset 
In the past, QEMU would create memory regions that could partially cover
hugetlb pages, making mmap() fail if we would use the mmap_offset as an
fd_offset. For that reason, we never used the mmap_offset as an offset into
the fd and instead always mapped the fd from the very start.

However, that can easily result in us mmap'ing a lot of unnecessary
parts of an fd, possibly repeatedly.

QEMU nowadays does not create memory regions that partially cover huge
pages -- it never really worked with postcopy. QEMU handles merging of
regions that partially cover huge pages (due to holes in boot memory) since
2018 in c1ece84e7c ("vhost: Huge page align and merge").

Let's be a bit careful and not unconditionally convert the
mmap_offset into an fd_offset. Instead, let's simply detect the hugetlb
size and pass as much as we can as fd_offset, making sure that we call
mmap() with a properly aligned offset.

With QEMU and a virtio-mem device that is fully plugged (50GiB using 50
memslots) the qemu-storage daemon process consumes in the VA space
1281GiB before this change and 58GiB after this change.

================ Vhost user message ================
Request: VHOST_USER_ADD_MEM_REG (37)
Flags:   0x9
Size:    40
Fds: 59
Adding region 4
    guest_phys_addr: 0x0000000200000000
    memory_size:     0x0000000040000000
    userspace_addr:  0x00007fb73bffe000
    old mmap_offset: 0x0000000080000000
    fd_offset:       0x0000000080000000
    new mmap_offset: 0x0000000000000000
    mmap_addr:       0x00007f02f1bdc000
Successfully added new region
================ Vhost user message ================
Request: VHOST_USER_ADD_MEM_REG (37)
Flags:   0x9
Size:    40
Fds: 59
Adding region 5
    guest_phys_addr: 0x0000000240000000
    memory_size:     0x0000000040000000
    userspace_addr:  0x00007fb77bffe000
    old mmap_offset: 0x00000000c0000000
    fd_offset:       0x00000000c0000000
    new mmap_offset: 0x0000000000000000
    mmap_addr:       0x00007f0284000000
Successfully added new region

Reviewed-by: Raphael Norwitz <raphael@enfabrica.net>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20240214151701.29906-12-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
2024-03-12 17:56:55 -04:00
..
include Use QEMU_SANITIZE_THREAD 2022-05-03 15:16:21 +04:00
standard-headers meson: fix meson 0.58 warning with libvhost-user subproject 2021-07-29 10:15:52 +02:00
libvhost-user-glib.c libvhost-user: make it a meson subproject 2020-12-08 13:48:58 -05:00
libvhost-user-glib.h libvhost-user: make it a meson subproject 2020-12-08 13:48:58 -05:00
libvhost-user.c libvhost-user: Use most of mmap_offset as fd_offset 2024-03-12 17:56:55 -04:00
libvhost-user.h libvhost-user: Bump up VHOST_USER_MAX_RAM_SLOTS to 509 2024-03-12 17:56:55 -04:00
link-test.c libvhost-user: add a simple link test without glib 2020-12-09 13:04:17 -05:00
meson.build libvhost-user: Add extra compiler warnings 2023-01-11 10:44:35 +01:00