From b2b63008b311126ade0a44ddb488d3704dffa51f Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Wed, 14 Feb 2024 16:16:58 +0100 Subject: [PATCH] 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 c1ece84e7c93 ("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 Acked-by: Stefano Garzarella Signed-off-by: David Hildenbrand Message-Id: <20240214151701.29906-12-david@redhat.com> Tested-by: Mario Casquero Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- subprojects/libvhost-user/libvhost-user.c | 54 ++++++++++++++++++++--- 1 file changed, 48 insertions(+), 6 deletions(-) diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c index ef6353d847..55aef5fcc6 100644 --- a/subprojects/libvhost-user/libvhost-user.c +++ b/subprojects/libvhost-user/libvhost-user.c @@ -43,6 +43,8 @@ #include #include #include +#include +#include #ifdef __NR_userfaultfd #include @@ -281,12 +283,32 @@ vu_remove_all_mem_regs(VuDev *dev) dev->nregions = 0; } +static size_t +get_fd_hugepagesize(int fd) +{ +#if defined(__linux__) + struct statfs fs; + int ret; + + do { + ret = fstatfs(fd, &fs); + } while (ret != 0 && errno == EINTR); + + if (!ret && (unsigned int)fs.f_type == HUGETLBFS_MAGIC) { + return fs.f_bsize; + } +#endif + return 0; +} + static void _vu_add_mem_reg(VuDev *dev, VhostUserMemoryRegion *msg_region, int fd) { const uint64_t start_gpa = msg_region->guest_phys_addr; const uint64_t end_gpa = start_gpa + msg_region->memory_size; int prot = PROT_READ | PROT_WRITE; + uint64_t mmap_offset, fd_offset; + size_t hugepagesize; VuDevRegion *r; void *mmap_addr; int low = 0; @@ -300,7 +322,7 @@ _vu_add_mem_reg(VuDev *dev, VhostUserMemoryRegion *msg_region, int fd) msg_region->memory_size); DPRINT(" userspace_addr: 0x%016"PRIx64"\n", msg_region->userspace_addr); - DPRINT(" mmap_offset: 0x%016"PRIx64"\n", + DPRINT(" old mmap_offset: 0x%016"PRIx64"\n", msg_region->mmap_offset); if (dev->postcopy_listening) { @@ -335,11 +357,31 @@ _vu_add_mem_reg(VuDev *dev, VhostUserMemoryRegion *msg_region, int fd) idx = low; /* - * We don't use offset argument of mmap() since the mapped address has - * to be page aligned, and we use huge pages. + * Convert most of msg_region->mmap_offset to fd_offset. In almost all + * cases, this will leave us with mmap_offset == 0, mmap()'ing only + * what we really need. Only if a memory region would partially cover + * hugetlb pages, we'd get mmap_offset != 0, which usually doesn't happen + * anymore (i.e., modern QEMU). + * + * Note that mmap() with hugetlb would fail if the offset into the file + * is not aligned to the huge page size. */ - mmap_addr = mmap(0, msg_region->memory_size + msg_region->mmap_offset, - prot, MAP_SHARED | MAP_NORESERVE, fd, 0); + hugepagesize = get_fd_hugepagesize(fd); + if (hugepagesize) { + fd_offset = ALIGN_DOWN(msg_region->mmap_offset, hugepagesize); + mmap_offset = msg_region->mmap_offset - fd_offset; + } else { + fd_offset = msg_region->mmap_offset; + mmap_offset = 0; + } + + DPRINT(" fd_offset: 0x%016"PRIx64"\n", + fd_offset); + DPRINT(" new mmap_offset: 0x%016"PRIx64"\n", + mmap_offset); + + mmap_addr = mmap(0, msg_region->memory_size + mmap_offset, + prot, MAP_SHARED | MAP_NORESERVE, fd, fd_offset); if (mmap_addr == MAP_FAILED) { vu_panic(dev, "region mmap error: %s", strerror(errno)); return; @@ -354,7 +396,7 @@ _vu_add_mem_reg(VuDev *dev, VhostUserMemoryRegion *msg_region, int fd) r->size = msg_region->memory_size; r->qva = msg_region->userspace_addr; r->mmap_addr = (uint64_t)(uintptr_t)mmap_addr; - r->mmap_offset = msg_region->mmap_offset; + r->mmap_offset = mmap_offset; dev->nregions++; if (dev->postcopy_listening) {