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.
Example debug output:
================ Vhost user message ================
Request: VHOST_USER_ADD_MEM_REG (37)
Flags: 0x9
Size: 40
Fds: 59
Adding region 50
guest_phys_addr: 0x0000000d80000000
memory_size: 0x0000000040000000
userspace_addr 0x00007f54ebffe000
mmap_offset 0x0000000c00000000
fd_offset: 0x0000000c00000000
new mmap_offset: 0x0000000000000000
mmap_addr: 0x00007f7ecc000000
Successfully added new region
================ Vhost user message ================
Request: VHOST_USER_ADD_MEM_REG (37)
Flags: 0x9
Size: 40
Fds: 59
Adding region 51
guest_phys_addr: 0x0000000dc0000000
memory_size: 0x0000000040000000
userspace_addr 0x00007f552bffe000
mmap_offset 0x0000000c40000000
fd_offset: 0x0000000c40000000
new mmap_offset: 0x0000000000000000
mmap_addr: 0x00007f7e8c000000
Successfully added new region
Signed-off-by: David Hildenbrand <da...@redhat.com>
---
subprojects/libvhost-user/libvhost-user.c | 50 ++++++++++++++++++++---
1 file changed, 45 insertions(+), 5 deletions(-)
diff --git a/subprojects/libvhost-user/libvhost-user.c
b/subprojects/libvhost-user/libvhost-user.c
index 75e47b7bb3..7d8293dc84 100644
--- a/subprojects/libvhost-user/libvhost-user.c
+++ b/subprojects/libvhost-user/libvhost-user.c
@@ -43,6 +43,8 @@
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/vhost.h>
+#include <sys/vfs.h>
+#include <linux/magic.h>
#ifdef __NR_userfaultfd
#include <linux/userfaultfd.h>
@@ -281,12 +283,36 @@ vu_remove_all_mem_regs(VuDev *dev)
dev->nregions = 0;
}
+static size_t
+get_fd_pagesize(int fd)
+{
+ static size_t pagesize;
+#if defined(__linux__)
+ struct statfs fs;
+ int ret;
+
+ do {
+ ret = fstatfs(fd, &fs);
+ } while (ret != 0 && errno == EINTR);
+
+ if (!ret && fs.f_type == HUGETLBFS_MAGIC) {
+ return fs.f_bsize;
+ }
+#endif
+
+ if (!pagesize) {
+ pagesize = getpagesize();
+ }
+ return pagesize;
+}
+
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;
VuDevRegion *r;
void *mmap_addr;
int low = 0;
@@ -335,11 +361,25 @@ _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);
+ fd_offset = ALIGN_DOWN(msg_region->mmap_offset, get_fd_pagesize(fd));
+ mmap_offset = msg_region->mmap_offset - fd_offset;
+
+ DPRINT(" fd_offset: 0x%016"PRIx64"\n",
+ fd_offset);
+ DPRINT(" adj 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 +394,7 @@ _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) {
--
2.43.0
