[PATCH v6 2/6] system/physmem: poisoned memory discard on reboot

“William Roche
2025-01-27T21:31:42Z

From: William Roche <william.roche@oracle.com>

Repair poisoned memory location(s), calling ram_block_discard_range():
punching a hole in the backend file when necessary and regenerating
a usable memory.
If the kernel doesn't support the madvise calls used by this function
and we are dealing with anonymous memory, fall back to remapping the
location(s).

Signed-off-by: William Roche <william.roche@oracle.com>
---
system/physmem.c | 54 ++++++++++++++++++++++++++++--------------------
1 file changed, 32 insertions(+), 22 deletions(-)

diff --git a/system/physmem.c b/system/physmem.c
index 3dd2adde73..3dc10ae27b 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2167,6 +2167,23 @@ void qemu_ram_free(RAMBlock *block)
}

#ifndef _WIN32
+/* Simply remap the given VM memory location from start to start+length */
+static int qemu_ram_remap_mmap(RAMBlock *block, uint64_t start, size_t length)
+{
+ int flags, prot;
+ void *area;
+ void *host_startaddr = block->host + start;
+
+ assert(block->fd < 0);
+ flags = MAP_FIXED | MAP_ANONYMOUS;
+ flags |= block->flags & RAM_SHARED ? MAP_SHARED : MAP_PRIVATE;
+ flags |= block->flags & RAM_NORESERVE ? MAP_NORESERVE : 0;
+ prot = PROT_READ;
+ prot |= block->flags & RAM_READONLY ? 0 : PROT_WRITE;
+ area = mmap(host_startaddr, length, prot, flags, -1, 0);
+ return area != host_startaddr ? -errno : 0;
+}
+
/*
* qemu_ram_remap - remap a single RAM page
*
@@ -2184,9 +2201,7 @@ void qemu_ram_remap(ram_addr_t addr)
{
RAMBlock *block;
uint64_t offset;
- int flags;
- void *area, *vaddr;
- int prot;
+ void *vaddr;
size_t page_size;

RAMBLOCK_FOREACH(block) {
@@ -2201,25 +2216,20 @@ void qemu_ram_remap(ram_addr_t addr)
;
} else if (xen_enabled()) {
abort();
- } else {
- flags = MAP_FIXED;
- flags |= block->flags & RAM_SHARED ?
- MAP_SHARED : MAP_PRIVATE;
- flags |= block->flags & RAM_NORESERVE ? MAP_NORESERVE : 0;
- prot = PROT_READ;
- prot |= block->flags & RAM_READONLY ? 0 : PROT_WRITE;
- if (block->fd >= 0) {
- area = mmap(vaddr, page_size, prot, flags, block->fd,
- offset + block->fd_offset);
- } else {
- flags |= MAP_ANONYMOUS;
- area = mmap(vaddr, page_size, prot, flags, -1, 0);
- }
- if (area != vaddr) {
- error_report("Could not remap RAM %s:%" PRIx64 "+%" PRIx64
- " +%zx", block->idstr, offset,
- block->fd_offset, page_size);
- exit(1);
+ if (ram_block_discard_range(block, offset, page_size) != 0) {
+ /*
+ * Fall back to using mmap() only for anonymous mapping,
+ * as if a backing file is associated we may not be able
+ * to recover the memory in all cases.
+ * So don't take the risk of using only mmap and fail now.
+ */
+ if (block->fd >= 0 ||
+ qemu_ram_remap_mmap(block, offset, page_size) != 0) {
+ error_report("Could not remap RAM %s:%" PRIx64 "+%"
+ PRIx64 " +%zx", block->idstr, offset,
+ block->fd_offset, page_size);
+ exit(1);
+ }
}
memory_try_enable_merging(vaddr, page_size);
qemu_ram_setup_dump(vaddr, page_size);
--
2.43.5