[v1,3/4] system/physmem: Largepage punch hole before reset of memory pages

Commit Message

“William Roche Oct. 22, 2024, 9:35 p.m. UTC

From: William Roche <[e-mail]>

When the VM reboots, a memory reset is performed calling
qemu_ram_remap() on all hwpoisoned pages.
While we take into account the recorded page sizes to repair the
memory locations, a large page also needs to punch a hole in the
backend file to regenerate a usable memory, cleaning the HW
poisoned section. This is mandatory for hugetlbfs case for example.

Signed-off-by: William Roche <[e-mail]>
---
 system/physmem.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

David Hildenbrand Oct. 23, 2024, 7:30 a.m. UTC | #1

On 22.10.24 23:35, “William Roche wrote:
> From: William Roche <[e-mail]>
> 
> When the VM reboots, a memory reset is performed calling
> qemu_ram_remap() on all hwpoisoned pages.
> While we take into account the recorded page sizes to repair the
> memory locations, a large page also needs to punch a hole in the
> backend file to regenerate a usable memory, cleaning the HW
> poisoned section. This is mandatory for hugetlbfs case for example.
> 
> Signed-off-by: William Roche <[e-mail]>
> ---
>   system/physmem.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/system/physmem.c b/system/physmem.c
> index 3757428336..3f6024a92d 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -2211,6 +2211,14 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
>                   prot = PROT_READ;
>                   prot |= block->flags & RAM_READONLY ? 0 : PROT_WRITE;
>                   if (block->fd >= 0) {
> +                    if (length > TARGET_PAGE_SIZE && fallocate(block->fd,
> +                        FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE,
> +                        offset + block->fd_offset, length) != 0) {
> +                        error_report("Could not recreate the file hole for "
> +                                     "addr: " RAM_ADDR_FMT "@" RAM_ADDR_FMT "",
> +                                     length, addr);
> +                        exit(1);
> +                    }
>                       area = mmap(vaddr, length, prot, flags, block->fd,
>                                   offset + block->fd_offset);
>                   } else {

Ah! Just what I commented to patch #3; we should be using 
ram_discard_range(). It might be better to avoid the mmap() completely 
if ram_discard_range() worked.

And as raised, there is the problem with memory preallocation (where we 
should fail if it doesn't work) and ram discards being disabled because 
something relies on long-term page pinning ...

Patch

diff --git a/system/physmem.c b/system/physmem.c
index 3757428336..3f6024a92d 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2211,6 +2211,14 @@  void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
                 prot = PROT_READ;
                 prot |= block->flags & RAM_READONLY ? 0 : PROT_WRITE;
                 if (block->fd >= 0) {
+                    if (length > TARGET_PAGE_SIZE && fallocate(block->fd,
+                        FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE,
+                        offset + block->fd_offset, length) != 0) {
+                        error_report("Could not recreate the file hole for "
+                                     "addr: " RAM_ADDR_FMT "@" RAM_ADDR_FMT "",
+                                     length, addr);
+                        exit(1);
+                    }
                     area = mmap(vaddr, length, prot, flags, block->fd,
                                 offset + block->fd_offset);
                 } else {