Re: [PATCH v5 2/6] system/physmem: poisoned memory discard on reboot

[David Hildenbrand <david@redhat.com>]

On 10.01.25 22:14, “William Roche wrote:
> 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 | 57 ++++++++++++++++++++++++++++++------------------
>   1 file changed, 36 insertions(+), 21 deletions(-)
> 
> diff --git a/system/physmem.c b/system/physmem.c
> index 7a87548f99..ae1caa97d8 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -2180,13 +2180,32 @@ void qemu_ram_free(RAMBlock *block)
>   }
>   
>   #ifndef _WIN32
> +/* Simply remap the given VM memory location from start to start+length */
> +static void 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);
> +    if (area != host_startaddr) {
> +        error_report("Could not remap addr: " RAM_ADDR_FMT "@" RAM_ADDR_FMT "",
> +                     length, start);
> +        exit(1);
> +    }

Can we return an error and have a single error printed in the caller?

return area != host_startaddr ? -errno : 0;

> +}
> +
>   void qemu_ram_remap(ram_addr_t addr)
>   {
>       RAMBlock *block;
>       ram_addr_t offset;
> -    int flags;
> -    void *area, *vaddr;
> -    int prot;
> +    void *vaddr;
>       size_t page_size;
>   
>       RAMBLOCK_FOREACH(block) {
> @@ -2202,24 +2221,20 @@ 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 addr: "
> -                                 RAM_ADDR_FMT "@" RAM_ADDR_FMT "",
> -                                 page_size, addr);
> -                    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) {
> +                        error_report("Memory poison recovery failure addr: "
> +                                     RAM_ADDR_FMT "@" RAM_ADDR_FMT "",
> +                                     page_size, addr);

See my error message proposal as reply to v4 patch #1: we should similarly
just print rb->idstr, offset and page_size like ram_block_discard_range() does.

ram_addr_t is weird and not helpful for users.


To have a single error

if (ram_block_discard_range(block, offset, page_size) != 0) {
	/* ...
	if (block->fd >= 0 || qemu_ram_remap_mmap(block, offset, page_size)) {
		error_report() ... // use proposal from my reply to v4 patch #1
		exit(1);
	}
}

-- 
Cheers,

David / dhildenb