[PATCH v2 0/1] system/physmem: Improve guest memory allocation failure diagnostics

[PATCH v2 0/1] system/physmem: Improve guest memory allocation failure diagnostics

[Bin Meng]

This revives Philippe Mathieu-Daudé's patch from 2021. The original
posting and discussion are available at:

https://lore.kernel.org/r/20210820155211.3153137-1-philmd@redhat.com

When host memory allocation fails, QEMU reports the affected memory
region and host error, but not the size of the failed allocation. This
makes accidental requests harder to spot. In particular, a bare value
passed to '-m' is interpreted as MiB, so a user expecting bytes can
unintentionally request tens of TiB.

The patch adds the human-readable RAMBlock size to the existing error.
This does not try to explain host-specific overcommit, cgroup, or NUMA
policy. It provides the most useful portable fact available at the
failure site and makes both user mistakes and support reports easier
to diagnose.

Changes in v2:
- Rebase onto current master
- Update the source path from softmmu/physmem.c to system/physmem.c
- Fix a typo in the commit message
- Collect Peter Xu's Reviewed-by and Bin Meng's Tested-by

Philippe Mathieu-Daudé (1):
  system/physmem: Improve guest memory allocation failure error message

 system/physmem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

---
base-commit: eea8fe61b8be8f3016e522e6af24924a0266ca95
branch: virt

-- 
2.53.0

[PATCH v2 1/1] system/physmem: Improve guest memory allocation failure error message

[Bin Meng]

From: Philippe Mathieu-Daudé <philmd@redhat.com>

When Linux refuses to overcommit a seriously wild allocation we get:

  $ qemu-system-i386 -m 40000000
  qemu-system-i386: cannot set up guest memory 'pc.ram':
  Cannot allocate memory

Slightly improve the error message, displaying the memory size
requested (in case the user didn't expect unspecified memory size
unit is in MiB):

  $ qemu-system-i386 -m 40000000
  qemu-system-i386: Cannot set up 38.1 TiB of guest memory 'pc.ram':
  Cannot allocate memory

Reported-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Bin Meng <bin.meng@processmission.com>

---

Changes in v2:
- Rebase onto current master
- Update the source path from softmmu/physmem.c to system/physmem.c
- Fix a typo in the commit message
- Collect Peter Xu's Reviewed-by and Bin Meng's Tested-by

 system/physmem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/system/physmem.c b/system/physmem.c
index 362a00f76c..89ce6479f1 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2171,8 +2171,10 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
                                                   &new_block->mr->align,
                                                   shared, noreserve);
             if (!new_block->host) {
+                g_autofree char *size_s = size_to_str(new_block->max_length);
                 error_setg_errno(errp, errno,
-                                 "cannot set up guest memory '%s'",
+                                 "Cannot set up %s of guest memory '%s'",
+                                 size_s,
                                  memory_region_name(new_block->mr));
                 qemu_mutex_unlock_ramlist();
                 return;
-- 
2.53.0

Re: [PATCH v2 1/1] system/physmem: Improve guest memory allocation failure error message

[Peter Xu]

On Sat, Aug 22, 2026 at 08:27:08PM +0800, Bin Meng wrote:
> From: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> When Linux refuses to overcommit a seriously wild allocation we get:
> 
>   $ qemu-system-i386 -m 40000000
>   qemu-system-i386: cannot set up guest memory 'pc.ram':
>   Cannot allocate memory
> 
> Slightly improve the error message, displaying the memory size
> requested (in case the user didn't expect unspecified memory size
> unit is in MiB):
> 
>   $ qemu-system-i386 -m 40000000
>   qemu-system-i386: Cannot set up 38.1 TiB of guest memory 'pc.ram':
>   Cannot allocate memory
> 
> Reported-by: Bin Meng <bmeng.cn@gmail.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Tested-by: Bin Meng <bmeng.cn@gmail.com>
> Signed-off-by: Bin Meng <bin.meng@processmission.com>
> 
> ---
> 
> Changes in v2:
> - Rebase onto current master
> - Update the source path from softmmu/physmem.c to system/physmem.c
> - Fix a typo in the commit message
> - Collect Peter Xu's Reviewed-by and Bin Meng's Tested-by

queued, thanks.

-- 
Peter Xu

Re: [PATCH v2 1/1] system/physmem: Improve guest memory allocation failure error message

[Philippe Mathieu-Daudé]

On 22/8/26 14:27, Bin Meng wrote:
> From: Philippe Mathieu-Daudé <philmd@redhat.com>
> 
> When Linux refuses to overcommit a seriously wild allocation we get:
> 
>    $ qemu-system-i386 -m 40000000
>    qemu-system-i386: cannot set up guest memory 'pc.ram':
>    Cannot allocate memory
> 
> Slightly improve the error message, displaying the memory size
> requested (in case the user didn't expect unspecified memory size
> unit is in MiB):
> 
>    $ qemu-system-i386 -m 40000000
>    qemu-system-i386: Cannot set up 38.1 TiB of guest memory 'pc.ram':
>    Cannot allocate memory
> 
> Reported-by: Bin Meng <bmeng.cn@gmail.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Tested-by: Bin Meng <bmeng.cn@gmail.com>
> Signed-off-by: Bin Meng <bin.meng@processmission.com>
> 
> ---
> 
> Changes in v2:
> - Rebase onto current master
> - Update the source path from softmmu/physmem.c to system/physmem.c
> - Fix a typo in the commit message
> - Collect Peter Xu's Reviewed-by and Bin Meng's Tested-by

Thanks! :)

Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>

> 
>   system/physmem.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)