Re: [PATCH 1/7] debug/mem: add utility to name anonymous memory mappings

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Nov 09, 2026 at 01:59:01PM -0500, John Snow wrote:
> Add support to name mmap'd memory regions using
> PR_SET_VMA_ANON_NAME. On supported platforms, the QEMU utility
> functions are no-ops.
> 
> This is useful primarily for debugging and profiling QEMU; beind able
> to see and correlate each VMA segment with a specific use within QEMU.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  include/qemu/mmap-vma.h | 18 +++++++++++
>  util/mmap-vma.c         | 67 +++++++++++++++++++++++++++++++++++++++++
>  util/meson.build        |  1 +
>  3 files changed, 86 insertions(+)
>  create mode 100644 include/qemu/mmap-vma.h
>  create mode 100644 util/mmap-vma.c
> 
> diff --git a/include/qemu/mmap-vma.h b/include/qemu/mmap-vma.h
> new file mode 100644
> index 00000000000..763ea5a5cab
> --- /dev/null
> +++ b/include/qemu/mmap-vma.h
> @@ -0,0 +1,18 @@
> +/*
> + * Utility for naming VMA allocations in linux
> + *
> + * Copyright (c) Red Hat, Inc 2026
> + *
> + * SPDX-License-Identifier: LGPL-2.1-or-later
> + */
> +
> +#ifndef __QEMU_MMAP_VMA_H
> +#define __QEMU_MMAP_VMA_H
> +
> +#include <stddef.h>
> +#include "osdep.h"
> +
> +int qemu_name_vma(void *ptr, size_t sz, const char *fmt, ...)
> +    G_GNUC_PRINTF(3, 4);
> +
> +#endif
> diff --git a/util/mmap-vma.c b/util/mmap-vma.c
> new file mode 100644
> index 00000000000..7dcef3b7455
> --- /dev/null
> +++ b/util/mmap-vma.c
> @@ -0,0 +1,67 @@
> +/*
> + * Utility for naming VMA allocations in linux
> + *
> + * Copyright (c) Red Hat, Inc 2026
> + *
> + * SPDX-License-Identifier: LGPL-2.1-or-later
> + */
> +#include "qemu/mmap-vma.h"
> +#include "qemu/osdep.h"
> +
> +#include <errno.h>
> +#include <stddef.h>
> +#include <stdio.h>
> +
> +#if defined(__linux__)
> +#include <sys/prctl.h>
> +#endif
> +
> +#define VMA_PREFIX " qemu: "
> +#define VMA_MAXLEN 80
> +
> +/*
> + * Give a human-readable name to a previously mmap'd memory segment.
> + * This feature is only available on Linux >= 5.17, and only when the
> + * kernel was configured with CONFIG_ANON_VMA_NAME=y.
> + *
> + * For systems without this option, prctl() will return -1 and set
> + * errno to EINVAL, and can be safely ignored.
> + *
> + * For systems with this option, these human-readable names will be
> + * visible in /proc/$pid/[s]maps, pmap output, etc.
> + */
> +G_GNUC_PRINTF(3, 0)
> +static int qemu_vname_vma(void *ptr, size_t sz, const char *fmt, va_list args)
> +{
> +    int rc = 0;
> +
> +#if defined(PR_SET_VMA_ANON_NAME)
> +    char strbuf[VMA_MAXLEN];
> +
> +    assert(sizeof(VMA_PREFIX) < sizeof(strbuf));
> +    strncpy(strbuf, VMA_PREFIX, sizeof(strbuf));
> +    rc = vsnprintf(strbuf + sizeof(VMA_PREFIX),
> +                   sizeof(strbuf) - sizeof(VMA_PREFIX), fmt, args);
> +    if (rc < 0) {
> +        return -errno;
> +    } else if (rc >= (sizeof(strbuf) - sizeof(VMA_PREFIX))) {
> +        sprintf(strbuf + sizeof(strbuf) - 4, "...");
> +    }
> +
> +    rc = prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, ptr, sz, strbuf);
> +    if (rc) {
> +        return -errno;
> +    }
> +#endif
> +
> +    return rc;
> +}
> +
> +G_GNUC_PRINTF(3, 4)
> +int qemu_name_vma(void *ptr, size_t sz, const char *fmt, ...)
> +{
> +    va_list args;
> +
> +    va_start(args, fmt);
> +    return qemu_vname_vma(ptr, sz, fmt, args);

We need a va_end here too.

> +}
> diff --git a/util/meson.build b/util/meson.build
> index fa174c07a57..861ee228d5b 100644
> --- a/util/meson.build
> +++ b/util/meson.build
> @@ -1,5 +1,6 @@
>  util_ss.add(files('osdep.c', 'cutils.c', 'unicode.c', 'qemu-timer-common.c'))
>  util_ss.add(files('thread-context.c'), numa)
> +util_ss.add(files('mmap-vma.c'))
>  if host_os != 'windows'
>    util_ss.add(files('aio-posix.c'))
>    util_ss.add(files('fdmon-poll.c'))
> -- 
> 2.56.0
> 

With regards,
Daniel
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