Re: [PATCH v2] osdep: add a qemu_close_all_open_fd() helper

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

On Tue, Jun 18, 2024 at 01:17:03PM +0200, Clément Léger wrote:
> Since commit 03e471c41d8b ("qemu_init: increase NOFILE soft limit on
> POSIX"), the maximum number of file descriptors that can be opened are
> raised to nofile.rlim_max. On recent debian distro, this yield a maximum
> of 1073741816 file descriptors. Now, when forking to start
> qemu-bridge-helper, this actually calls close() on the full possible file
> descriptor range (more precisely [3 - sysconf(_SC_OPEN_MAX)]) which
> takes a considerable amount of time. In order to reduce that time,
> factorize existing code to close all open files descriptors in a new
> qemu_close_all_open_fd() function. This function uses various methods
> to close all the open file descriptors ranging from the most efficient
> one to the least one. It also accepts an ordered array of file
> descriptors that should not be closed since this is required by the
> callers that calls it after forking.
> 
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> 
> ----
> 
> v2:
>  - Factorize async_teardown.c close_fds implementation as well as tap.c ones
>  - Apply checkpatch
>  - v1: https://lore.kernel.org/qemu-devel/20240617162520.4045016-1-cleger@rivosinc.com/
> 
> ---
>  include/qemu/osdep.h    |   8 +++
>  net/tap.c               |  31 ++++++-----
>  system/async-teardown.c |  37 +------------
>  util/osdep.c            | 115 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 141 insertions(+), 50 deletions(-)
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index f61edcfdc2..9369a97d3d 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -755,6 +755,14 @@ static inline void qemu_reset_optind(void)
>  
>  int qemu_fdatasync(int fd);
>  
> +/**
> + * Close all open file descriptors except the ones supplied in the @skip array
> + *
> + * @skip: ordered array of distinct file descriptors that should not be closed
> + * @nskip: number of entries in the @skip array.
> + */
> +void qemu_close_all_open_fd(const int *skip, unsigned int nskip);
> +
>  /**
>   * Sync changes made to the memory mapped file back to the backing
>   * storage. For POSIX compliant systems this will fallback
> diff --git a/net/tap.c b/net/tap.c
> index 51f7aec39d..6fc3939078 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -385,6 +385,21 @@ static TAPState *net_tap_fd_init(NetClientState *peer,
>      return s;
>  }
>  
> +static void close_all_fds_after_fork(int excluded_fd)
> +{
> +        const int skip_fd[] = {0, 1, 2, 3, excluded_fd};
> +        unsigned int nskip = ARRAY_SIZE(skip_fd);
> +
> +        /*
> +         * skip_fd must be an ordered array of distinct fds, exclude
> +         * excluded_fd if already included in the [0 - 3] range
> +         */
> +        if (excluded_fd <= 3) {
> +            nskip--;
> +        }
> +        qemu_close_all_open_fd(skip_fd, nskip);
> +}

This is slightly over-indented - 4 space is QEMU normal style.

> diff --git a/util/osdep.c b/util/osdep.c
> index 5d23bbfbec..f3710710e3 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -625,3 +625,118 @@ int qemu_fdatasync(int fd)
>      return fsync(fd);
>  #endif
>  }
> +
> +#ifdef CONFIG_LINUX
> +static bool qemu_close_all_open_fd_proc(const int *skip, unsigned int nskip)
> +{
> +    struct dirent *de;
> +    int fd, dfd;
> +    bool close_fd;
> +    DIR *dir;
> +    int i;
> +
> +    dir = opendir("/proc/self/fd");
> +    if (!dir) {
> +        /* If /proc is not mounted, there is nothing that can be done. */
> +        return false;
> +    }
> +    /* Avoid closing the directory. */
> +    dfd = dirfd(dir);
> +
> +    for (de = readdir(dir); de; de = readdir(dir)) {

Don't we need

   if (de->d_name[0] == '.') {
       continue;
   }

otherwise atoi will fail and we'll try to close(0) multiple
times.

> +        fd = atoi(de->d_name);
> +        close_fd = true;
> +        if (fd == dfd) {
> +            close_fd = false;
> +        } else {
> +            for (i = 0; i < nskip; i++) {
> +                if (fd == skip[i]) {
> +                    close_fd = false;
> +                    break;
> +                }
> +            }
> +        }
> +        if (close_fd) {
> +            close(fd);
> +        }
> +    }
> +    closedir(dir);
> +
> +    return true;
> +}

With regards,
Daniel
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