Re: [PATCH v4] osdep: add a qemu_close_all_open_fd() helper

[Philippe Mathieu-Daudé]

Hi Clément,

On 17/7/24 14:45, 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>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> ----

FYI git tools parse 3 '-', not 4.

> v4:
>   - Add a comment saying that qemu_close_all_fds() can take a NULL skip
>     array and nskip == 0
>   - Added an assert in qemu_close_all_fds() to check for skip/nskip
>     parameters
>   - Fix spurious tabs instead of spaces
>   - Applied checkpatch
>   - v3: 
> 20240716144006.6571-1-cleger@rivosinc.com/">https://lore.kernel.org/qemu-devel/20240716144006.6571-1-cleger@rivosinc.com/


> +void qemu_close_all_open_fd(const int *skip, unsigned int nskip)
> +{
> +    int open_max = sysconf(_SC_OPEN_MAX);
> +    unsigned int cur_skip = 0;
> +    int i;
> +
> +    assert(skip != NULL || nskip == 0);
> +
> +    if (qemu_close_all_open_fd_close_range(skip, nskip)) {
> +        return;
> +    }
> +
> +    if (qemu_close_all_open_fd_proc(skip, nskip)) {
> +        return;
> +    }
> +
> +    /* Fallback */
> +    for (i = 0; i < open_max; i++) {
> +        if (cur_skip < nskip && i == skip[cur_skip]) {
> +            cur_skip++;
> +            continue;
> +        }
> +        close(i);
> +    }
> +}

Build failure on windows:

../util/osdep.c: In function 'qemu_close_all_open_fd':
../util/osdep.c:725:20: error: implicit declaration of function 
'sysconf'; did you mean 'swscanf'? [-Wimplicit-function-declaration]
  725 |     int open_max = sysconf(_SC_OPEN_MAX);
      |                    ^~~~~~~
      |                    swscanf
../util/osdep.c:725:20: error: nested extern declaration of 'sysconf' 
[-Werror=nested-externs]
../util/osdep.c:725:28: error: '_SC_OPEN_MAX' undeclared (first use in 
this function); did you mean 'FOPEN_MAX'?
  725 |     int open_max = sysconf(_SC_OPEN_MAX);
      |                            ^~~~~~~~~~~~
      |                            FOPEN_MAX
../util/osdep.c:725:28: note: each undeclared identifier is reported 
only once for each function it appears in