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

[Clément Léger]

On 12/07/2024 17:12, Daniel P. Berrangé wrote:
> 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.

Indeed, my bad.

> 
>> 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.

Yes, that seems like it was the case for the previous code. I'll fix that.

Thanks,

Clément

> 
>> +        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