Re: [PATCH] io: Do not implement fstat with fstatat

[Mateusz Guzik <mjguzik@gmail.com>]

On 9/5/23, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> AT_EMPTY_PATH is a requirement to implement fstat over fstatat,
> however it does not prevent the kernel to read the path argument.
> It is not an issue, but it has performance implications on x86_64
> with SMAP enable, since each newfstatat syscall generates a pagefault
> even with an empty path.
>

This does not generate page faults in the usual case.

How about:

On x86-64 with SMAP-capable CPUs (vast majority today) the kernel is
forced to perform expensive user memory access. After that regular
lookup is performed which adds even more overhead.

I have no commentso n the patch itself.

Thanks for quick turn around.


> Instead, issue the fstat syscall directly on LFS fstat implementation
> (32 bit architectures will still continue to use statx, which is
> required to have 64 bit time_t support).  it should be even a
> small performance gain on non x86_64, since there is no need
> to handle the path argument.
>
> Checked on x86_64-linux-gnu.
> ---
>  sysdeps/unix/sysv/linux/fstat64.c       | 37 +++++++++++++++++++++++--
>  sysdeps/unix/sysv/linux/fstatat64.c     | 12 ++------
>  sysdeps/unix/sysv/linux/internal-stat.h | 31 +++++++++++++++++++++
>  3 files changed, 68 insertions(+), 12 deletions(-)
>  create mode 100644 sysdeps/unix/sysv/linux/internal-stat.h
>
> diff --git a/sysdeps/unix/sysv/linux/fstat64.c
> b/sysdeps/unix/sysv/linux/fstat64.c
> index 124384e57f..a7f4935a01 100644
> --- a/sysdeps/unix/sysv/linux/fstat64.c
> +++ b/sysdeps/unix/sysv/linux/fstat64.c
> @@ -19,20 +19,53 @@
>  #define __fstat __redirect___fstat
>  #define fstat   __redirect_fstat
>  #include <sys/stat.h>
> +#undef __fstat
> +#undef fstat
>  #include <fcntl.h>
> -#include <kernel_stat.h>
> -#include <stat_t64_cp.h>
> +#include <internal-stat.h>
>  #include <errno.h>
>
>  int
>  __fstat64_time64 (int fd, struct __stat64_t64 *buf)
>  {
> +#if !FSTATAT_USE_STATX
> +# if XSTAT_IS_XSTAT64
> +#  ifdef __NR_newfstatat
> +  /* 64-bit kABI, e.g. aarch64, ia64, powerpc64*, s390x, riscv64, and
> +     x86_64.  */
> +  return INLINE_SYSCALL_CALL (fstat, fd, buf);
> +#  elif defined __NR_fstatat64
> +#   if STAT64_IS_KERNEL_STAT64
> +  /* 64-bit kABI outlier, e.g. alpha  */
> +  return INLINE_SYSCALL_CALL (fstat64, fd, buf);
> +#   else
> +  /* 64-bit kABI outlier, e.g. sparc64.  */
> +  struct kernel_stat64 kst64;
> +  int r = INLINE_SYSCALL_CALL (fstat64, fd, &kst64);
> +  if (r == 0)
> +    __cp_stat64_kstat64 (buf, &kst64);
> +  return r;
> +#   endif /* STAT64_IS_KERNEL_STAT64 */
> +#  endif
> +# else /* XSTAT_IS_XSTAT64 */
> +  /* 64-bit kabi outlier, e.g. mips64 and mips64-n32.  */
> +  struct kernel_stat kst;
> +  int r = INLINE_SYSCALL_CALL (fstat, fd, &kst);
> +  if (r == 0)
> +    __cp_kstat_stat64_t64 (&kst, buf);
> +  return r;
> +# endif
> +#else /* !FSTATAT_USE_STATX  */
> +  /* All kABIs with non-LFS support and with old 32-bit time_t support
> +     e.g. arm, csky, i386, hppa, m68k, microblaze, nios2, sh, powerpc32,
> +     and sparc32.  */
>    if (fd < 0)
>      {
>        __set_errno (EBADF);
>        return -1;
>      }
>    return __fstatat64_time64 (fd, "", buf, AT_EMPTY_PATH);
> +#endif
>  }
>  #if __TIMESIZE != 64
>  hidden_def (__fstat64_time64)
> diff --git a/sysdeps/unix/sysv/linux/fstatat64.c
> b/sysdeps/unix/sysv/linux/fstatat64.c
> index 3509d3ca6d..127c6ff601 100644
> --- a/sysdeps/unix/sysv/linux/fstatat64.c
> +++ b/sysdeps/unix/sysv/linux/fstatat64.c
> @@ -21,12 +21,10 @@
>  #include <sys/stat.h>
>  #include <fcntl.h>
>  #include <string.h>
> -#include <kernel_stat.h>
>  #include <sysdep.h>
>  #include <time.h>
> -#include <kstat_cp.h>
> -#include <stat_t64_cp.h>
>  #include <sys/sysmacros.h>
> +#include <internal-stat.h>
>
>  #if __TIMESIZE == 64 \
>       && (__WORDSIZE == 32 \
> @@ -40,11 +38,7 @@ _Static_assert (sizeof (__blkcnt_t) == sizeof
> (__blkcnt64_t),
>                  "__blkcnt_t and __blkcnt64_t must match");
>  #endif
>
> -#if (__WORDSIZE == 32 \
> -     && (!defined __SYSCALL_WORDSIZE || __SYSCALL_WORDSIZE == 32)) \
> -     || defined STAT_HAS_TIME32 \
> -     || (!defined __NR_newfstatat && !defined __NR_fstatat64)
> -# define FSTATAT_USE_STATX 1
> +#if FSTATAT_USE_STATX
>
>  static inline int
>  fstatat64_time64_statx (int fd, const char *file, struct __stat64_t64
> *buf,
> @@ -79,8 +73,6 @@ fstatat64_time64_statx (int fd, const char *file, struct
> __stat64_t64 *buf,
>
>    return r;
>  }
> -#else
> -# define FSTATAT_USE_STATX 0
>  #endif
>
>  /* Only statx supports 64-bit timestamps for 32-bit architectures with
> diff --git a/sysdeps/unix/sysv/linux/internal-stat.h
> b/sysdeps/unix/sysv/linux/internal-stat.h
> new file mode 100644
> index 0000000000..e3b0569853
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/internal-stat.h
> @@ -0,0 +1,31 @@
> +/* Internal stat definitions.
> +   Copyright (C) 2023 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <sysdep.h>
> +#include <stat_t64_cp.h>
> +#include <kernel_stat.h>
> +#include <kstat_cp.h>
> +
> +#if (__WORDSIZE == 32 \
> +     && (!defined __SYSCALL_WORDSIZE || __SYSCALL_WORDSIZE == 32)) \
> +     || defined STAT_HAS_TIME32 \
> +     || (!defined __NR_newfstatat && !defined __NR_fstatat64)
> +# define FSTATAT_USE_STATX 1
> +#else
> +# define FSTATAT_USE_STATX 0
> +#endif
> --
> 2.34.1
>
>


-- 
Mateusz Guzik <mjguzik gmail.com>