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

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 05/09/23 17:48, Mateusz Guzik wrote:
> 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.

Alright, I have changed to:

  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 on x86-64 with SMAP-capable CPUs 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
>>
>>
> 
>