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

[Mateusz Guzik <mjguzik@gmail.com>]

On 9/5/23, Adhemerval Zanella Netto <adhemerval.zanella@linaro.org> wrote:
>
>
> 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.
>

LGTM

>>
>> 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>