Re: [PATCH v7 3/5] Add preadv and pwritev system calls.

[Michael Kerrisk <mtk.manpages@googlemail.com>]

Gerd,

On Mon, Jan 26, 2009 at 2:26 PM, Gerd Hoffmann <kraxel@redhat.com> wrote:
> This patch adds preadv and pwritev system calls.  These syscalls are a
> pretty straightforward combination of pread and readv (same for write).
> They are quite useful for doing vectored I/O in threaded applications.
> Using lseek+readv instead opens race windows you'll have to plug with
> locking.
>
> Other systems have such system calls too, for example NetBSD, check
> here: http://www.daemon-systems.org/man/preadv.2.html
>
> The application-visible interface provided by glibc should look like
> this to be compatible to the existing implementations in the *BSD family:
>
>  ssize_t preadv(int d, const struct iovec *iov, int iovcnt, off_t offset);
>  ssize_t pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);

I earlier asked if you could provide some userspace example code using
this API.  If there was a response, I missed it.  Could you please
provide some working test using this interface.

Cheers,

Michael

> This prototype has one problem though:  On 32bit archs is the (64bit)
> offset argument unaligned, which the syscall ABI of several archs
> doesn't allow to do.  At least s390 needs a wrapper in glibc to handle
> this.  As we'll need a wrappers in glibc anyway I've decided to push
> problem to glibc entriely and use a syscall prototype which works
> without arch-specific wrappers inside the kernel:  The offset argument
> is explicitly splitted into two 32bit values.
>
> The patch sports the actual system call implementation and the windup in
> the x86 system call tables.  Other archs follow as separate patches.
>
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  arch/x86/ia32/ia32entry.S          |    2 +
>  arch/x86/include/asm/unistd_32.h   |    2 +
>  arch/x86/include/asm/unistd_64.h   |    4 +++
>  arch/x86/kernel/syscall_table_32.S |    2 +
>  fs/compat.c                        |   36 ++++++++++++++++++++++++++
>  fs/read_write.c                    |   50 ++++++++++++++++++++++++++++++++++++
>  include/linux/compat.h             |    6 ++++
>  include/linux/syscalls.h           |    4 +++
>  8 files changed, 106 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> index 256b00b..9a8501b 100644
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -826,4 +826,6 @@ ia32_sys_call_table:
>        .quad sys_dup3                  /* 330 */
>        .quad sys_pipe2
>        .quad sys_inotify_init1
> +       .quad compat_sys_preadv
> +       .quad compat_sys_pwritev
>  ia32_syscall_end:
> diff --git a/arch/x86/include/asm/unistd_32.h b/arch/x86/include/asm/unistd_32.h
> index f2bba78..6e72d74 100644
> --- a/arch/x86/include/asm/unistd_32.h
> +++ b/arch/x86/include/asm/unistd_32.h
> @@ -338,6 +338,8 @@
>  #define __NR_dup3              330
>  #define __NR_pipe2             331
>  #define __NR_inotify_init1     332
> +#define __NR_preadv            333
> +#define __NR_pwritev           334
>
>  #ifdef __KERNEL__
>
> diff --git a/arch/x86/include/asm/unistd_64.h b/arch/x86/include/asm/unistd_64.h
> index d2e415e..f818294 100644
> --- a/arch/x86/include/asm/unistd_64.h
> +++ b/arch/x86/include/asm/unistd_64.h
> @@ -653,6 +653,10 @@ __SYSCALL(__NR_dup3, sys_dup3)
>  __SYSCALL(__NR_pipe2, sys_pipe2)
>  #define __NR_inotify_init1                     294
>  __SYSCALL(__NR_inotify_init1, sys_inotify_init1)
> +#define __NR_preadv                            295
> +__SYSCALL(__NR_preadv, sys_preadv)
> +#define __NR_pwritev                           296
> +__SYSCALL(__NR_pwritev, sys_pwritev)
>
>
>  #ifndef __NO_STUBS
> diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S
> index e2e86a0..106204c 100644
> --- a/arch/x86/kernel/syscall_table_32.S
> +++ b/arch/x86/kernel/syscall_table_32.S
> @@ -332,3 +332,5 @@ ENTRY(sys_call_table)
>        .long sys_dup3                  /* 330 */
>        .long sys_pipe2
>        .long sys_inotify_init1
> +       .long sys_preadv
> +       .long sys_pwritev
> diff --git a/fs/compat.c b/fs/compat.c
> index 4e65644..1a67eee 100644
> --- a/fs/compat.c
> +++ b/fs/compat.c
> @@ -1204,6 +1204,24 @@ compat_sys_readv(unsigned long fd, const struct compat_iovec __user *vec,
>        return ret;
>  }
>
> +asmlinkage ssize_t
> +compat_sys_preadv(unsigned long fd, const struct compat_iovec __user *vec,
> +                 unsigned long vlen, u32 pos_high, u32 pos_low)
> +{
> +       loff_t pos = ((loff_t)pos_high << 32) | pos_low;
> +       struct file *file;
> +       ssize_t ret;
> +
> +       if (pos < 0)
> +               return -EINVAL;
> +       file = fget(fd);
> +       if (!file)
> +               return -EBADF;
> +       ret = compat_readv(file, vec, vlen, &pos);
> +       fput(file);
> +       return ret;
> +}
> +
>  static size_t compat_writev(struct file *file,
>                            const struct compat_iovec __user *vec,
>                            unsigned long vlen, loff_t *pos)
> @@ -1241,6 +1259,24 @@ compat_sys_writev(unsigned long fd, const struct compat_iovec __user *vec,
>        return ret;
>  }
>
> +asmlinkage ssize_t
> +compat_sys_pwritev(unsigned long fd, const struct compat_iovec __user *vec,
> +                  unsigned long vlen, u32 pos_high, u32 pos_low)
> +{
> +       loff_t pos = ((loff_t)pos_high << 32) | pos_low;
> +       struct file *file;
> +       ssize_t ret;
> +
> +       if (pos < 0)
> +               return -EINVAL;
> +       file = fget(fd);
> +       if (!file)
> +               return -EBADF;
> +       ret = compat_writev(file, vec, vlen, &pos);
> +       fput(file);
> +       return ret;
> +}
> +
>  asmlinkage long
>  compat_sys_vmsplice(int fd, const struct compat_iovec __user *iov32,
>                    unsigned int nr_segs, unsigned int flags)
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 400fe81..6d5d8ff 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -731,6 +731,56 @@ SYSCALL_DEFINE3(writev, unsigned long, fd, const struct iovec __user *, vec,
>        return ret;
>  }
>
> +SYSCALL_DEFINE5(preadv, unsigned long, fd, const struct iovec __user *, vec,
> +               unsigned long, vlen, u32, pos_high, u32, pos_low)
> +{
> +       loff_t pos = ((loff_t)pos_high << 32) | pos_low;
> +       struct file *file;
> +       ssize_t ret = -EBADF;
> +       int fput_needed;
> +
> +       if (pos < 0)
> +               return -EINVAL;
> +
> +       file = fget_light(fd, &fput_needed);
> +       if (file) {
> +               ret = -ESPIPE;
> +               if (file->f_mode & FMODE_PREAD)
> +                       ret = vfs_readv(file, vec, vlen, &pos);
> +               fput_light(file, fput_needed);
> +       }
> +
> +       if (ret > 0)
> +               add_rchar(current, ret);
> +       inc_syscr(current);
> +       return ret;
> +}
> +
> +SYSCALL_DEFINE5(pwritev, unsigned long, fd, const struct iovec __user *, vec,
> +               unsigned long, vlen, u32, pos_high, u32, pos_low)
> +{
> +       loff_t pos = ((loff_t)pos_high << 32) | pos_low;
> +       struct file *file;
> +       ssize_t ret = -EBADF;
> +       int fput_needed;
> +
> +       if (pos < 0)
> +               return -EINVAL;
> +
> +       file = fget_light(fd, &fput_needed);
> +       if (file) {
> +               ret = -ESPIPE;
> +               if (file->f_mode & FMODE_PWRITE)
> +                       ret = vfs_writev(file, vec, vlen, &pos);
> +               fput_light(file, fput_needed);
> +       }
> +
> +       if (ret > 0)
> +               add_wchar(current, ret);
> +       inc_syscw(current);
> +       return ret;
> +}
> +
>  static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
>                           size_t count, loff_t max)
>  {
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index 3fd2194..79dba49 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -183,6 +183,12 @@ asmlinkage ssize_t compat_sys_readv(unsigned long fd,
>                const struct compat_iovec __user *vec, unsigned long vlen);
>  asmlinkage ssize_t compat_sys_writev(unsigned long fd,
>                const struct compat_iovec __user *vec, unsigned long vlen);
> +asmlinkage ssize_t compat_sys_preadv(unsigned long fd,
> +               const struct compat_iovec __user *vec,
> +               unsigned long vlen, u32 pos_high, u32 pos_low);
> +asmlinkage ssize_t compat_sys_pwritev(unsigned long fd,
> +               const struct compat_iovec __user *vec,
> +               unsigned long vlen, u32 pos_high, u32 pos_low);
>
>  int compat_do_execve(char * filename, compat_uptr_t __user *argv,
>                compat_uptr_t __user *envp, struct pt_regs * regs);
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 16875f8..b63e93d 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -456,6 +456,10 @@ asmlinkage long sys_pread64(unsigned int fd, char __user *buf,
>                            size_t count, loff_t pos);
>  asmlinkage long sys_pwrite64(unsigned int fd, const char __user *buf,
>                             size_t count, loff_t pos);
> +asmlinkage long sys_preadv(unsigned long fd, const struct iovec __user *vec,
> +                          unsigned long vlen, u32 pos_high, u32 pos_low);
> +asmlinkage long sys_pwritev(unsigned long fd, const struct iovec __user *vec,
> +                           unsigned long vlen, u32 pos_high, u32 pos_low);
>  asmlinkage long sys_getcwd(char __user *buf, unsigned long size);
>  asmlinkage long sys_mkdir(const char __user *pathname, int mode);
>  asmlinkage long sys_chdir(const char __user *filename);
> --
> 1.6.1
>
> --
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-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
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