Re: [PATCH v8 3/4] mm/madvise: introduce process_madvise() syscall: an external memory hinting API

[Jens Axboe <axboe@kernel.dk>]

On 8/28/20 11:40 AM, Arnd Bergmann wrote:
> On Mon, Jun 22, 2020 at 9:29 PM Minchan Kim <minchan@kernel.org> wrote:
>> So finally, the API is as follows,
>>
>>      ssize_t process_madvise(int pidfd, const struct iovec *iovec,
>>                unsigned long vlen, int advice, unsigned int flags);
> 
> I had not followed the discussion earlier and only now came across
> the syscall in linux-next, sorry for stirring things up this late.
> 
>> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
>> index 94bf4958d114..8f959d90338a 100644
>> --- a/arch/x86/entry/syscalls/syscall_64.tbl
>> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
>> @@ -364,6 +364,7 @@
>>  440    common  watch_mount             sys_watch_mount
>>  441    common  watch_sb                sys_watch_sb
>>  442    common  fsinfo                  sys_fsinfo
>> +443    64      process_madvise         sys_process_madvise
>>
>>  #
>>  # x32-specific system call numbers start at 512 to avoid cache impact
>> @@ -407,3 +408,4 @@
>>  545    x32     execveat                compat_sys_execveat
>>  546    x32     preadv2                 compat_sys_preadv64v2
>>  547    x32     pwritev2                compat_sys_pwritev64v2
>> +548    x32     process_madvise         compat_sys_process_madvise
> 
> I think we should not add any new x32-specific syscalls. Instead I think
> the compat_sys_process_madvise/sys_process_madvise can be
> merged into one.
> 
>> +       mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
>> +       if (IS_ERR_OR_NULL(mm)) {
>> +               ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
>> +               goto release_task;
>> +       }
> 
> Minor point: Having to use IS_ERR_OR_NULL() tends to be fragile,
> and I would try to avoid that. Can mm_access() be changed to
> itself return PTR_ERR(-ESRCH) instead of NULL to improve its
> calling conventions? I see there are only three other callers.
> 
> 
>> +       ret = import_iovec(READ, vec, vlen, ARRAY_SIZE(iovstack), &iov, &iter);
>> +       if (ret >= 0) {
>> +               ret = do_process_madvise(pidfd, &iter, behavior, flags);
>> +               kfree(iov);
>> +       }
>> +       return ret;
>> +}
>> +
>> +#ifdef CONFIG_COMPAT
> ...
>> +
>> +       ret = compat_import_iovec(READ, vec, vlen, ARRAY_SIZE(iovstack),
>> +                               &iov, &iter);
>> +       if (ret >= 0) {
>> +               ret = do_process_madvise(pidfd, &iter, behavior, flags);
>> +               kfree(iov);
>> +       }
> 
> Every syscall that passes an iovec seems to do this. If we make import_iovec()
> handle both cases directly, this syscall and a number of others can
> be simplified, and you avoid the x32 entry point I mentioned above
> 
> Something like (untested)
> 
> index dad8d0cfaaf7..0de4ddff24c1 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1683,8 +1683,13 @@ ssize_t import_iovec(int type, const struct
> iovec __user * uvector,
>  {
>         ssize_t n;
>         struct iovec *p;
> -       n = rw_copy_check_uvector(type, uvector, nr_segs, fast_segs,
> -                                 *iov, &p);
> +
> +       if (in_compat_syscall())
> +               n = compat_rw_copy_check_uvector(type, uvector, nr_segs,
> +                                                fast_segs, *iov, &p);
> +       else
> +               n = rw_copy_check_uvector(type, uvector, nr_segs,
> +                                         fast_segs, *iov, &p);
>         if (n < 0) {
>                 if (p != *iov)
>                         kfree(p);

Doesn't work for the async case, where you want to be holding on to the
allocated iovec. But in general I think it's a good helper for the sync
case, which is by far the majority.

-- 
Jens Axboe