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

Jens Axboe
2020-08-28T18:24:17Z

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