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

Minchan Kim
2020-08-28T19:04:07Z

On Fri, Aug 28, 2020 at 08:25:34PM +0200, Christian Brauner wrote:
> On Fri, Aug 28, 2020 at 8:24 PM Jens Axboe <axboe@kernel.dk> wrote:
> >
> > 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())
>
> I suggested the exact same solutions roughly 1.5 weeks ago. :)
> Fun when I saw you mentioning this in BBB I knew exactly what you were
> referring too. :)
>

https://lore.kernel.org/linux-man/20200816081227.ngw3l45c5uncesmr@wittgenstein/

Yes, Christian suggested the idea but mostly for only this new syscall.
I don't have the time to revise the patchset yet but may have next week.
I will follow Christian's suggestion.

Thanks.