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

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Date:   Fri, 28 Aug 2020 22:15:24 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Oleksandr Natalenko <oleksandr@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Dias <joaodias@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jann Horn <jannh@...gle.com>,
        alexander.h.duyck@...ux.intel.com,
        SeongJae Park <sj38.park@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Arjun Roy <arjunroy@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Brauner <christian@...uner.io>,
        Daniel Colascione <dancol@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        SeongJae Park <sjpark@...zon.de>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH v8 3/4] mm/madvise: introduce process_madvise() syscall:
 an external memory hinting API

On Fri, Aug 28, 2020 at 9:27 PM Jens Axboe <axboe@...nel.dk> wrote:
> On 8/28/20 12:24 PM, Jens Axboe wrote:

> >> @@ -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.
>
> Nevermind, I'm an idiot for reading this totally wrong.
>

I think you are right about the need to pick the compat vs native
behavior based on req->ctx->compat instead of in_compat_syscall()
inside of io_import_iovec().

That one can probably call a lower-level version and when all other
callers get changed to calling

ssize_t import_iovec(int type, const struct iovec __user * uvector,
                 unsigned nr_segs, unsigned fast_segs,
                 struct iovec **iov, struct iov_iter *i)
{
       return __import_iovec(type, uvector, nr_segs, fast_segs, iov,
i, in_compat_syscall());
}

      Arnd