linux-kernel - Re: [RFC PATCH v2] xen/privcmd: Convert get_user_pages*() to pin_user

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Message-ID: <CAFqt6zZo4ZZ9sHGqMGiYoGoA8HQ2z_ijwnpr_b+PHuAzq31scw@mail.gmail.com>
Date:   Wed, 24 Jun 2020 07:06:01 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Juergen Gross <jgross@...e.com>, sstabellini@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC PATCH v2] xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

On Tue, Jun 23, 2020 at 11:11 PM Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
>
> On 6/23/20 7:58 AM, Souptick Joarder wrote:
> > In 2019, we introduced pin_user_pages*() and now we are converting
> > get_user_pages*() to the new API as appropriate. [1] & [2] could
> > be referred for more information. This is case 5 as per document [1].
> >
> > As discussed, pages need to be marked as dirty before unpinned it.
> >
> > Previously, if lock_pages() end up partially mapping pages, it used
> > to return -ERRNO due to which unlock_pages() have to go through
> > each pages[i] till *nr_pages* to validate them. This can be avoided
> > by passing correct number partially mapped pages & -ERRNO separately
> > while returning from lock_pages() due to error.
> > With this fix unlock_pages() doesn't need to validate pages[i] till
> > *nr_pages* for error scenario.
>
>
> This should be split into two patches please. The first one will fix the
> return value bug (and will need to go to stable branches) and the second
> will use new routine to pin pages.

Initially I split the patches into 2 commits. But at last moment I
figure out that,
this bug fix ( better to call coding error, doesn't looks like lead to
any runtime bug) is tightly coupled to 2nd commit for
pin_user_pages*() conversion,
which means we don't need the bug fix patch if we are not converting the API to
pin_user_pages*()/ unpin_user_pages_dirty_lock(). That's the reason to
clubbed these two
commits into a single one.

If this looks unreasonable, will split it into 2 patches again.


>
>
> > @@ -580,25 +580,30 @@ static long privcmd_ioctl_mmap_batch(
> >
> >  static int lock_pages(
> >       struct privcmd_dm_op_buf kbufs[], unsigned int num,
> > -     struct page *pages[], unsigned int nr_pages)
> > +     struct page *pages[], unsigned int nr_pages, int *errno)
>
>
> I'd prefer if you used more traditional way of returning error code by
> the function, and pass the number of pinned pages as an argument. This
> will also make call site simpler.

Sure, Will do it.

>
>
> -boris
>