linux-kernel - Re: [PATCH 1/2] xen/privcmd: Corrected error handling path and mark pages dirty

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Date:   Fri, 26 Jun 2020 12:18:23 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Jürgen Groß <jgross@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        sstabellini@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Paul Durrant <xadimgnik@...il.com>
Subject: Re: [PATCH 1/2] xen/privcmd: Corrected error handling path and mark
 pages dirty

On Fri, Jun 26, 2020 at 11:22 AM Jürgen Groß <jgross@...e.com> wrote:
>
> On 25.06.20 05:02, Souptick Joarder wrote:
> > 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 of 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 and few condition checks can be ignored.
> >
> > As discussed, pages need to be marked as dirty before unpinned it in
> > unlock_pages() which was oversight.
> >
> > Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > Cc: Paul Durrant <xadimgnik@...il.com>
> > ---
> > Hi,
> >
> > I'm compile tested this, but unable to run-time test, so any testing
> > help is much appriciated.
> >
> >   drivers/xen/privcmd.c | 34 +++++++++++++++++++---------------
> >   1 file changed, 19 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > index a250d11..0da417c 100644
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -580,43 +580,44 @@ 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 *pinned)
>
> unsigned int *pinned, please.
>
> >   {
> >       unsigned int i;
> > +     int errno = 0, page_count = 0;
>
> Please drop the errno variable. It is misnamed (you never assign an
> errno value to it) and not needed, as you can ...
>
> >
> >       for (i = 0; i < num; i++) {
> >               unsigned int requested;
> > -             int pinned;
> >
> > +             *pinned += page_count;
> >               requested = DIV_ROUND_UP(
> >                       offset_in_page(kbufs[i].uptr) + kbufs[i].size,
> >                       PAGE_SIZE);
> >               if (requested > nr_pages)
> >                       return -ENOSPC;
> >
> > -             pinned = get_user_pages_fast(
> > +             page_count = get_user_pages_fast(
> >                       (unsigned long) kbufs[i].uptr,
> >                       requested, FOLL_WRITE, pages);
> > -             if (pinned < 0)
> > -                     return pinned;
> > +             if (page_count < 0) {
> > +                     errno = page_count;
> > +                     return errno;
>
> ... just return page_count her, and ...
>
> > +             }
> >
> > -             nr_pages -= pinned;
> > -             pages += pinned;
> > +             nr_pages -= page_count;
> > +             pages += page_count;
> >       }
> >
> > -     return 0;
> > +     return errno;
>
> ... leave this as it was.
>
> >   }
> >
> >   static void unlock_pages(struct page *pages[], unsigned int nr_pages)
> >   {
> >       unsigned int i;
> >
> > -     if (!pages)
> > -             return;
> > -
> >       for (i = 0; i < nr_pages; i++) {
> > -             if (pages[i])
> > -                     put_page(pages[i]);
> > +             if (!PageDirty(page))
> > +                     set_page_dirty_lock(page);
>
> page? Not pages[i]?

I fixed it in compile branch, but missed it in patch creation work
space at the time of posting.
I think, this is the compile error Boris was pointing to.
Sorry about it. I will fix it and post the v2.

>
> > +             put_page(pages[i]);
> >       }
> >   }
> >
> > @@ -630,6 +631,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
> >       struct xen_dm_op_buf *xbufs = NULL;
> >       unsigned int i;
> >       long rc;
> > +     int pinned = 0;
> >
> >       if (copy_from_user(&kdata, udata, sizeof(kdata)))
> >               return -EFAULT;
> > @@ -683,9 +685,11 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
> >               goto out;
> >       }
> >
> > -     rc = lock_pages(kbufs, kdata.num, pages, nr_pages);
> > -     if (rc)
> > +     rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &pinned);
> > +     if (rc < 0) {
> > +             nr_pages = pinned;
> >               goto out;
> > +     }
> >
> >       for (i = 0; i < kdata.num; i++) {
> >               set_xen_guest_handle(xbufs[i].h, kbufs[i].uptr);
> >
>
>
> Juergen