linux-kernel - Re: [linux-next PATCH v3] drivers/virt/fsl

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Date:   Wed, 2 Sep 2020 01:44:28 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, timur@...escale.com,
        galak@...nel.crashing.org, linux-kernel@...r.kernel.org
Subject: Re: [linux-next PATCH v3] drivers/virt/fsl_hypervisor: Fix error
 handling path

Hi John,

On Tue, Sep 1, 2020 at 4:28 AM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 8/31/20 3:07 PM, Souptick Joarder wrote:
> > First, when memory allocation for sg_list_unaligned failed, there
> > is a bug of calling put_pages() as we haven't pinned any pages.
>
> "we should unpin"

will it be "we shouldn't unpin" ? can you please clarify this ?
>
> ...
> >
> > @@ -250,7 +250,7 @@ static long ioctl_memcpy(struct fsl_hv_ioctl_memcpy __user *p)
> >               num_pages, param.source != -1 ? FOLL_WRITE : 0, pages);
> >
> >       if (num_pinned != num_pages) {
> > -             /* get_user_pages() failed */
> > +             /* get_user_pages_fast() failed */
>
> Let's please just delete that particular comment entirely. It's of
> questionable accuracy (partial success is allowed with this API), and it
> is echoing the code too closely to be worth the line that it consumes.
>
> More importantly, though, we need to split up the cases of gup_fast
> returning a negative value, and a zero or positive value. Either here,
> or at "exit:", the negative return case should just skip any attempt to
> do any put_page() calls at all. Because it's a maintenance hazard to
> leave in a loop that depends on looping from zero, to -ERRNO, and *not*
> doing any loops--especially in the signed/unsigned soupy mess around gup
> calls.
>
>
> >               pr_debug("fsl-hv: could not lock source buffer\n");
> >               ret = (num_pinned < 0) ? num_pinned : -EFAULT;
> >               goto exit;
> > @@ -293,12 +293,12 @@ static long ioctl_memcpy(struct fsl_hv_ioctl_memcpy __user *p)
> >
> >   exit:
> >       if (pages) {
> > -             for (i = 0; i < num_pages; i++)
> > -                     if (pages[i])
> > -                             put_page(pages[i]);
> > +             for (i = 0; i < num_pinned; i++)
> > +                     put_page(pages[i]);
>
> Looks correct. I sometimes wonder why more callers don't use
> release_pages() in situations like this, but that's beyond the scope of
> your work here.
>
>
> thanks,
> --
> John Hubbard
> NVIDIA