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From:   Jann Horn <jannh@google.com>
Date:   Tue, 13 Aug 2019 17:40:58 +0200
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Subject: Re: [PATCH v5 1/6] mm/page_idle: Add per-pid idle page tracking using
 virtual index
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     kernel list <linux-kernel@vger.kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
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On Tue, Aug 13, 2019 at 5:30 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> On Mon, Aug 12, 2019 at 08:14:38PM +0200, Jann Horn wrote:
> [snip]
> > > +/* Helper to get the start and end frame given a pos and count */
> > > +static int page_idle_get_frames(loff_t pos, size_t count, struct mm_struct *mm,
> > > +                               unsigned long *start, unsigned long *end)
> > > +{
> > > +       unsigned long max_frame;
> > > +
> > > +       /* If an mm is not given, assume we want physical frames */
> > > +       max_frame = mm ? (mm->task_size >> PAGE_SHIFT) : max_pfn;
> > > +
> > > +       if (pos % BITMAP_CHUNK_SIZE || count % BITMAP_CHUNK_SIZE)
> > > +               return -EINVAL;
> > > +
> > > +       *start = pos * BITS_PER_BYTE;
> > > +       if (*start >= max_frame)
> > > +               return -ENXIO;
> > > +
> > > +       *end = *start + count * BITS_PER_BYTE;
> > > +       if (*end > max_frame)
> > > +               *end = max_frame;
> > > +       return 0;
> > > +}
> >
> > You could add some overflow checks for the multiplications. I haven't
> > seen any place where it actually matters, but it seems unclean; and in
> > particular, on a 32-bit architecture where the maximum user address is
> > very high (like with a 4G:4G split), it looks like this function might
> > theoretically return with `*start > *end`, which could be confusing to
> > callers.
>
> I could store the multiplication result in unsigned long long (since we are
> bounds checking with max_frame, start > end would not occur). Something like
> the following (with extraneous casts). But I'll think some more about the
> point you are raising.

check_mul_overflow() exists and could make that a bit cleaner.


> > This means that BITMAP_CHUNK_SIZE is UAPI on big-endian systems,
> > right? My opinion is that it would be slightly nicer to design the
> > UAPI such that incrementing virtual addresses are mapped to
> > incrementing offsets in the buffer (iow, either use bytewise access or
> > use little-endian), but I'm not going to ask you to redesign the UAPI
> > this late.
>
> That would also be slow and consume more memory in userspace buffers.
> Currently, a 64-bit (8 byte) chunk accounts for 64 pages worth or 256KB.

I still wanted to use one bit per page; I just wanted to rearrange the
bits. So the first byte would always contain 8 bits corresponding to
the first 8 pages, instead of corresponding to pages 56-63 on some
systems depending on endianness. Anyway, this is a moot point, since
as you said...

> Also I wanted to keep the interface consistent with the global
> /sys/kernel/mm/page_idle interface.

Sorry, I missed that this is already UAPI in the global interface. I
agree, using a different API for the per-process interface would be a
bad idea.