Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

[David Hildenbrand]

On 11.08.23 17:58, Peter Xu wrote:
> On Fri, Aug 11, 2023 at 05:32:37PM +0200, David Hildenbrand wrote:
> > On 11.08.23 17:18, Peter Xu wrote:
> > > On Fri, Aug 11, 2023 at 12:27:13AM +0200, David Hildenbrand wrote:
> > > > On 10.08.23 23:48, Matthew Wilcox wrote:
> > > > > On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
> > > > > > AFAICS if that patch was all correct (while I'm not yet sure..), you can
> > > > > > actually fit your new total mapcount field into page 1 so even avoid the
> > > > > > extra cacheline access.  You can have a look: the trick is refcount for
> > > > > > tail page 1 is still seems to be free on 32 bits (if that was your worry
> > > > > > before).  Then it'll be very nice if to keep Hugh's counter all in tail 1.
> > > > >
> > > > > No, refcount must be 0 on all tail pages.  We rely on this in many places
> > > > > in the MM.
> > > >
> > > > Very right.
> > >
> > > Obviously I could have missed this in the past.. can I ask for an example
> > > explaining why refcount will be referenced before knowing it's a head?
> >
> > I think the issue is, when coming from a PFN walker (or GUP-fast), you might
> > see "oh, this is a folio, let's lookup the head page". And you do that.
> >
> > Then, you try taking a reference on that head page. (see try_get_folio()).
> >
> > But as you didn't hold a reference on the folio yet, it can happily get
> > freed + repurposed in the meantime, so maybe it's not a head page anymore.
> >
> > So if the field would get reused for something else, grabbing a reference
> > would corrupt whatever is now stored in there.
>
> Not an issue before large folios, am I right?  Because having a head page
> reused as tail cannot happen iiuc with current thps if only pmd-sized,
> because the head page is guaranteed to be pmd aligned physically.

There are other users of compound pages, no? THP and hugetlb are just 
two examples I think. For example, I can spot __GFP_COMP in slab code.

Must such compound pages would not be applicable to GUP, though, but to 
PFN walkers could end up trying to grab them.

> I don't really know, where a hugetlb 2M head can be reused by a 1G huge
> later right during the window of fast-gup walking. But obviously that's not
> common either if that could ever happen.
>
> Maybe Matthew was referring to something else (per "in many places")?

There are some other cases where PFN walkers want to identify tail pages 
to skip over them. See the comment in has_unmovable_pages().

--
Cheers,

David / dhildenb