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

[Peter Xu <peterx@redhat.com>]

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.

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")?

Thanks,

-- 
Peter Xu