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

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 11, 2023 at 12:11:55PM -0400, Zi Yan wrote:
> On 11 Aug 2023, at 12:08, David Hildenbrand wrote:
> 
> > 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.
> >
> For FS supporting large folios, their page cache pages can be any order <= PMD_ORDER.
> See page_cache_ra_order() in mm/readahead.c

Ah yes..

> 
> >>
> >> 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().

Indeed.

Thanks!

-- 
Peter Xu