Re: [PATCH v2] mm/hugetlb: mm/memory_hotplug: use a folio in scan_movable_pages()

[Sidhartha Kumar]

On 5/31/24 6:05 AM, David Hildenbrand wrote:
> On 31.05.24 15:04, David Hildenbrand wrote:
> > On 30.05.24 19:14, Sidhartha Kumar wrote:
> > > By using a folio in scan_movable_pages() we convert the last user of the
> > > page-based hugetlb information macro functions to the folio version.
> > > After this conversion, we can safely remove the page-based definitions
> > > from include/linux/hugetlb.h.
> > >
> > > Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
> > > ---
> > >
> > > v1 -> v2:
> > >     simplify pfn skipping logic with pfn |= folio_nr_pages(folio) - 1
> > >     per Matthew
> > >
> > >    include/linux/hugetlb.h |  6 +-----
> > >    mm/memory_hotplug.c     | 11 +++++------
> > >    2 files changed, 6 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > > index 15a58f69782c..279aca379b95 100644
> > > --- a/include/linux/hugetlb.h
> > > +++ b/include/linux/hugetlb.h
> > > @@ -616,9 +616,7 @@ static __always_inline                        \
> > >    bool folio_test_hugetlb_##flname(struct folio *folio)        \
> > >        {    void *private = &folio->private;        \
> > >            return test_bit(HPG_##flname, private);        \
> > > -    }                            \
> > > -static inline int HPage##uname(struct page *page)        \
> > > -    { return test_bit(HPG_##flname, &(page->private)); }
> > > +    }
> > >    #define SETHPAGEFLAG(uname, flname)                \
> > >    static __always_inline                        \
> > > @@ -637,8 +635,6 @@ void folio_clear_hugetlb_##flname(struct folio 
> > > *folio)        \
> > >    #define TESTHPAGEFLAG(uname, flname)                \
> > >    static inline bool                        \
> > >    folio_test_hugetlb_##flname(struct folio *folio)        \
> > > -    { return 0; }                        \
> > > -static inline int HPage##uname(struct page *page)        \
> > >        { return 0; }
> > >    #define SETHPAGEFLAG(uname, flname)                \
> > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > > index 431b1f6753c0..9c36eb3bbd3b 100644
> > > --- a/mm/memory_hotplug.c
> > > +++ b/mm/memory_hotplug.c
> > > @@ -1731,8 +1731,8 @@ static int scan_movable_pages(unsigned long start, 
> > > unsigned long end,
> > >        unsigned long pfn;
> > >        for (pfn = start; pfn < end; pfn++) {
> > > -        struct page *page, *head;
> > > -        unsigned long skip;
> > > +        struct page *page;
> > > +        struct folio *folio;
> > >            if (!pfn_valid(pfn))
> > >                continue;
> > > @@ -1753,7 +1753,7 @@ static int scan_movable_pages(unsigned long start, 
> > > unsigned long end,
> > >            if (!PageHuge(page))
> > >                continue;
> > > -        head = compound_head(page);
> > > +        folio = page_folio(page);
> > >            /*
> > >             * This test is racy as we hold no reference or lock.  The
> > >             * hugetlb page could have been free'ed and head is no longer
> > > @@ -1761,10 +1761,9 @@ static int scan_movable_pages(unsigned long start, 
> > > unsigned long end,
> > >             * cases false positives and negatives are possible.  Calling
> > >             * code must deal with these scenarios.
> > >             */
> > > -        if (HPageMigratable(head))
> > > +        if (folio_test_hugetlb_migratable(folio))
> > >                goto found;
> > > -        skip = compound_nr(head) - (pfn - page_to_pfn(head));
> > > -        pfn += skip - 1;
> > > +        pfn |= folio_nr_pages(folio) - 1;
> >
> > Likely not exactly what we want?
> >
> > pfn |= folio_nr_pages(folio);
> >
> > Would make sure that we are "one PFN before the start of the next
> > folio". The pfn++ before the next loop iteration would move us to the
> > next folio.
> >
> > Or am I missing something?
>
> Okay, I got it wrong.
>
> "folio_nr_pages(folio) - 1" gives us the bitmask to land one PFN before the end.

ya because folio_nr_pages() will be a power of 2, subtracting 1 turns it into a 
bitmask.

> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks for taking a look at this.

> > It might be cleaner if we would handle the "pfn++;" on the "continue;"
> > paths inmstead, and simply here do something like
> >
> >     pfn = ALIGN(pfn + 1, folio_nr_pages(folio));
> >
> > instead.