Re: [PATCH v2 07/13] mm: Remove HUGETLB_PAGE_DTOR

[Mike Kravetz <mike.kravetz@oracle.com>]

On 08/22/23 04:32, Matthew Wilcox wrote:
> On Mon, Aug 21, 2023 at 08:13:00PM -0700, Mike Kravetz wrote:
> > On 08/16/23 16:11, Matthew Wilcox (Oracle) wrote:
> > > We can use a bit in page[1].flags to indicate that this folio belongs
> > > to hugetlb instead of using a value in page[1].dtors.  That lets
> > > folio_test_hugetlb() become an inline function like it should be.
> > > We can also get rid of NULL_COMPOUND_DTOR.
> > 
> > Not 100% sure yet, but I suspect this patch/series causes the following
> > BUG in today's linux-next.  I can do a deeper dive tomorrow.
> > 
> > # echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
> > # echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
> > 
> > [  352.631099] page:ffffea0007a30000 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1e8c00
> > [  352.633674] head:ffffea0007a30000 order:8 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> 
> order 8?  Well, that's exciting.  This is clearly x86, so it should be
> order 9.  Did we mistakenly clear bit 0 of tail->flags?
> 
> Oh.  Oh yes, we do.
> 
> __update_and_free_hugetlb_folio:
> 
>         for (i = 0; i < pages_per_huge_page(h); i++) {
>                 subpage = folio_page(folio, i);
>                 subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
>                                 1 << PG_referenced | 1 << PG_dirty |
>                                 1 << PG_active | 1 << PG_private |
>                                 1 << PG_writeback);
>         }
> 
> locked		PF_NO_TAIL
> error		PF_NO_TAIL
> referenced	PF_HEAD
> dirty		PF_HEAD
> active		PF_HEAD
> private		PF_ANY
> writeback	PF_NO_TAIL
> 
> So it seems to me there's no way to set any of these bits other than
> PG_private.  And, well, you control PG_private in hugetlbfs.  Do you
> ever set it on tail pages?

Nope

> 
> I think you can remove this entire loop and be happy?

I believe you are correct.  Although, this is clearing flags in the head
page as well as tail pages.  So, I think we still need to clear referenced,
dirty and active as well as private on the head page.

Will take a look today.
-- 
Mike Kravetz