Re: [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map gone

[Kirill A. Shutemov]

On Fri, Oct 25, 2019 at 08:58:22AM -0700, Yang Shi wrote:
> 
> 
> On 10/25/19 8:36 AM, Kirill A. Shutemov wrote:
> > On Fri, Oct 25, 2019 at 01:27:46AM +0800, Yang Shi wrote:
> > > File THP sets PageDoubleMap flag when the first it gets PTE mapped, but
> > > the flag is never cleared until the THP is freed.  This result in
> > > unbalanced state although it is not a big deal.
> > > 
> > > Clear the flag when the last compound_mapcount is gone.  It should be
> > > cleared when all the PTE maps are gone (become PMD mapped only) as well,
> > > but this needs check all subpage's _mapcount every time any subpage's
> > > rmap is removed, the overhead may be not worth.  The anonymous THP also
> > > just clears PageDoubleMap flag when the last PMD map is gone.
> > NAK, sorry.
> > 
> > The key difference with anon THP that file THP can be mapped again with
> > PMD after all PMD (or all) mappings are gone.
> > 
> > Your patch breaks the case when you map the page with PMD again while the
> > page is still mapped with PTEs. Who would set PageDoubleMap() in this
> > case?
> 
> Aha, yes, you are right. I missed that point. However, I'm wondering we
> might move this up a little bit like this:
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d17cbf3..ac046fd 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1230,15 +1230,17 @@ static void page_remove_file_rmap(struct page *page,
> bool compound)
>                         if (atomic_add_negative(-1, &page[i]._mapcount))
>                                 nr++;
>                 }
> +
> +               /* No PTE map anymore */
> +               if (nr == HPAGE_PMD_NR)
> +                       ClearPageDoubleMap(compound_head(page));
> +
>                 if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
>                         goto out;
>                 if (PageSwapBacked(page))
>                         __dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
>                 else
>                         __dec_node_page_state(page, NR_FILE_PMDMAPPED);
> -
> -               /* The last PMD map is gone */
> -               ClearPageDoubleMap(compound_head(page));
>         } else {
>                 if (!atomic_add_negative(-1, &page->_mapcount))
>                         goto out;
> 
> 
> This should guarantee no PTE map anymore, it should be safe to clear the
> flag.

At first glance looks safe, but let me think more about it. I didn't
expect it be that easy :P

-- 
 Kirill A. Shutemov