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

[Yang Shi]

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)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (atomic_add_negative(-1, &page[i]._mapcount))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ nr++;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ }
+
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* No PTE map anymore */
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (nr == HPAGE_PMD_NR)
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ClearPageDoubleMap(compound_head(page));
+
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto out;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (PageSwapBacked(page))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ __dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ else
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ __dec_node_page_state(page, NR_FILE_PMDMAPPED);
-
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* The last PMD map is gone */
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ClearPageDoubleMap(compound_head(page));
ÂÂÂÂÂÂÂ } else {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!atomic_add_negative(-1, &page->_mapcount))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto out;


This should guarantee no PTE map anymore, it should be safe to clear the 
flag.

>