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

From: Yang Shi
Date: Fri Oct 25 2019 - 14:49:42 EST

On 10/25/19 9:39 AM, Kirill A. Shutemov wrote:
On Fri, Oct 25, 2019 at 07:32:33PM +0300, Kirill A. Shutemov wrote:
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
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))
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* No PTE map anymore */
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ClearPageDoubleMap(compound_head(page));
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (PageSwapBacked(page))
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* The last PMD map is gone */
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ClearPageDoubleMap(compound_head(page));
ÂÂÂÂÂÂÂ } else {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!atomic_add_negative(-1, &page->_mapcount))

This should guarantee no PTE map anymore, it should be safe to clear the
At first glance looks safe, but let me think more about it. I didn't
expect it be that easy :P
How do you protect from races? What prevents other thread/process to map
the page as PTE after you've calculated 'nr'?

I don't remember the code that well, but I believe we don't require
PageLock for all cases... Or do we?

No, page lock is required by adding PTE rmap, but not required when removing rmap, i.e. huge pmd split. It looks we can't prevent from the races for processes, threads are protected by ptl.