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Subject: Re: [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map
 gone
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     hughd@google.com, kirill.shutemov@linux.intel.com,
        aarcange@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1571938066-29031-1-git-send-email-yang.shi@linux.alibaba.com>
 <20191025153618.ajcecye3bjm5abax@box>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <74becfc0-3c34-bdd2-02cd-25b763c92f3b@linux.alibaba.com>
Date:   Fri, 25 Oct 2019 08:58:22 -0700
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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.

>