From: Yang Shi <yang.shi@linux.alibaba.com>
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
Subject: Re: [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map gone
Date: Fri, 25 Oct 2019 08:58:22 -0700 [thread overview]
Message-ID: <74becfc0-3c34-bdd2-02cd-25b763c92f3b@linux.alibaba.com> (raw)
In-Reply-To: <20191025153618.ajcecye3bjm5abax@box>
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.
>
next prev parent reply other threads:[~2019-10-25 15:58 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-24 17:27 [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map gone Yang Shi
2019-10-24 19:26 ` Song Liu
2019-10-25 15:36 ` Kirill A. Shutemov
2019-10-25 15:58 ` Yang Shi [this message]
2019-10-25 16:32 ` Kirill A. Shutemov
2019-10-25 16:39 ` Kirill A. Shutemov
2019-10-25 18:49 ` Yang Shi
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