On 4/8/20 11:51 AM, Yang Shi wrote:
On 4/8/20 6:10 AM, Kirill A. Shutemov wrote:
On Mon, Apr 06, 2020 at 01:50:56PM -0700, Yang Shi wrote:
On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
The page can be included into collapse as long as it doesn't have
extra
pins (from GUP or otherwise).
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
mm/khugepaged.c | 25 ++++++++++++++-----------
1 file changed, 14 insertions(+), 11 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 57ff287caf6b..1e7e6543ebca 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -581,11 +581,18 @@ static int
__collapse_huge_page_isolate(struct vm_area_struct *vma,
}
/*
- * cannot use mapcount: can't collapse if there's a gup pin.
- * The page must only be referenced by the scanned process
- * and page swap cache.
+ * Check if the page has any GUP (or other external) pins.
+ *
+ * The page table that maps the page has been already
unlinked
+ * from the page table tree and this process cannot get
+ * additinal pin on the page.
+ *
+ * New pins can come later if the page is shared across fork,
+ * but not for the this process. It is fine. The other
process
+ * cannot write to the page, only trigger CoW.
*/
- if (page_count(page) != 1 + PageSwapCache(page)) {
+ if (total_mapcount(page) + PageSwapCache(page) !=
+ page_count(page)) {
This check looks fine for base page, but what if the page is
PTE-mapped THP?
The following patch made this possible.
If it is PTE-mapped THP and the page is in swap cache, the refcount
would be
512 + the number of PTE-mapped pages.
Shall we do the below change in the following patch?
extra_pins = PageSwapCache(page) ? nr_ccompound(page) - 1 : 0;
if (total_mapcount(page) + PageSwapCache(page) != page_count(page) -
extra_pins) {
...
Looks like you're right.
It would be nice to have a test case to demonstrate the issue.
Is there any way to trigger moving the page to swap cache? I don't
see it
immediately.
It sounds not easy to trigger since it totally depends on timing, I'm
wondering we may have to use MADV_PAGEOUT? Something below off the top
of my head may trigger this?
CPU A CPU B
CPU C
In parent:
MADV_HUGEPAGE
page fault to fill with THP
fork
In Child:
MADV_NOHUGEPAGE
MADV_DONTNEED (split pmd)
MADV_PAGEOUT
->
add_to_swap
khugepaged scan parent and try to
collapse PTE-mapped
->
try_to_unmap
When doing MADV_DONTNEED we need make sure head page is unmapped since
MADV_PAGEOUT would call page_mapcount(page) to skip shared mapping.
This can't trigger it since MADV_PAGEOUT would isolate the page from lru
so khugepaged_scan_pmd() can't reach the refcount check at all.
If try_to_unmap() or pageout() fails, the page may stay in swap cache.
But I didn't think of an easy way to make this happen.
