Re: [PATCH v2 4/4] hugetlb: Do early cow when page pinned on src mm

On 2/4/21 5:43 PM, Peter Xu wrote:

On Thu, Feb 04, 2021 at 03:25:37PM -0800, Mike Kravetz wrote:

quoted

On 2/4/21 6:50 AM, Peter Xu wrote:

quoted

This is the last missing piece of the COW-during-fork effort when there're
pinned pages found.  One can reference 70e806e4e645 ("mm: Do early cow for
pinned pages during fork() for ptes", 2020-09-27) for more information, since
we do similar things here rather than pte this time, but just for hugetlb.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/hugetlb.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 56 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9e6ea96bf33b..5793936e00ef 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
+	__SetPageUptodate(new_page);
+	ClearPagePrivate(new_page);
+	set_page_huge_active(new_page);

Code to replace the above ClearPagePrivate and set_page_huge_active is
in Andrew's tree.  With changes in Andrew's tree, this would be:

	ClearHPageRestoreReserve(new_page);
	SetHPageMigratable(new_page);

Indeed these names are much better than using the default ones.  At the
meantime I'll rebase to linux-next/akpm.  Sorry it's always not easy for me to
find the right branch...

No worries.  I only know because I recently changed these.

...

quoted

quoted

@@ -3787,7 +3803,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 		dst_entry = huge_ptep_get(dst_pte);
 		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry))
 			continue;
-
+again:
 		dst_ptl = huge_pte_lock(h, dst, dst_pte);
 		src_ptl = huge_pte_lockptr(h, src, src_pte);
 		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);

Side question: Mike, do you know why we need this lock_nested()?  Could the src
lock be taken due to any reason already?  It confused me when I read the chunk.

I see that it was added with commit 4647875819aa.  That was when huge pages
used the single per-mm ptl.  Lockdep seemed to complain about taking
&mm->page_table_lock twice.   Certainly, source and destination mm can not
be the same.  Right?  I do not have the full history, but it 'looks' like
lockdep might have been confused and this was added to keep it quiet.

BTW - Copy page range for 'normal' pages has the same spin_lock_nested().
-- 
Mike Kravetz