Re: [PATCH 1/6] mm/thp: Simplify copying of huge zero page pmd when fork

From: David Hildenbrand
Date: Thu Apr 29 2021 - 04:04:12 EST

On 29.04.21 00:50, Peter Xu wrote:
Huge zero page is handled in a special path in copy_huge_pmd(), however it
should share most codes with a normal thp page. Trying to share more code with
it by removing the special path. The only leftover so far is the huge zero
page refcounting (mm_get_huge_zero_page()), because that's separately done with
a global counter.

This prepares for a future patch to modify the huge pmd to be installed, so
that we don't need to duplicate it explicitly into huge zero page case too.

Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
mm/huge_memory.c | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 98456017744d6..22bf2d0fff79b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1076,17 +1076,13 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
* a page table.
if (is_huge_zero_pmd(pmd)) {
- struct page *zero_page;
* get_huge_zero_page() will never allocate a new page here,
* since we already have a zero page to copy. It just takes a
* reference.
- zero_page = mm_get_huge_zero_page(dst_mm);
- set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
- zero_page);
- ret = 0;
- goto out_unlock;
+ mm_get_huge_zero_page(dst_mm);
+ goto out_zero_page;
src_page = pmd_page(pmd);
@@ -1110,6 +1106,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
page_dup_rmap(src_page, true);
add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);

AFAIKs, the only change is that we're now doing an additional

pmdp_set_wrprotect(src_mm, addr, src_pmd)
pmd = pmd_mkold(pmd_wrprotect(pmd));

But as we are copying the zeropage, it should already be write-protected, so no effective change.



David / dhildenb