Re: [PATCHv2 6/8] khugepaged: Allow to collapse PTE-mapped compound pages
From: Yang Shi
Date: Wed Apr 08 2020 - 14:57:38 EST
On 4/8/20 6:29 AM, Kirill A. Shutemov wrote:
On Mon, Apr 06, 2020 at 02:29:35PM -0700, Yang Shi wrote:
On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
We can collapse PTE-mapped compound pages. We only need to avoid
handling them more than once: lock/unlock page only once if it's present
in the PMD range multiple times as it handled on compound level. The
same goes for LRU isolation and putback.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
mm/khugepaged.c | 103 ++++++++++++++++++++++++++++++++----------------
1 file changed, 68 insertions(+), 35 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1e7e6543ebca..49e56e4e30d1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -515,23 +515,37 @@ void __khugepaged_exit(struct mm_struct *mm)
static void release_pte_page(struct page *page)
{
- dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
+ mod_node_page_state(page_pgdat(page),
+ NR_ISOLATED_ANON + page_is_file_cache(page),
+ -compound_nr(page));
unlock_page(page);
putback_lru_page(page);
}
-static void release_pte_pages(pte_t *pte, pte_t *_pte)
+static void release_pte_pages(pte_t *pte, pte_t *_pte,
+ struct list_head *compound_pagelist)
{
+ struct page *page, *tmp;
+
while (--_pte >= pte) {
pte_t pteval = *_pte;
- if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
- release_pte_page(pte_page(pteval));
+
+ page = pte_page(pteval);
+ if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
+ !PageCompound(page))
+ release_pte_page(page);
+ }
+
+ list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
+ list_del(&page->lru);
+ release_pte_page(page);
}
}
static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
unsigned long address,
- pte_t *pte)
+ pte_t *pte,
+ struct list_head *compound_pagelist)
{
struct page *page = NULL;
pte_t *_pte;
@@ -561,13 +575,21 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
goto out;
}
- /* TODO: teach khugepaged to collapse THP mapped with pte */
+ VM_BUG_ON_PAGE(!PageAnon(page), page);
+
if (PageCompound(page)) {
- result = SCAN_PAGE_COMPOUND;
- goto out;
- }
+ struct page *p;
+ page = compound_head(page);
- VM_BUG_ON_PAGE(!PageAnon(page), page);
+ /*
+ * Check if we have dealt with the compound page
+ * already
+ */
+ list_for_each_entry(p, compound_pagelist, lru) {
+ if (page == p)
+ goto next;
+ }
+ }
/*
* We can do it before isolate_lru_page because the
@@ -597,19 +619,15 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
result = SCAN_PAGE_COUNT;
goto out;
}
- if (pte_write(pteval)) {
- writable = true;
- } else {
- if (PageSwapCache(page) &&
- !reuse_swap_page(page, NULL)) {
- unlock_page(page);
- result = SCAN_SWAP_CACHE_PAGE;
- goto out;
- }
+ if (!pte_write(pteval) && PageSwapCache(page) &&
+ !reuse_swap_page(page, NULL)) {
/*
- * Page is not in the swap cache. It can be collapsed
- * into a THP.
+ * Page is in the swap cache and cannot be re-used.
+ * It cannot be collapsed into a THP.
*/
+ unlock_page(page);
+ result = SCAN_SWAP_CACHE_PAGE;
+ goto out;
}
/*
@@ -621,16 +639,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
result = SCAN_DEL_PAGE_LRU;
goto out;
}
- inc_node_page_state(page,
- NR_ISOLATED_ANON + page_is_file_cache(page));
+ mod_node_page_state(page_pgdat(page),
+ NR_ISOLATED_ANON + page_is_file_cache(page),
+ compound_nr(page));
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(PageLRU(page), page);
+ if (PageCompound(page))
+ list_add_tail(&page->lru, compound_pagelist);
+next:
/* There should be enough young pte to collapse the page */
if (pte_young(pteval) ||
page_is_young(page) || PageReferenced(page) ||
mmu_notifier_test_young(vma->vm_mm, address))
referenced++;
+
+ if (pte_write(pteval))
+ writable = true;
}
if (likely(writable)) {
if (likely(referenced)) {
@@ -644,7 +669,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
}
out:
- release_pte_pages(pte, _pte);
+ release_pte_pages(pte, _pte, compound_pagelist);
trace_mm_collapse_huge_page_isolate(page, none_or_zero,
referenced, writable, result);
return 0;
@@ -653,13 +678,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
struct vm_area_struct *vma,
unsigned long address,
- spinlock_t *ptl)
+ spinlock_t *ptl,
+ struct list_head *compound_pagelist)
{
+ struct page *src_page, *tmp;
pte_t *_pte;
for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
_pte++, page++, address += PAGE_SIZE) {
pte_t pteval = *_pte;
- struct page *src_page;
if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
clear_user_highpage(page, address);
@@ -679,7 +705,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
} else {
src_page = pte_page(pteval);
copy_user_highpage(page, src_page, address, vma);
- release_pte_page(src_page);
/*
* ptl mostly unnecessary, but preempt has to
* be disabled to update the per-cpu stats
@@ -693,9 +718,18 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
pte_clear(vma->vm_mm, address, _pte);
page_remove_rmap(src_page, false);
spin_unlock(ptl);
- free_page_and_swap_cache(src_page);
+ if (!PageCompound(src_page)) {
+ release_pte_page(src_page);
+ free_page_and_swap_cache(src_page);
+ }
}
}
+
+ list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
+ list_del(&src_page->lru);
+ release_pte_page(src_page);
+ free_page_and_swap_cache(src_page);
+ }
It looks this may mess up the PTE-mapped THP's refcount if it has multiple
PTE-mapped subpages since put_page() is not called for every PTE-mapped
subpages.
Good catch!
I *think* something like this should fix the issue (untested):
Is this an incremental fix?
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index bfb6155f1d69..9a96f9bff798 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -570,6 +570,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
list_del(&page->lru);
release_pte_page(page);
+ put_page(page);
}
}
@@ -682,8 +683,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(PageLRU(page), page);
- if (PageCompound(page))
+ if (PageCompound(page)) {
list_add_tail(&page->lru, compound_pagelist);
+ get_page(page);
I don't get why we have to bump refcount here? The THP won't go away
since it is pinned by isolation. I'm supposed calling
free_page_and_swap_cache(src_page) in each loop looks good enough to dec
refcount for each PTE-mapped subpage.
Then calling release_pte_page() for each entry on compund_pagelist
outside the loop to release the pin from isolation. The compound page
would get freed via pagevec finally.
+ }
next:
/* There should be enough young pte to collapse the page */
if (pte_young(pteval) ||
@@ -742,6 +745,8 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
} else {
src_page = pte_page(pteval);
copy_user_highpage(page, src_page, address, vma);
+ if (!PageCompound(src_page))
+ release_pte_page(src_page);
/*
* ptl mostly unnecessary, but preempt has to
* be disabled to update the per-cpu stats
@@ -755,10 +760,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
pte_clear(vma->vm_mm, address, _pte);
page_remove_rmap(src_page, false);
spin_unlock(ptl);
- if (!PageCompound(src_page)) {
- release_pte_page(src_page);
- free_page_and_swap_cache(src_page);
- }
+ free_page_and_swap_cache(src_page);
}
}