Re: [PATCH v4 4/4] mm: introduce MADV_PAGEOUT

On Fri, Jul 12, 2019 at 02:18:28PM +0900, Minchan Kim wrote:

Hi Johannes,

On Thu, Jul 11, 2019 at 02:42:23PM -0400, Johannes Weiner wrote:

quoted

On Thu, Jul 11, 2019 at 10:25:28AM +0900, Minchan Kim wrote:

quoted

@@ -480,6 +482,198 @@ static long madvise_cold(struct vm_area_struct *vma,
 	return 0;
 }
 
+static int madvise_pageout_pte_range(pmd_t *pmd, unsigned long addr,
+				unsigned long end, struct mm_walk *walk)
+{
+	struct mmu_gather *tlb = walk->private;
+	struct mm_struct *mm = tlb->mm;
+	struct vm_area_struct *vma = walk->vma;
+	pte_t *orig_pte, *pte, ptent;
+	spinlock_t *ptl;
+	LIST_HEAD(page_list);
+	struct page *page;
+	unsigned long next;
+
+	if (fatal_signal_pending(current))
+		return -EINTR;
+
+	next = pmd_addr_end(addr, end);
+	if (pmd_trans_huge(*pmd)) {
+		pmd_t orig_pmd;
+
+		tlb_change_page_size(tlb, HPAGE_PMD_SIZE);
+		ptl = pmd_trans_huge_lock(pmd, vma);
+		if (!ptl)
+			return 0;
+
+		orig_pmd = *pmd;
+		if (is_huge_zero_pmd(orig_pmd))
+			goto huge_unlock;
+
+		if (unlikely(!pmd_present(orig_pmd))) {
+			VM_BUG_ON(thp_migration_supported() &&
+					!is_pmd_migration_entry(orig_pmd));
+			goto huge_unlock;
+		}
+
+		page = pmd_page(orig_pmd);
+		if (next - addr != HPAGE_PMD_SIZE) {
+			int err;
+
+			if (page_mapcount(page) != 1)
+				goto huge_unlock;
+			get_page(page);
+			spin_unlock(ptl);
+			lock_page(page);
+			err = split_huge_page(page);
+			unlock_page(page);
+			put_page(page);
+			if (!err)
+				goto regular_page;
+			return 0;
+		}
+
+		if (isolate_lru_page(page))
+			goto huge_unlock;
+
+		if (pmd_young(orig_pmd)) {
+			pmdp_invalidate(vma, addr, pmd);
+			orig_pmd = pmd_mkold(orig_pmd);
+
+			set_pmd_at(mm, addr, pmd, orig_pmd);
+			tlb_remove_tlb_entry(tlb, pmd, addr);
+		}
+
+		ClearPageReferenced(page);
+		test_and_clear_page_young(page);
+		list_add(&page->lru, &page_list);
+huge_unlock:
+		spin_unlock(ptl);
+		reclaim_pages(&page_list);
+		return 0;
+	}
+
+	if (pmd_trans_unstable(pmd))
+		return 0;
+regular_page:
+	tlb_change_page_size(tlb, PAGE_SIZE);
+	orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
+	flush_tlb_batched_pending(mm);
+	arch_enter_lazy_mmu_mode();
+	for (; addr < end; pte++, addr += PAGE_SIZE) {
+		ptent = *pte;
+		if (!pte_present(ptent))
+			continue;
+
+		page = vm_normal_page(vma, addr, ptent);
+		if (!page)
+			continue;
+
+		/*
+		 * creating a THP page is expensive so split it only if we
+		 * are sure it's worth. Split it if we are only owner.
+		 */
+		if (PageTransCompound(page)) {
+			if (page_mapcount(page) != 1)
+				break;
+			get_page(page);
+			if (!trylock_page(page)) {
+				put_page(page);
+				break;
+			}
+			pte_unmap_unlock(orig_pte, ptl);
+			if (split_huge_page(page)) {
+				unlock_page(page);
+				put_page(page);
+				pte_offset_map_lock(mm, pmd, addr, &ptl);
+				break;
+			}
+			unlock_page(page);
+			put_page(page);
+			pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
+			pte--;
+			addr -= PAGE_SIZE;
+			continue;
+		}
+
+		VM_BUG_ON_PAGE(PageTransCompound(page), page);
+
+		if (isolate_lru_page(page))
+			continue;
+
+		if (pte_young(ptent)) {
+			ptent = ptep_get_and_clear_full(mm, addr, pte,
+							tlb->fullmm);
+			ptent = pte_mkold(ptent);
+			set_pte_at(mm, addr, pte, ptent);
+			tlb_remove_tlb_entry(tlb, pte, addr);
+		}
+		ClearPageReferenced(page);
+		test_and_clear_page_young(page);
+		list_add(&page->lru, &page_list);
+	}
+
+	arch_leave_lazy_mmu_mode();
+	pte_unmap_unlock(orig_pte, ptl);
+	reclaim_pages(&page_list);
+	cond_resched();
+
+	return 0;
+}

I know you have briefly talked about code sharing already.

While I agree that sharing with MADV_FREE is maybe a stretch, I
applied these patches and compared the pageout and the cold page table
functions, and they are line for line the same EXCEPT for 2-3 lines at
the very end, where one reclaims and the other deactivates. It would
be good to share here, it shouldn't be hard or result in fragile code.

Fair enough if we leave MADV_FREE.

quoted

Something like int madvise_cold_or_pageout_range(..., bool pageout)?

How about this?

From 41592f23e876ec21e49dc3c76dc89538e2bb16be Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Fri, 12 Jul 2019 14:05:36 +0900
Subject: [PATCH] mm: factor out common parts between MADV_COLD and
 MADV_PAGEOUT

There are many common parts between MADV_COLD and MADV_PAGEOUT.
This patch factor them out to save code duplication.

Signed-off-by: Minchan Kim <minchan@kernel.org>

This looks much better, thanks!

One thought on the ordering here.

When LRU isolation fails, it would still make sense to clear the young
bit: we cannot reclaim the page as we wanted to, but the user still
provided a clear hint that the page is cold and she won't be touching
it for a while. MADV_PAGEOUT is basically MADV_COLD + try_to_reclaim.
So IMO isolation should go to the end next to deactivate_page().