linux-kernel - [PATCH RFC v2 3/9] mm: slightly clarify KSM logic in do_swap

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Message-Id: <20220126095557.32392-4-david@redhat.com>
Date:   Wed, 26 Jan 2022 10:55:51 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>
Subject: [PATCH RFC v2 3/9] mm: slightly clarify KSM logic in do_swap_page()

Let's make it clearer that KSM might only have to copy a page
in case we have a page in the swapcache, not if we allocated a fresh
page and bypassed the swapcache. While at it, add a comment why this is
usually necessary and merge the two swapcache conditions.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/memory.c | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 61d67ceef734..ab3153252cfe 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3617,21 +3617,29 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		goto out_release;
 	}
 
-	/*
-	 * Make sure try_to_free_swap or reuse_swap_page or swapoff did not
-	 * release the swapcache from under us.  The page pin, and pte_same
-	 * test below, are not enough to exclude that.  Even if it is still
-	 * swapcache, we need to check that the page's swap has not changed.
-	 */
-	if (unlikely((!PageSwapCache(page) ||
-			page_private(page) != entry.val)) && swapcache)
-		goto out_page;
-
-	page = ksm_might_need_to_copy(page, vma, vmf->address);
-	if (unlikely(!page)) {
-		ret = VM_FAULT_OOM;
-		page = swapcache;
-		goto out_page;
+	if (swapcache) {
+		/*
+		 * Make sure try_to_free_swap or reuse_swap_page or swapoff did
+		 * not release the swapcache from under us.  The page pin, and
+		 * pte_same test below, are not enough to exclude that.  Even if
+		 * it is still swapcache, we need to check that the page's swap
+		 * has not changed.
+		 */
+		if (unlikely(!PageSwapCache(page) ||
+			     page_private(page) != entry.val))
+			goto out_page;
+
+		/*
+		 * KSM sometimes has to copy on read faults, for example, if
+		 * page->index of !PageKSM() pages would be nonlinear inside the
+		 * anon VMA -- PageKSM() is lost on actual swapout.
+		 */
+		page = ksm_might_need_to_copy(page, vma, vmf->address);
+		if (unlikely(!page)) {
+			ret = VM_FAULT_OOM;
+			page = swapcache;
+			goto out_page;
+		}
 	}
 
 	cgroup_throttle_swaprate(page, GFP_KERNEL);
-- 
2.34.1