linux-kernel - Re: [PATCH v3 3/9] mm: slightly clarify KSM logic in do_swap

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Date:   Wed, 9 Mar 2022 12:50:45 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Yang Shi <shy828301@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.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>,
        "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 <linux-mm@...ck.org>
Subject: Re: [PATCH v3 3/9] mm: slightly clarify KSM logic in do_swap_page()

On Wed, 9 Mar 2022 20:15:54 +0100 David Hildenbrand <david@...hat.com> wrote:

> On 09.03.22 19:48, Yang Shi wrote:
> > On Mon, Jan 31, 2022 at 8:33 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> 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 923165b4c27e..3c91294cca98 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -3615,21 +3615,29 @@ 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
> > 
> > We could remove the reference to "reuse_swap_page", right?
> >
> Yes, I noticed this a couple of days ago as well and already have a
> patch prepared for that ("mm: adjust stale comment in do_swap_page()
> mentioning reuse_swap_page()" at
> https://github.com/davidhildenbrand/linux/commits/cow_fixes_part_3)
> 
> If Andrew wants, we can fix that up directly before sending upstream or
> I'll simply include that patch when sending out part2 v2.
> 
> (I want to avoid sending another series just for this)

Thanks, I did this.  The same change plus gratuitous comment reflowing.

--- a/mm/memory.c~mm-slightly-clarify-ksm-logic-in-do_swap_page-fix
+++ a/mm/memory.c
@@ -3609,11 +3609,11 @@ vm_fault_t do_swap_page(struct vm_fault
 
 	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.
+		 * Make sure try_to_free_swap 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))
_