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Wed, 02 Dec 2020 18:44:05 -0800 (PST) MIME-Version: 1.0 References: <20201202122114.75316-1-songmuchun@bytedance.com> <1505b548-968b-2053-fd17-0cc4ae240a2f@suse.cz> In-Reply-To: <1505b548-968b-2053-fd17-0cc4ae240a2f@suse.cz> From: Muchun Song Date: Thu, 3 Dec 2020 10:43:29 +0800 Message-ID: Subject: Re: [External] Re: [PATCH] mm/page_isolation: do not isolate the max order page To: Vlastimil Babka Cc: Andrew Morton , Linux Memory Management List , LKML , Joonsoo Kim Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Dec 3, 2020 at 8:03 AM Vlastimil Babka wrote: > > On 12/2/20 1:21 PM, Muchun Song wrote: > > The max order page has no buddy page and never merge to other order. > > So isolating and then freeing it is pointless. > > > > Signed-off-by: Muchun Song > > Acked-by: Vlastimil Babka > > > --- > > mm/page_isolation.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/page_isolation.c b/mm/page_isolation.c > > index a254e1f370a3..bddf788f45bf 100644 > > --- a/mm/page_isolation.c > > +++ b/mm/page_isolation.c > > @@ -88,7 +88,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype) > > */ > > if (PageBuddy(page)) { > > order = buddy_order(page); > > - if (order >= pageblock_order) { > > + if (order >= pageblock_order && order < MAX_ORDER - 1) { > > pfn = page_to_pfn(page); > > buddy_pfn = __find_buddy_pfn(pfn, order); > > buddy = page + (buddy_pfn - pfn); > > Hm I wonder if order == MAX_ORDER - 1, then the buddy can actually be a > !pfn_valid() in some corner case? pfn_valid_within(buddy_pfn) that follows would > only catch it on archs with holes in zone. Then is_migrate_isolate_page(buddy) > might access an invalid buddy. So this might be actually a bug fix and not just > optimization, just the bug hasn't been observed in practice. Agree. Should we add a Fixes tag in the commit log? Thanks. > > > > -- Yours, Muchun