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Sun, 29 Sep 2024 09:16:37 +0800 Subject: Re: [PATCH v3 1/5] mm: memory_hotplug: remove head variable in do_migrate_range() To: David Hildenbrand , Matthew Wilcox , Kefeng Wang CC: Andrew Morton , Oscar Salvador , Naoya Horiguchi , , , Jonathan Cameron References: <20240827114728.3212578-1-wangkefeng.wang@huawei.com> <20240827114728.3212578-2-wangkefeng.wang@huawei.com> <20a75b57-12a6-468f-bd7c-0aeb2f259228@redhat.com> From: Miaohe Lin Message-ID: <170546a8-e442-91e9-31e8-60a91018172a@huawei.com> Date: Sun, 29 Sep 2024 09:16:18 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.173.127.72] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To kwepemd200019.china.huawei.com (7.221.188.193) X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: BE2BA1A0006 X-Stat-Signature: 5wtao9qiiijjj781gs9sjiqfd3wbwc6o X-HE-Tag: 1727572601-722201 X-HE-Meta: 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 SFrLG3yq ySpPDrdCwwNlqB/mTyWBput2CJHtqkNnO1OV26G6rIKWhErOB7Jeg/Ivb8MHERGJV35H7hLjpEymj8Dn0atWu8EoveP76MdcYo8ZZh6L9GhkniBeWEBVOxVnNh5ebwahO+fi3Nlq6gKHIQEVomY5DY/Qt4690gG+/BVnqtMxAXNEn6iQiOSpjUiuwooBefcbX6BhtYFqXD0rI1CFZ6INADuqnwzIJGOUyK3HvIvcYQq2cO6YY0QoFnErf0KVXN5xRtaNWwwNC966Tb1QRfrmeCJNC9g== 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: List-Subscribe: List-Unsubscribe: On 2024/9/28 16:39, David Hildenbrand wrote: > On 28.09.24 10:34, David Hildenbrand wrote: >> On 28.09.24 06:55, Matthew Wilcox wrote: >>> On Tue, Aug 27, 2024 at 07:47:24PM +0800, Kefeng Wang wrote: >>>> Directly use a folio for HugeTLB and THP when calculate the next pfn, then >>>> remove unused head variable. >>> >>> I just noticed this got merged.  You're going to hit BUG_ON with it. >>> >>>> -        if (PageHuge(page)) { >>>> -            pfn = page_to_pfn(head) + compound_nr(head) - 1; >>>> -            isolate_hugetlb(folio, &source); >>>> -            continue; >>>> -        } else if (PageTransHuge(page)) >>>> -            pfn = page_to_pfn(head) + thp_nr_pages(page) - 1; >>>> +        /* >>>> +         * No reference or lock is held on the folio, so it might >>>> +         * be modified concurrently (e.g. split).  As such, >>>> +         * folio_nr_pages() may read garbage.  This is fine as the outer >>>> +         * loop will revisit the split folio later. >>>> +         */ >>>> +        if (folio_test_large(folio)) { >>> >>> But it's not fine.  Look at the implementation of folio_test_large(): >>> >>> static inline bool folio_test_large(const struct folio *folio) >>> { >>>           return folio_test_head(folio); >>> } >>> >>> That's going to be provided by: >>> >>> #define FOLIO_TEST_FLAG(name, page)                                     \ >>> static __always_inline bool folio_test_##name(const struct folio *folio) \ >>> { return test_bit(PG_##name, const_folio_flags(folio, page)); } >>> >>> and here's the BUG: >>> >>> static const unsigned long *const_folio_flags(const struct folio *folio, >>>                   unsigned n) >>> { >>>           const struct page *page = &folio->page; >>> >>>           VM_BUG_ON_PGFLAGS(PageTail(page), page); >>>           VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page); >>>           return &page[n].flags; >>> } >>> >>> (this page can be transformed from a head page to a tail page because, >>> as the comment notes, we don't hold a reference. >>> >>> Please back this out. >> >> Should we generalize the approach in dump_folio() to locally copy a >> folio, so we can safely perform checks before deciding whether we want >> to try grabbing a reference on the real folio (if it's still a folio :) )? >> > > Oh, and I forgot: isn't the existing code already racy? > > PageTransHuge() -> VM_BUG_ON_PAGE(PageTail(page), page); do_migrate_range is called after start_isolate_page_range(). So a page might not be able to transform from a head page to a tail page as it's isolated? Thanks.