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[2003:cb:c735:f200:cb49:cb8f:88fc:9446]) by smtp.gmail.com with ESMTPSA id t9-20020a7bc3c9000000b003feae747ff2sm7016568wmj.35.2023.10.02.04.25.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 02 Oct 2023 04:25:11 -0700 (PDT) Message-ID: <3057dab3-19f2-99ca-f125-e91a094975ed@redhat.com> Date: Mon, 2 Oct 2023 13:25:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY Content-Language: en-US To: Mike Rapoport Cc: Yajun Deng , akpm@linux-foundation.org, mike.kravetz@oracle.com, muchun.song@linux.dev, willy@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20230928083302.386202-1-yajun.deng@linux.dev> <20230928083302.386202-3-yajun.deng@linux.dev> <20230929083018.GU3303@kernel.org> <20230929100252.GW3303@kernel.org> <15233624-f32e-172e-b2f6-7ca7bffbc96d@linux.dev> <20231001185934.GX3303@kernel.org> <90342474-432a-9fe3-2f11-915a04f0053f@linux.dev> <20231002084708.GZ3303@kernel.org> <20231002111051.GA3303@kernel.org> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20231002111051.GA3303@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02.10.23 13:10, Mike Rapoport wrote: > On Mon, Oct 02, 2023 at 10:56:51AM +0200, David Hildenbrand wrote: >> On 02.10.23 10:47, Mike Rapoport wrote: >>> On Mon, Oct 02, 2023 at 03:03:56PM +0800, Yajun Deng wrote: >>>> >>>> On 2023/10/2 02:59, Mike Rapoport wrote: >>>>> On Fri, Sep 29, 2023 at 06:27:25PM +0800, Yajun Deng wrote: >>>>>> On 2023/9/29 18:02, Mike Rapoport wrote: >>>>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>>>>>>>> index 06be8821d833..b868caabe8dc 100644 >>>>>>>>>> --- a/mm/page_alloc.c >>>>>>>>>> +++ b/mm/page_alloc.c >>>>>>>>>> @@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order) >>>>>>>>>> unsigned int loop; >>>>>>>>>> /* >>>>>>>>>> - * When initializing the memmap, __init_single_page() sets the refcount >>>>>>>>>> - * of all pages to 1 ("allocated"/"not free"). We have to set the >>>>>>>>>> - * refcount of all involved pages to 0. >>>>>>>>>> + * When initializing the memmap, memmap_init_range sets the refcount >>>>>>>>>> + * of all pages to 1 ("reserved" and "free") in hotplug context. We >>>>>>>>>> + * have to set the refcount of all involved pages to 0. Otherwise, >>>>>>>>>> + * we don't do it, as reserve_bootmem_region only set the refcount on >>>>>>>>>> + * reserve region ("reserved") in early context. >>>>>>>>>> */ >>>>>>>>> Again, why hotplug and early init should be different? >>>>>>>> I will add a comment that describes it will save boot time. >>>>>>> But why do we need initialize struct pages differently at boot time vs >>>>>>> memory hotplug? >>>>>>> Is there a reason memory hotplug cannot have page count set to 0 just like >>>>>>> for pages reserved at boot time? >>>>>> This patch just save boot time in MEMINIT_EARLY. If someone finds out that >>>>>> it can save time in >>>>>> >>>>>> MEMINIT_HOTPLUG, I think it can be done in another patch later. I just >>>>>> keeping it in the same. >>>>> But it's not the same. It becomes slower after your patch and the code that >>>>> frees the pages for MEMINIT_EARLY and MEMINIT_HOTPLUG becomes non-uniform >>>>> for no apparent reason. >>>> >>>> __free_pages_core will also be called by others, such as: >>>> deferred_free_range, do_collection and memblock_free_late. >>>> >>>> We couldn't remove  'if (page_count(page))' even if we set page count to 0 >>>> when MEMINIT_HOTPLUG. >>> >>> That 'if' breaks the invariant that __free_pages_core is always called for >>> pages with initialized page count. Adding it may lead to subtle bugs and >>> random memory corruption so we don't want to add it at the first place. >> >> As long as we have to special-case memory hotplug, we know that we are >> always coming via generic_online_page() in that case. We could either move >> some logic over there, or let __free_pages_core() know what it should do. > > Looks like the patch rather special cases MEMINIT_EARLY, although I didn't > check throughfully other code paths. > Anyway, relying on page_count() to be correct in different ways for > different callers of __free_pages_core() does not sound right to me. Absolutely agreed. -- Cheers, David / dhildenb