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[2003:cb:c735:f200:cb49:cb8f:88fc:9446]) by smtp.gmail.com with ESMTPSA id s28-20020adfa29c000000b003232f167df5sm17124815wra.108.2023.10.02.01.30.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 02 Oct 2023 01:30:22 -0700 (PDT) Message-ID: Date: Mon, 2 Oct 2023 10:30:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Content-Language: en-US To: Mike Rapoport , Yajun Deng Cc: 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> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY In-Reply-To: <20230929083018.GU3303@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29.09.23 10:30, Mike Rapoport wrote: > On Thu, Sep 28, 2023 at 04:33:02PM +0800, Yajun Deng wrote: >> memmap_init_range() would init page count of all pages, but the free >> pages count would be reset in __free_pages_core(). There are opposite >> operations. It's unnecessary and time-consuming when it's MEMINIT_EARLY >> context. >> >> Init page count in reserve_bootmem_region when in MEMINIT_EARLY context, >> and check the page count before reset it. >> >> At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't >> need, as it already done in __init_single_page. >> >> The following data was tested on an x86 machine with 190GB of RAM. >> >> before: >> free_low_memory_core_early() 341ms >> >> after: >> free_low_memory_core_early() 285ms >> >> Signed-off-by: Yajun Deng >> --- >> v4: same with v2. >> v3: same with v2. >> v2: check page count instead of check context before reset it. >> v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/ >> --- >> mm/mm_init.c | 18 +++++++++++++----- >> mm/page_alloc.c | 20 ++++++++++++-------- >> 2 files changed, 25 insertions(+), 13 deletions(-) >> >> diff --git a/mm/mm_init.c b/mm/mm_init.c >> index 9716c8a7ade9..3ab8861e1ef3 100644 >> --- a/mm/mm_init.c >> +++ b/mm/mm_init.c >> @@ -718,7 +718,7 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid) >> if (zone_spans_pfn(zone, pfn)) >> break; >> } >> - __init_single_page(pfn_to_page(pfn), pfn, zid, nid, INIT_PAGE_COUNT); >> + __init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0); >> } >> #else >> static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {} >> @@ -756,8 +756,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start, >> >> init_reserved_page(start_pfn, nid); >> >> - /* Avoid false-positive PageTail() */ >> - INIT_LIST_HEAD(&page->lru); >> + /* Init page count for reserved region */ > > Please add a comment that describes _why_ we initialize the page count here. > >> + init_page_count(page); >> >> /* >> * no need for atomic set_bit because the struct >> @@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone >> } >> >> page = pfn_to_page(pfn); >> - __init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT); >> - if (context == MEMINIT_HOTPLUG) >> + >> + /* If the context is MEMINIT_EARLY, we will init page count and >> + * mark page reserved in reserve_bootmem_region, the free region >> + * wouldn't have page count and we will check the pages count >> + * in __free_pages_core. >> + */ >> + __init_single_page(page, pfn, zone, nid, 0); >> + if (context == MEMINIT_HOTPLUG) { >> + init_page_count(page); >> __SetPageReserved(page); > > Rather than calling init_page_count() and __SetPageReserved() for > MEMINIT_HOTPLUG you can set flags to INIT_PAGE_COUNT | INIT_PAGE_RESERVED > an call __init_single_page() after the check for MEMINIT_HOTPLUG. > > But more generally, I wonder if we have to differentiate HOTPLUG here at all. > @David, can you comment please? There are a lot of details to that, and I'll share some I can briefly think of. 1) __SetPageReserved() I tried removing that a while ago, but there was a blocker (IIRC something about ZONE_DEVICE). I still have the patches at [1] and I could probably take a look if that blocker still exists (I recall that something changed at some point, but I never had the time to follow up). But once we stop setting the pages reserved, we might run into issues with ... 2) init_page_count() virtio-mem, XEN balloon and HV-balloon add memory blocks that can contain holes. set_online_page_callback() is used to intercept memory onlining and to expose only the pages that are not holes to the buddy: calling generic_online_page() on !hole. Holes are PageReserved but with an initialized page count. Memory offlining will fail on PageReserved pages -- has_unmovable_pages(). At least virtio-mem clears the PageReserved flag of holes when onlining memory, and currently relies in the page count to be reasonable (so memory offlining can work). static void virtio_mem_set_fake_offline(unsigned long pfn, unsigned long nr_pages, bool onlined) { page_offline_begin(); for (; nr_pages--; pfn++) { struct page *page = pfn_to_page(pfn); __SetPageOffline(page); if (!onlined) { SetPageDirty(page); /* FIXME: remove after cleanups */ ClearPageReserved(page); } } page_offline_end(); } For virtio-mem, we could initialize the page count there instead. The other PV drivers might require a bit more thought. [1] https://github.com/davidhildenbrand/linux/tree/online_reserved_cleanup > >> + } >> >> /* >> * Usually, we want to mark the pageblock MIGRATE_MOVABLE, >> 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? > >> - prefetchw(p); >> - for (loop = 0; loop < (nr_pages - 1); loop++, p++) { >> - prefetchw(p + 1); >> + if (page_count(page)) { >> + prefetchw(p); >> + for (loop = 0; loop < (nr_pages - 1); loop++, p++) { >> + prefetchw(p + 1); >> + __ClearPageReserved(p); >> + set_page_count(p, 0); >> + } >> __ClearPageReserved(p); >> set_page_count(p, 0); That looks wrong. if the page count would by pure luck be 0 already for hotplugged memory, you wouldn't clear the reserved flag. These changes make me a bit nervous. -- Cheers, David / dhildenb