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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v3 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() To: "Huang, Ying" Cc: akpm@linux-foundation.org, rppt@kernel.org, mgorman@techsingularity.net, vbabka@suse.cz, mhocko@suse.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <50b5e05dbb007e3a969ac946bc9ee0b2b77b185f.1682342634.git.baolin.wang@linux.alibaba.com> <87zg6wkdy8.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Baolin Wang In-Reply-To: <87zg6wkdy8.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 04B5220012 X-Stat-Signature: sg7f9xqp555zstkdrhifnbg4oaebsa16 X-HE-Tag: 1682386049-435619 X-HE-Meta: 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 35yHxY+9 zKW2gfp1NDVVX5gtfsg+azQXFNhIImkcKxYo1smN/0uQ/hy8qou/eHgup/eXballWZcMXxd1rNJkvVW6zUVHPCLerrJfa85sIRWgAfp36mvZMnWuIcxmk8sAitmf4gYukF+B/2wB2OFjIZHPbBH7XKOkHIKyySG5gwJpWGoRcHyIHGKmOB0GZUb+6TLZvn8YLdRRg69KLR7SHsFBuxyp1dRbNf5lnRkU4508wRGkzBUBpQWwxiCrqr0vH5GrCXhf7Rc7W7kuMYhujp5Y= 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 4/25/2023 8:22 AM, Huang, Ying wrote: > Baolin Wang writes: > >> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which >> checks whether the given zone contains holes, and uses pfn_to_online_page() >> to validate if the start pfn is online and valid, as well as using pfn_valid() >> to validate the end pfn. >> >> However, the __pageblock_pfn_to_page() function may return non-NULL even >> if the end pfn of a pageblock is in a memory hole in some situations. For >> example, if the pageblock order is MAX_ORDER, which will fall into 2 >> sub-sections, and the end pfn of the pageblock may be hole even though >> the start pfn is online and valid. >> >> See below memory layout as an example and suppose the pageblock order >> is MAX_ORDER. >> >> [ 0.000000] Zone ranges: >> [ 0.000000] DMA [mem 0x0000000040000000-0x00000000ffffffff] >> [ 0.000000] DMA32 empty >> [ 0.000000] Normal [mem 0x0000000100000000-0x0000001fa7ffffff] >> [ 0.000000] Movable zone start for each node >> [ 0.000000] Early memory node ranges >> [ 0.000000] node 0: [mem 0x0000000040000000-0x0000001fa3c7ffff] >> [ 0.000000] node 0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff] >> [ 0.000000] node 0: [mem 0x0000001fa4000000-0x0000001fa402ffff] >> [ 0.000000] node 0: [mem 0x0000001fa4030000-0x0000001fa40effff] >> [ 0.000000] node 0: [mem 0x0000001fa40f0000-0x0000001fa73cffff] >> [ 0.000000] node 0: [mem 0x0000001fa73d0000-0x0000001fa745ffff] >> [ 0.000000] node 0: [mem 0x0000001fa7460000-0x0000001fa746ffff] >> [ 0.000000] node 0: [mem 0x0000001fa7470000-0x0000001fa758ffff] >> [ 0.000000] node 0: [mem 0x0000001fa7590000-0x0000001fa7dfffff] >> >> Focus on the last memory range, and there is a hole for the range [mem >> 0x0000001fa7590000-0x0000001fa7dfffff]. That means the last pageblock >> will contain the range from 0x1fa7c00000 to 0x1fa7ffffff, since the >> pageblock must be 4M aligned. And in this pageblock, these pfns will >> fall into 2 sub-section (the sub-section size is 2M aligned). >> >> So, the 1st sub-section (indicates pfn range: 0x1fa7c00000 - >> 0x1fa7dfffff ) in this pageblock is valid by calling subsection_map_init() >> in free_area_init(), but the 2nd sub-section (indicates pfn range: >> 0x1fa7e00000 - 0x1fa7ffffff ) in this pageblock is not valid. >> >> This did not break anything until now, but the zone continuous is fragile >> in this possible scenario. So as previous discussion[1], it is better to >> add some comments to explain this possible issue in case there are some >> future pfn walkers that rely on this. >> >> [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@yhuang6-desk2.ccr.corp.intel.com/ >> >> Signed-off-by: Baolin Wang >> --- >> Changes from v2: >> - Update the commit log and comments per Michal, thanks. >> Changes from v1: >> - Update the comments per Ying and Mike, thanks. >> >> Note, I did not add Huang Ying's reviewed tag, since there are some >> updates per Michal's suggestion. Ying, please review the v3. Thanks. >> --- >> mm/page_alloc.c | 9 +++++++++ >> 1 file changed, 9 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6457b64fe562..bd124390c79b 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1502,6 +1502,15 @@ void __free_pages_core(struct page *page, unsigned int order) >> * interleaving within a single pageblock. It is therefore sufficient to check >> * the first and last page of a pageblock and avoid checking each individual >> * page in a pageblock. >> + * >> + * Note: the function may return non-NULL struct page even for a page block >> + * which contains a memory hole (i.e. there is no physical memory for a subset >> + * of the pfn range). For example, if the pageblock order is MAX_ORDER, which >> + * will fall into 2 sub-sections, and the end pfn of the pageblock may be hole >> + * even though the start pfn is online and valid. This should be safe most of >> + * the time because struct pages are still zero pre-filled and pfn walkers > > I don't think the pfn is just zero-filled even it's a hole. Can you > confirm that? In memmap_init() and memmap_init_zone_range(), > init_unavailable_range() is called to initialize the struct page. Yes, what I mean is the page frames were initialized to zero firstly, and some fields were initialized to default value. The "zero pre-filled" seems confusing, may be change to "initialized"?