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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH 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, mgorman@techsingularity.net, vbabka@suse.cz, mhocko@suse.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <02defcbe9d7a797a2257e5f6a28ff7ea78e394e5.1682158312.git.baolin.wang@linux.alibaba.com> <87cz3vs8nn.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Baolin Wang In-Reply-To: <87cz3vs8nn.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: rspam04 X-Rspamd-Queue-Id: 7F809100008 X-Stat-Signature: 7k36sjmmfwc5my9e9xr4b8tawwzq8hsj X-HE-Tag: 1682213253-919401 X-HE-Meta: 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 Z4STlbuH ka4KItjNLAf/OuIuewX6/hn69AbdTs+/oDBzSoctrbPreuuL05/6svLqdSs9aIRGck7arITIn2TA1dl1ok9OQRAZH84kOd1mLZmJc2fN4M9pbviq95yLuL7dXnUomVivoK7YWA5QWBkjkjo5TgeeDkcoTf9uNnaL7Ptaidai+4MVKBL+1U06B65ZTGuwVPsD78/N0NRIzzvhrTiv7yuCnDYmpWwVPR0epOdaCtVsiRz8Y/ZX7JIzvl1jdo/IIL8xGwE0Px3s+MxbE04U= 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 4/23/2023 9:13 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, though the start pfn of a pageblock is valid, it can not always >> guarantee the end pfn of the pageblock is also valid (may be holes) in some >> cases. For example, if the pageblock order is MAX_ORDER - 1, 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 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 >> --- >> mm/page_alloc.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6457b64fe562..dc4005b32ae0 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1502,6 +1502,14 @@ 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: if the start pfn of a pageblock is valid, but it can not always guarantee >> + * the end pfn of the pageblock is also valid (may be holes) in some cases. For > > "valid" sounds confusing here. pfn_valid() is true, but the pfn is > considered invalid at some degree. How about the following? > > Note: the 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 - 1, 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 did not break anything until now, but be >> + * careful this possible issue when checking if the whole pfns are valid of a > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > whether all pfns of a pageblock are valid. ? > >> + * pageblock. >> */ >> struct page *__pageblock_pfn_to_page(unsigned long start_pfn, >> unsigned long end_pfn, struct zone *zone) > > My English is poor. So, feel free to ignore the comments. Better than me:) . Will do in next version. Thanks.