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X-IronPort-AV: E=McAfee;i="6600,9927,10688"; a="343707640" X-IronPort-AV: E=Sophos;i="5.99,219,1677571200"; d="scan'208";a="343707640" Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Apr 2023 18:14:23 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10688"; a="836534930" X-IronPort-AV: E=Sophos;i="5.99,219,1677571200"; d="scan'208";a="836534930" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by fmsmga001-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Apr 2023 18:14:21 -0700 From: "Huang, Ying" To: Baolin Wang Cc: , , , , , , Subject: Re: [PATCH 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() References: <02defcbe9d7a797a2257e5f6a28ff7ea78e394e5.1682158312.git.baolin.wang@linux.alibaba.com> Date: Sun, 23 Apr 2023 09:13:16 +0800 In-Reply-To: <02defcbe9d7a797a2257e5f6a28ff7ea78e394e5.1682158312.git.baolin.wang@linux.alibaba.com> (Baolin Wang's message of "Sat, 22 Apr 2023 18:15:18 +0800") Message-ID: <87cz3vs8nn.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspam-User: X-Rspamd-Queue-Id: 4D7634001F X-Rspamd-Server: rspam01 X-Stat-Signature: 5731rzt33zkmicyxgusus3utzmf4phqh X-HE-Tag: 1682212465-944303 X-HE-Meta: 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 ykgV1BG6 gHWnjK9dweiRl1jVKho5wrxknG9P+UKSnaCLEZMlJhM17RWZANOdI8TP7m0XtwIBD0Ziibo68CLZTQFPVcYDy7mVoYNabhXoxi3cnVbEjEgMUULaRGd51OSzb2weSr0MYyAbt+YYc3Vexdp5iQzyLCwcfb+dUTnDOJohN+C3KTHp4OuJqbI0AvCfYbbFsAwE+yYYR70bCQf/EeY/2LWLSsO15KEPx4bNC/heAmjiSdQytJc3PORzNQqDaAx+D90jADIvrJa/UKgTRRVWh0ysxI011yK2Mb6vpnbk3xDTrGWeRVLx1gKpKt+KsRrb+cq1z+kw8 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: 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. Best Regards, Huang, Ying