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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v2 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() To: Michal Hocko Cc: akpm@linux-foundation.org, rppt@kernel.org, ying.huang@intel.com, mgorman@techsingularity.net, vbabka@suse.cz, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <9fc85cce8908938f4fd75ff50bc981c073779aa5.1682229876.git.baolin.wang@linux.alibaba.com> <0733a4cf57109a4136de5ae46fac83fb15bdd528.1682229876.git.baolin.wang@linux.alibaba.com> <9a20c0b5-9d8a-2b1d-570a-61c17a4ce5e8@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 5F97E14000A X-Stat-Signature: hhn4x5kn8yhk9dkfiwss5xss6urh7cw4 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1682336435-413855 X-HE-Meta: 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 3JTMkjDi 8ewgl87IdSJ8DlaPLh3NUgG4usPwtPN2DE4s7fFp0T8sILs6qvRc3Fn2g+eRCzafL1c1hg7WqkkTzvH5FmRps2yMhYxY8cPk9NgyK7RQ8HHGJPos1ykPY32U2dliTCtfRChPw5VBmBzBW1XufbonimO5p6FUEyCcfGawGsaMjlIzPx3vI5mSWRmAgmQQOMMTpqp3MmdJoVpmXIZubMvIDtVCBvQAHIr7CSLhDFfIAnBCz2Ft0DN4uealjAbiaoiBRPRIxRQ2ozIu6RAI= 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 4/24/2023 7:34 PM, Michal Hocko wrote: > On Mon 24-04-23 19:20:43, Baolin Wang wrote: >> >> >> On 4/24/2023 5:54 PM, Michal Hocko wrote: >>> On Sun 23-04-23 18:59:11, Baolin Wang wrote: >>>> 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. >>>> >>>> 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/ >>> >>> Do I remember correctly you've had a specific configuration that would >>> trigger this case? >> >> Yes, I provided an example in previous thread [2] so show the >> __pageblock_pfn_to_page() is fragile in some cases. >> >> [2] https://lore.kernel.org/all/52dfdd2e-9c99-eac4-233e-59919a24323e@linux.alibaba.com/ > > Please make it a part of the changelog. Sure. > >>> >>>> Signed-off-by: Baolin Wang >>>> --- >>>> Changes from v1: >>>> - Update the comments per Ying and Mike, thanks. >>>> --- >>>> mm/page_alloc.c | 7 +++++++ >>>> 1 file changed, 7 insertions(+) >>>> >>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>> index 6457b64fe562..9756d66f471c 100644 >>>> --- a/mm/page_alloc.c >>>> +++ b/mm/page_alloc.c >>>> @@ -1502,6 +1502,13 @@ 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 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. >>>> + * This did not break anything until now, but be careful about this possible >>>> + * issue when checking whether all pfns of a pageblock are valid. >>> >>> It is not really clear what you should be doing (other than to be >>> careful which is not helpful much TBH) when you encounter this >>> situation. If the reality changes and this would break in the future >>> what would breakage look like? What should be done about that? >> >> That depends on what the future pfn walkers do, which may access some hole >> memory with zero-init page frame. For example, if checking the >> __PageMovable() for a zero-init page frame, that will crash the system. But >> I can not list all the possible cases. >> >> So how about below words? >> >> * 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, 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 about this possible >> * issue when checking whether all pfns of a pageblock are valid, that may >> * lead to accessing empty page frame, and the worst case can crash the >> system. >> * So you should use pfn_to_onlie_page() instead of pfn_valid() to valid the >> * pfns in a pageblock if such case happens. > > Does that mean that struct page is not initialized and PagePoisoned will > trigger or it is just zero-prefilled? In the example I provided[2], these page frames of the hole memory are zero-prefilled. [2] https://lore.kernel.org/all/52dfdd2e-9c99-eac4-233e-59919a24323e@linux.alibaba.com/