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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> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 840AF40019 X-Stat-Signature: 7pn6x78bz5tgyxed67edbrcehkjpmikn X-HE-Tag: 1682335249-979263 X-HE-Meta: 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 YQrLkJCL YSGt2QrF+4euPNzyt8ElOetjO5gxlwPdnyExw+uEwsemeUDzxlTMzTiIQHofVM0f0KldXAISlY3Bb/nL7TRMRfolVEJ4azEvHMYqt9fX2ZVeln70SxiuzD7DyL/ngZOSjyGa4MfoDtF+jdehIDmawvsCZ4vArsOSrw5HgEodFIzPQSPspOek+1qMmz+EbNloijwawSxshw4OWYv6YBadiuOjW8sluZQ+bYIOz2epP8/6cI6tC7dxoBrEGMInctpLWQSW0kLQ3Jpfo57w= 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 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/ > >> 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.