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b=UhGel73birqVS59+C/A/qJF4ZvqB42EErURgteqbJNSQRXzEVENe709aQPdpLkuJW z07gPHtgAOJzq6xD3TMihyzzzcbEGjz7diSII+m+3a+yP9fs+0mUNK9gUokGaaCLPG sKBZqfv8NtQVI6vudwTGAvX6nB/pAhLAFe1hhhwXLKHT57Lgu4F5otfyODoNdaaL+1 L5xGcCnR0Reo7J0SD0fok0oQZBNellpmARMZSRkxKlYzGAeVQ8qePzTKOlw01MF+zX 3KNqHgni83JwQXPw5ATx5MwSrC+gCWrlFSyYDUbWjaIfjqEkXypKHGu+66a+AUz5v2 s+O2Gk31XHQ2g== Message-ID: <3149fcee-eded-49b0-baf4-ab69c27ff063@kernel.org> Date: Fri, 19 Jun 2026 10:00:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: page_ext: validate section in for_each_page_ext iterator To: Luiz Capitulino , Ketan , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan Cc: kernel@oss.qualcomm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260617-page_ext-v1-1-37ad802b1a38@oss.qualcomm.com> <1dbebf73-6c46-44e1-ac6a-f5ffa9849b37@kernel.org> <698e5cdc-8f1d-4984-bb54-a4b7979b4dc8@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 100372000B X-Stat-Signature: y9byztjbh7i9qtpogyzni473cfg5xhi9 X-Rspam-User: X-HE-Tag: 1781856064-274244 X-HE-Meta: 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 a+8IYBo1 gUWRvPPQCbaCZ2wmw4hFu4EcJjJTtqEAaeOgNHFNV4r/2WltxoT2998SXJ7PRY5ytHoEi0Q7vlt72m9zYA0+n/H0yk842NuxdU8JlP0C1+S7+CwkXzkT6NIbP7Af/sDONRYGPnP04++VRIHMjXMuY9TidlqGUQOVCw/lo3rblMP3SFqosL5Q9jvc4NGGEYnpRdBqGtkIW5OgaJi8PQB7MzjLgjajWvzN/O7I77YtwpW9hq6sTNFV5lHTU++Glv4XPIFl81xAeza+e7oCRhwFKY+iVYfIJcPGvu7LfnC6x8QZ1KgNQP8tct7m6+IUaGVPHosbufrD+LfVG4t/3Ki8hdoaKkfYRK5zneMXe Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/19/26 04:38, Luiz Capitulino wrote: > On 2026-06-18 05:04, David Hildenbrand (Arm) wrote: >> On 6/17/26 17:09, Ketan wrote: >>> The page_ext iteration API do not validate if the PFN still >>> belongs to a valid section while advancing the iterator. >>> >>> When dynamically adding memory in hotplug path, it can lead >>> to a NULL pointer dereference during page_ext_lookup at the >>> boundary of the last valid section. >>> >>> [   14.555124][  T846] Call trace: >>> [   14.555125][  T846]  lookup_page_ext+0x6c/0x108 (P) >>> [   14.555127][  T846]  page_ext_lookup+0x30/0x3c >>> [   14.555129][  T846]   +0x11c/0x260 >>> [   14.571201][  T846]  __free_pages_ok+0x5e8/0x8e0 >>> [   14.571204][  T846]  __free_pages_core+0x78/0xf0 >>> [   14.571206][  T846]  generic_online_page+0x14/0x24 >>> [   14.597782][  T846]  online_pages+0x178/0x30c >>> [   14.597784][  T846]  memory_block_change_state+0x284/0x32c >>> [   14.597787][  T846]  memory_subsys_online+0x4c/0x64 >>> [   14.597789][  T846]  device_online+0x88/0xb0 >>> [   14.597791][  T846]  online_memory_block+0x30/0x40 >>> [   14.597793][  T846]  walk_memory_blocks+0xac/0xe8 >>> [   14.597794][  T846]  add_memory_resource+0x280/0x298 >>> [   14.656161][  T846]  add_memory+0x60/0x98 >> >> So, we do allocate the page_ext in memory_notify(MEM_GOING_ONLINE) -> ... >> page_ext_callback() -> online_page_ext(). >> >> Which makes sure that all page_ext is actually allocated (for the full section). >> >> So when we later end up in generic_online_page(), the page_ext should be there >> and the memory section should be valid. >> >> >> How can online_pages(), which onlines memory part of present memory sections, >> online (free) memory that suddenly does not belong to a present memory section? >> >> Something doesn't make sense here. > > Ketan, would you please share the full OOPs message and maybe tell us > how you reproduce this issue? Ah, I think I figured it out. for_each_page_ext() does a "__page_ext = page_ext_iter_next(&__iter)" and the end. That is the real problem and should be spelled out in the patch description. Likely we should just handle that in the iterator, looking up page-ext for something that doesn't exist is weird. >From ce41607a4e4c1d872b6d3050da2e24d41a30d822 Mon Sep 17 00:00:00 2001 From: "David Hildenbrand (Arm)" Date: Fri, 19 Jun 2026 09:58:46 +0200 Subject: [PATCH] tmp Signed-off-by: David Hildenbrand (Arm) --- include/linux/page_ext.h | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-) diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h index 61e876e255e8..468c074582e1 100644 --- a/include/linux/page_ext.h +++ b/include/linux/page_ext.h @@ -112,6 +112,7 @@ static inline struct page_ext *page_ext_next(struct page_ext *curr) struct page_ext_iter { unsigned long index; + unsigned long pgcount; unsigned long start_pfn; struct page_ext *page_ext; }; @@ -120,15 +121,19 @@ struct page_ext_iter { * page_ext_iter_begin() - Prepare for iterating through page extensions. * @iter: page extension iterator. * @pfn: PFN of the page we're interested in. + * @pgcount: number of page_ext entries to iterate * * Must be called with RCU read lock taken. * * Return: NULL if no page_ext exists for this page. */ static inline struct page_ext *page_ext_iter_begin(struct page_ext_iter *iter, - unsigned long pfn) + unsigned long pfn, unsigned long pgcount) { + VM_WARN_ON_ONCE(!pgcount); + iter->index = 0; + iter->pgcount = pgcount; iter->start_pfn = pfn; iter->page_ext = page_ext_lookup(pfn); @@ -149,8 +154,9 @@ static inline struct page_ext *page_ext_iter_next(struct page_ext_iter *iter) if (WARN_ON_ONCE(!iter->page_ext)) return NULL; + if (++iter->index >= iter->pgcount) + return NULL; - iter->index++; pfn = iter->start_pfn + iter->index; if (page_ext_iter_next_fast_possible(pfn)) @@ -183,9 +189,9 @@ static inline struct page_ext *page_ext_iter_get(const struct page_ext_iter *ite * IMPORTANT: must be called with RCU read lock taken. */ #define for_each_page_ext(__page, __pgcount, __page_ext, __iter) \ - for (__page_ext = page_ext_iter_begin(&__iter, page_to_pfn(__page));\ - __page_ext && __iter.index < __pgcount; \ - __page_ext = page_ext_iter_next(&__iter)) + for (__page_ext = page_ext_iter_begin(&__iter, page_to_pfn(__page), __pgcount);\ + __page_ext; \ + __page_ext = page_ext_iter_next(&__iter)) #else /* !CONFIG_PAGE_EXTENSION */ struct page_ext; -- 2.43.0 -- Cheers, David