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Thu, 22 Jan 2026 07:42:59 -0500 (EST) Date: Thu, 22 Jan 2026 12:42:54 +0000 From: Kiryl Shutsemau To: Muchun Song Cc: Andrew Morton , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCHv4 07/14] mm/sparse: Check memmap alignment for compound_info_has_mask() Message-ID: References: <20260121162253.2216580-1-kas@kernel.org> <20260121162253.2216580-8-kas@kernel.org> <71F051F2-5F3B-40A5-9347-BA2D93F2FF3F@linux.dev> <35B81EA5-D719-4FC4-93C5-674DD5BFDA4F@linux.dev> <45434FC3-455E-4CE8-9F43-F398D5EC73A9@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <45434FC3-455E-4CE8-9F43-F398D5EC73A9@linux.dev> On Thu, Jan 22, 2026 at 07:42:47PM +0800, Muchun Song wrote: > > > > On Jan 22, 2026, at 19:33, Muchun Song wrote: > > > > > > > >> On Jan 22, 2026, at 19:28, Kiryl Shutsemau wrote: > >> > >> On Thu, Jan 22, 2026 at 11:10:26AM +0800, Muchun Song wrote: > >>> > >>> > >>>> On Jan 22, 2026, at 00:22, Kiryl Shutsemau wrote: > >>>> > >>>> If page->compound_info encodes a mask, it is expected that memmap to be > >>>> naturally aligned to the maximum folio size. > >>>> > >>>> Add a warning if it is not. > >>>> > >>>> A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the > >>>> kernel is still likely to be functional if this strict check fails. > >>>> > >>>> Signed-off-by: Kiryl Shutsemau > >>>> --- > >>>> include/linux/mmzone.h | 1 + > >>>> mm/sparse.c | 5 +++++ > >>>> 2 files changed, 6 insertions(+) > >>>> > >>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > >>>> index 390ce11b3765..7e4f69b9d760 100644 > >>>> --- a/include/linux/mmzone.h > >>>> +++ b/include/linux/mmzone.h > >>>> @@ -91,6 +91,7 @@ > >>>> #endif > >>>> > >>>> #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) > >>>> +#define MAX_FOLIO_SIZE (PAGE_SIZE << MAX_FOLIO_ORDER) > >>>> > >>>> enum migratetype { > >>>> MIGRATE_UNMOVABLE, > >>>> diff --git a/mm/sparse.c b/mm/sparse.c > >>>> index 17c50a6415c2..5f41a3edcc24 100644 > >>>> --- a/mm/sparse.c > >>>> +++ b/mm/sparse.c > >>>> @@ -600,6 +600,11 @@ void __init sparse_init(void) > >>>> BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section))); > >>>> memblocks_present(); > >>>> > >>>> + if (compound_info_has_mask()) { > >>>> + WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0), > >>>> + MAX_FOLIO_SIZE / sizeof(struct page))); > >>> > >>> I still have concerns about this. If certain architectures or configurations, > >>> especially when KASLR is enabled, do not meet the requirements during the > >>> boot stage, only specific folios larger than a certain size might end up with > >>> incorrect struct page entries as the system runs. How can we detect issues > >>> arising from either updating the struct page or making incorrect logical > >>> judgments based on information retrieved from the struct page? > >>> > >>> After all, when we see this warning, we don't know when or if a problem will > >>> occur in the future. It's like a time bomb in the system, isn't it? Therefore, > >>> I would like to add a warning check to the memory allocation place, for > >>> example: > >>> > >>> WARN_ON(!IS_ALIGNED((unsigned long)&folio->page, folio_size / sizeof(struct page))); > >> > >> I don't think it is needed. Any compound page usage would trigger the > >> problem. It should happen pretty early. > > > > Why would you think it would be discovered early? If the alignment of struct page > > can only meet the needs of 4M pages (i.e., the largest pages that buddy can > > allocate), how can you be sure that there will be a similar path using CMA > > early on if the system allocates through CMA in the future (after all, CMA > > is used much less than buddy)? True. > Suppose we are more aggressive. If the alignment requirement of struct page > cannot meet the needs of 2GB pages (which is an uncommon memory allocation > requirement), then users might not care about such a warning message after > the system boots. And if there is no allocation of pages greater than or > equal to 2GB for a period of time in the future, the system will have no > problems. But once some path allocates pages greater than or equal to 2GB, > the system will go into chaos. And by that time, the system log may no > longer have this warning message. Is that not the case? It is. I expect the warning to be reported early if we have configurations that do not satisfy the alignment requirement even in absence of the crash. Adding a check to the allocation path if way too high price for a theoretical problem. -- Kiryl Shutsemau / Kirill A. Shutemov