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From: Muchun Song In-Reply-To: Date: Fri, 23 Jan 2026 10:32:28 +0800 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 Content-Transfer-Encoding: quoted-printable Message-Id: References: <554FD2AA-16B5-498B-9F79-296798194DF7@linux.dev> To: Kiryl Shutsemau X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 7D77DA0003 X-Stat-Signature: h6rhuz1315t76exymmz3rmcboswbyy9e X-HE-Tag: 1769135592-735066 X-HE-Meta: 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 xV4qhlcZ aYXCVxPA5sWFv3kCCQm5usMy9kpqdg49lvbFS0cLrb0NKi0td7jX3p2KaDqYQS0j1b3744Hum7xdwKRww8XDZMleqJAbqvbhvr3/byDd+toYVa0JP0QfAxQjMMmtPx0xaCHjYVPl5E/T0eSmZfm23H2IaCHFKHyzcnQRmB+z3BWonXnH29DS76va6pOiObfON9AVGHZDHgCi3LXZVzZyvxga4Inifu+fIcxuq+5N2IFEet4qeJ3Mw9Zew+q+8Cywz+pDDn2qkaHy1qnIdwJvedJg7+9vQHAJ4BGPubo+DK8wNF+XX8woAK987AA== 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: List-Subscribe: List-Unsubscribe: > On Jan 23, 2026, at 01:59, Kiryl Shutsemau wrote: >=20 > On Thu, Jan 22, 2026 at 10:02:24PM +0800, Muchun Song wrote: >>=20 >>=20 >>> On Jan 22, 2026, at 20:43, Kiryl Shutsemau wrote: >>>=20 >>> =EF=BB=BFOn Thu, Jan 22, 2026 at 07:42:47PM +0800, Muchun Song = wrote: >>>>=20 >>>>=20 >>>>>> On Jan 22, 2026, at 19:33, Muchun Song = wrote: >>>>>=20 >>>>>=20 >>>>>=20 >>>>>> On Jan 22, 2026, at 19:28, Kiryl Shutsemau = wrote: >>>>>>=20 >>>>>> On Thu, Jan 22, 2026 at 11:10:26AM +0800, Muchun Song wrote: >>>>>>>=20 >>>>>>>=20 >>>>>>>> On Jan 22, 2026, at 00:22, Kiryl Shutsemau = wrote: >>>>>>>>=20 >>>>>>>> If page->compound_info encodes a mask, it is expected that = memmap to be >>>>>>>> naturally aligned to the maximum folio size. >>>>>>>>=20 >>>>>>>> Add a warning if it is not. >>>>>>>>=20 >>>>>>>> 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. >>>>>>>>=20 >>>>>>>> Signed-off-by: Kiryl Shutsemau >>>>>>>> --- >>>>>>>> include/linux/mmzone.h | 1 + >>>>>>>> mm/sparse.c | 5 +++++ >>>>>>>> 2 files changed, 6 insertions(+) >>>>>>>>=20 >>>>>>>> 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 >>>>>>>>=20 >>>>>>>> #define MAX_FOLIO_NR_PAGES (1UL << MAX_FOLIO_ORDER) >>>>>>>> +#define MAX_FOLIO_SIZE (PAGE_SIZE << MAX_FOLIO_ORDER) >>>>>>>>=20 >>>>>>>> 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(); >>>>>>>>=20 >>>>>>>> + if (compound_info_has_mask()) { >>>>>>>> + WARN_ON(!IS_ALIGNED((unsigned long)pfn_to_page(0), >>>>>>>> + MAX_FOLIO_SIZE / sizeof(struct page))); >>>>>>>=20 >>>>>>> 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? >>>>>>>=20 >>>>>>> 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: >>>>>>>=20 >>>>>>> WARN_ON(!IS_ALIGNED((unsigned long)&folio->page, folio_size / = sizeof(struct page))); >>>>>>=20 >>>>>> I don't think it is needed. Any compound page usage would trigger = the >>>>>> problem. It should happen pretty early. >>>>>=20 >>>>> 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)? >>>=20 >>> True. >>>=20 >>>> 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? >>>=20 >>> It is. >>>=20 >>> 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. >>=20 >> If you=E2=80=99re saying the issue was only caught during >> testing, keep in mind that with KASLR enabled the >> warning is triggered at run-time; you can=E2=80=99t assume it >> will never appear in production. >=20 > Let's look at what architectures actually do with vmemmap. >=20 > On 64-bit machines, we want vmemmap to be naturally aligned to > accommodate 16GiB pages. >=20 > Assuming 64 byte struct page, it requires 256 MiB alignment for 4K > PAGE_SIZE, 64MiB for 16K PAGE_SIZE and 16MiB for 64K PAGE_SIZE. >=20 > Only 3 architectures support HVO (select = ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP): > loongarch, riscv and x86. We should make the feature conditional to = HVO > to limit exposure. >=20 > I am not sure why arm64 is not in the club. >=20 > x86 aligns vmemmap to 1G - OK. >=20 > loongarch aligns vmemmap to PMD_SIZE does not fit us with 4K and 16K > PAGE_SIZE. It should be easily fixable. No KALSR. >=20 > riscv aligns vmemmap to section size (128MiB) which is not enough. > Again, easily fixable. >=20 OK. After we fix all problems, I think changing WARN_ON to BUG_ON is = fine. > --=20 > Kiryl Shutsemau / Kirill A. Shutemov