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Howlett" , Linus Torvalds , linux-arm-kernel@axis.com, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-ide@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mmc@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Marco Elver , Marek Szyprowski , Michal Hocko , Muchun Song , netdev@vger.kernel.org, Oscar Salvador , Peter Xu , Robin Murphy , Suren Baghdasaryan , Tejun Heo , virtualization@lists.linux.dev, Vlastimil Babka , wireguard@lists.zx2c4.com, x86@kernel.org Subject: Re: [PATCH v1 11/36] mm: limit folio/compound page sizes in problematic kernel configs Message-ID: <32fbe774-d0e4-498e-873f-f028347c1fcb@lucifer.local> References: <20250827220141.262669-1-david@redhat.com> <20250827220141.262669-12-david@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: 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sfid-20250829_050232_778476_2778AD6F X-CRM114-Status: GOOD ( 24.19 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Aug 29, 2025 at 01:57:22PM +0200, David Hildenbrand wrote: > On 28.08.25 17:10, Lorenzo Stoakes wrote: > > On Thu, Aug 28, 2025 at 12:01:15AM +0200, David Hildenbrand wrote: > > > Let's limit the maximum folio size in problematic kernel config where > > > the memmap is allocated per memory section (SPARSEMEM without > > > SPARSEMEM_VMEMMAP) to a single memory section. > > > > > > Currently, only a single architectures supports ARCH_HAS_GIGANTIC_PAGE > > > but not SPARSEMEM_VMEMMAP: sh. > > > > > > Fortunately, the biggest hugetlb size sh supports is 64 MiB > > > (HUGETLB_PAGE_SIZE_64MB) and the section size is at least 64 MiB > > > (SECTION_SIZE_BITS == 26), so their use case is not degraded. > > > > > > As folios and memory sections are naturally aligned to their order-2 size > > > in memory, consequently a single folio can no longer span multiple memory > > > sections on these problematic kernel configs. > > > > > > nth_page() is no longer required when operating within a single compound > > > page / folio. > > > > > > Reviewed-by: Zi Yan > > > Acked-by: Mike Rapoport (Microsoft) > > > Signed-off-by: David Hildenbrand > > > > Realy great comments, like this! > > > > I wonder if we could have this be part of the first patch where you fiddle > > with MAX_FOLIO_ORDER etc. but not a big deal. > > I think it belongs into this patch where we actually impose the > restrictions. Sure it's not a big deal. > > [...] > > > > +/* > > > + * Only pages within a single memory section are guaranteed to be > > > + * contiguous. By limiting folios to a single memory section, all folio > > > + * pages are guaranteed to be contiguous. > > > + */ > > > +#define MAX_FOLIO_ORDER PFN_SECTION_SHIFT > > > > Hmmm, was this implicit before somehow? I mean surely by the fact as you say > > that physical contiguity would not otherwise be guaranteed :)) > > Well, my patches until this point made sure that any attempt to use a larger > folio would fail in a way that we could spot now if there is any offender. Ack yeah. > > That is why before this change, nth_page() was required within a folio. > > Hope that clarifies it, thanks! Yes thanks! :) > > -- > Cheers > > David / dhildenb > Cheers, Lorenzo