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Howlett" , ankita@nvidia.com, aniketa@nvidia.com, vsethi@nvidia.com, mochs@nvidia.com, skolothumtho@nvidia.com, linmiaohe@huawei.com, nao.horiguchi@gmail.com, akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, tony.luck@intel.com, bp@alien8.de, rafael@kernel.org, guohanjun@huawei.com, mchehab@kernel.org, lenb@kernel.org, kevin.tian@intel.com, alex@shazbot.org, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, zhiw@nvidia.com, dnigam@nvidia.com, kjaju@nvidia.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-edac@vger.kernel.org, Jonathan.Cameron@huawei.com, ira.weiny@intel.com, Smita.KoralahalliChannabasappa@amd.com, u.kleine-koenig@baylibre.com, peterz@infradead.org, linux-acpi@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v3 0/3] mm: Implement ECC handling for pfn with no struct page Message-ID: <20251021164444.GB699957@nvidia.com> References: <20251021102327.199099-1-ankita@nvidia.com> Content-Type: text/plain; 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Howlett wrote: > > enables VM_PFNMAP vmas to map at PMD level. Otherwise, a poison to a PFN > > would need breaking the PMD mapping into PTEs to unmap only the poisoned > > PFN. This can have a major performance impact. > > Is the performance impact really a concern in the event of failed > memory? Yes, something like the KVM S2 is very sensitive to page size for TLB performace. > Does this happen enough to warrant this special case? If you have a 100k sized cluster it happens constantly :\ > Surely it's not failing hardware that may cause performance impacts, so > is this triggered in some other way that I'm missing or a conversation > pointer? It is the splitting of a pgd/pmd level into PTEs that gets mirrored into the S2 and then greatly increases the cost of table walks inside a guest. The HW caches are sized for 1G S2 PTEs, not 4k. Jason