The patch titled Subject: mm: don't install PMD mappings when THPs are disabled by the hw/process/vma has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-dont-install-pmd-mappings-when-thps-are-disabled-by-the-hw-process-vma.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: David Hildenbrand david@redhat.com Subject: mm: don't install PMD mappings when THPs are disabled by the hw/process/vma Date: Fri, 11 Oct 2024 12:24:45 +0200

We (or rather, readahead logic :) ) might be allocating a THP in the pagecache and then try mapping it into a process that explicitly disabled THP: we might end up installing PMD mappings.

This is a problem for s390x KVM, which explicitly remaps all PMD-mapped THPs to be PTE-mapped in s390_enable_sie()->thp_split_mm(), before starting the VM.

For example, starting a VM backed on a file system with large folios supported makes the VM crash when the VM tries accessing such a mapping using KVM.

Is it also a problem when the HW disabled THP using TRANSPARENT_HUGEPAGE_UNSUPPORTED? At least on x86 this would be the case without X86_FEATURE_PSE.

In the future, we might be able to do better on s390x and only disallow PMD mappings -- what s390x and likely TRANSPARENT_HUGEPAGE_UNSUPPORTED really wants. For now, fix it by essentially performing the same check as would be done in __thp_vma_allowable_orders() or in shmem code, where this works as expected, and disallow PMD mappings, making us fallback to PTE mappings.

Link: https://lkml.kernel.org/r/20241011102445.934409-3-david@redhat.com Fixes: 793917d997df ("mm/readahead: Add large folio readahead") Signed-off-by: David Hildenbrand david@redhat.com Reported-by: Leo Fu bfu@redhat.com Tested-by: Thomas Huth thuth@redhat.com Cc: Thomas Huth thuth@redhat.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Ryan Roberts ryan.roberts@arm.com Cc: Christian Borntraeger borntraeger@linux.ibm.com Cc: Janosch Frank frankja@linux.ibm.com Cc: Claudio Imbrenda imbrenda@linux.ibm.com Cc: Hugh Dickins hughd@google.com Cc: Kefeng Wang wangkefeng.wang@huawei.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memory.c | 9 +++++++++ 1 file changed, 9 insertions(+)

--- a/mm/memory.c~mm-dont-install-pmd-mappings-when-thps-are-disabled-by-the-hw-process-vma +++ a/mm/memory.c @@ -4931,6 +4931,15 @@ vm_fault_t do_set_pmd(struct vm_fault *v pmd_t entry; vm_fault_t ret = VM_FAULT_FALLBACK;

+ /* + * It is too late to allocate a small folio, we already have a large + * folio in the pagecache: especially s390 KVM cannot tolerate any + * PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any + * PMD mappings if THPs are disabled. + */ + if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags)) + return ret; + if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER)) return ret;

_

Patches currently in -mm which might be from david@redhat.com are

mm-dont-install-pmd-mappings-when-thps-are-disabled-by-the-hw-process-vma.patch selftests-mm-hugetlb_fault_after_madv-use-default-hguetlb-page-size.patch selftests-mm-hugetlb_fault_after_madv-improve-test-output.patch