mm-dont-install-pmd-mappings-when-thps-are-disabled-by-the-hw-process-vma.patch removed from -mm tree

The quilt patch titled
Subject: mm: don't install PMD mappings when THPs are disabled by the hw/process/vma
has been removed from the -mm tree. Its filename was
mm-dont-install-pmd-mappings-when-thps-are-disabled-by-the-hw-process-vma.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: David Hildenbrand <[email protected]>
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.

---

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

@@ -4920,6 +4920,15 @@ 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 [email protected] are

mm-pagewalk-fix-usage-of-pmd_leaf-pud_leaf-without-present-check.patch selftests-mm-hugetlb_fault_after_madv-use-default-hguetlb-page-size.patch selftests-mm-hugetlb_fault_after_madv-improve-test-output.patch