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/mm-dont-install-pmd-mappings-when-thps-are-disabled-by-the-hw-process-vma.patch
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@xxxxxxxxxx>
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@xxxxxxxxxx
Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Reported-by: Leo Fu <bfu@xxxxxxxxxx>
Tested-by: Thomas Huth <thuth@xxxxxxxxxx>
Cc: Thomas Huth <thuth@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
Cc: Janosch Frank <frankja@xxxxxxxxxxxxx>
Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxx 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
