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> (cherry picked from commit 2b0f922323ccfa76219bcaacd35cd50aeaa13592) Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> --- mm/memory.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/mm/memory.c b/mm/memory.c index b6ddfe22c5d5c..742c2f65c2c85 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4293,6 +4293,15 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page) 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 (!transhuge_vma_suitable(vma, haddr)) return ret; -- 2.47.0
