Patch "mm: don't install PMD mappings when THPs are disabled by the hw/process/vma" has been added to the 6.11-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    mm: don't install PMD mappings when THPs are disabled by the hw/process/vma

to the 6.11-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mm-don-t-install-pmd-mappings-when-thps-are-disabled.patch
and it can be found in the queue-6.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit b13b8a326a0c338769aa6ea6633e039a9632f95b
Author: David Hildenbrand <david@xxxxxxxxxx>
Date:   Fri Oct 11 12:24:45 2024 +0200

    mm: don't install PMD mappings when THPs are disabled by the hw/process/vma
    
    [ Upstream commit 2b0f922323ccfa76219bcaacd35cd50aeaa13592 ]
    
    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>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/mm/memory.c b/mm/memory.c
index cda2c12c500b8..cb66345e398d2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4719,6 +4719,15 @@ 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 (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
 		return ret;