On 11.10.24 13:29, Ryan Roberts wrote:
On 11/10/2024 11:24, David Hildenbrand wrote:
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.
Reported-by: Leo Fu <bfu@xxxxxxxxxx>
Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
Will this patch be difficult to backport given it depends on the previous patch
and that doesn't have a Fixes tag?
"difficult" -- not really. Andrew might want to tag patch #1 with
"Fixes:" as well, but I can also send simple stable backports that avoid
patch #1.
(Thinking again, I assume we want to Cc:stable)
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>
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 2366578015ad..a2e501489517 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4925,6 +4925,15 @@ 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;
Why not just call thp_vma_allowable_orders()?
Why call thp_vma_allowable_orders() that does a lot more work that
doesn't really apply here? :)
I'd say, just like shmem, we handle this separately here.
--
Cheers,
David / dhildenb
