Re: [PATCH 6.1.y] mm: huge_memory: add vma_thp_disabled() and thp_disabled_by_hw()

[David Hildenbrand <david@redhat.com>]

On 22.10.24 11:00, David Hildenbrand wrote:
> From: Kefeng Wang <wangkefeng.wang@huawei.com>
> 
> Patch series "mm: don't install PMD mappings when THPs are disabled by the
> hw/process/vma".
> 
> During testing, it was found that we can get PMD mappings in processes
> where THP (and more precisely, PMD mappings) are supposed to be disabled.
> While it works as expected for anon+shmem, the pagecache is the
> problematic bit.
> 
> For s390 KVM this currently means that a VM backed by a file located on
> filesystem with large folio support can crash when KVM tries accessing the
> problematic page, because the readahead logic might decide to use a
> PMD-sized THP and faulting it into the page tables will install a PMD
> mapping, something that s390 KVM cannot tolerate.
> 
> This might also be a problem with HW that does not support PMD mappings,
> but I did not try reproducing it.
> 
> Fix it by respecting the ways to disable THPs when deciding whether we can
> install a PMD mapping.  khugepaged should already be taking care of not
> collapsing if THPs are effectively disabled for the hw/process/vma.
> 
> This patch (of 2):
> 
> Add vma_thp_disabled() and thp_disabled_by_hw() helpers to be shared by
> shmem_allowable_huge_orders() and __thp_vma_allowable_orders().
> 
> [david@redhat.com: rename to vma_thp_disabled(), split out thp_disabled_by_hw() ]
> Link: https://lkml.kernel.org/r/20241011102445.934409-2-david@redhat.com
> Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reported-by: Leo Fu <bfu@redhat.com>
> Tested-by: Thomas Huth <thuth@redhat.com>
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Boqiao Fu <bfu@redhat.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Janosch Frank <frankja@linux.ibm.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 963756aac1f011d904ddd9548ae82286d3a91f96)
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> 
> Only contextual differences in shmem_allowable_huge_orders(). Note that
> this patch is required to backport the fix
> 2b0f922323ccfa76219bcaacd35cd50aeaa13592, which can be cleanly cherry
> picked on top.

ARG my backporting skills (or rather patch sending skills) are not 
strong today. This is the 6.11.y variant. Please ignore this mail ... :(

-- 
Cheers,

David / dhildenb