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SubjectRe: [PATCH v4 1/2] khugepaged: enable collapse pmd for pte-mapped THP
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> On Aug 6, 2019, at 3:02 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 08/02, Song Liu wrote:
>>
>> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
>> +{
>> + unsigned long haddr = addr & HPAGE_PMD_MASK;
>> + struct vm_area_struct *vma = find_vma(mm, haddr);
>> + pmd_t *pmd = mm_find_pmd(mm, haddr);
>> + struct page *hpage = NULL;
>> + spinlock_t *ptl;
>> + int count = 0;
>> + pmd_t _pmd;
>> + int i;
>> +
>> + if (!vma || !vma->vm_file || !pmd ||
>> + vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE)
>> + return;
>
> I still can't understand why is it safe to blindly use mm_find_pmd().
>
> Say, what pmd_offset(pud, address) will return to this function if
> pud_huge() == T? IIUC, this is possible if is_file_hugepages(vm_file).
> How the code below can use this result?

IIUC, the concern is matching files in hugetlbfs. Maybe we can exclude
that specifically?

>
> I think you need something like hugepage_vma_check() or even
> hugepage_vma_revalidate().

To use hugepage_vma_check(), we will need to set VM_HUGEPAGE for the
following case:

mount shm with huge=always
copy app to shm
start app and enable uprobe
disable uprobe

This vma will not have VM_HUGEPAGE, so it will fail hugepage_vma_check().

How about something like:

diff --git i/kernel/events/uprobes.c w/kernel/events/uprobes.c
index 94d38a39d72e..f0d3e367f907 100644
--- i/kernel/events/uprobes.c
+++ w/kernel/events/uprobes.c
@@ -532,8 +532,10 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
put_page(new_page);
new_page = NULL;

- if (PageCompound(orig_page))
+ if (PageCompound(orig_page)) {
orig_page_huge = true;
+ vma->vm_flags |= VM_HUGEPAGE;
+ }
}
put_page(orig_page);
}
Thanks,
Song
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