Re: [PATCH v4 1/2] khugepaged: enable collapse pmd for pte-mapped THP

[Song Liu]

> On Aug 6, 2019, at 3:02 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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