> On Aug 12, 2019, at 6:06 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> On 08/09, 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);
>> + struct page *hpage = NULL;
>> + pmd_t *pmd, _pmd;
>> + spinlock_t *ptl;
>> + int count = 0;
>> + int i;
>> +
>> + if (!vma || !vma->vm_file ||
>> + vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE)
>> + return;
>> +
>> + /*
>> + * This vm_flags may not have VM_HUGEPAGE if the page was not
>> + * collapsed by this mm. But we can still collapse if the page is
>> + * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
>> + * will not fail the vma for missing VM_HUGEPAGE
>> + */
>> + if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
>> + return;
>> +
>> + pmd = mm_find_pmd(mm, haddr);
>> + if (!pmd)
>> + return;
>> +
>> + /* step 1: check all mapped PTEs are to the right huge page */
>> + for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
>> + pte_t *pte = pte_offset_map(pmd, addr);
>> + struct page *page;
>> +
>> + if (pte_none(*pte) || !pte_present(*pte))
>> + continue;
>
> if (!pte_present(*pte))
> return;
>
> you can't simply flush pmd if this page is swapped out.
hmm... how about
if (pte_none(*pte))
continue;
if (!pte_present(*pte))
return;
If the page hasn't faulted in for this mm, i.e. pte_none(), we
can flush the pmd.
>
>> +
>> + page = vm_normal_page(vma, addr, *pte);
>> +
>> + if (!page || !PageCompound(page))
>> + return;
>> +
>> + if (!hpage) {
>> + hpage = compound_head(page);
>> + /*
>> + * The mapping of the THP should not change.
>> + *
>> + * Note that uprobe may change the page table,
>
> Not only uprobe can cow the page. Debugger can do. Or mmap(PROT_WRITE, MAP_PRIVATE).
>
> uprobe() is "special" because it a) it works with a foreign mm and b)
> it can't stop the process which uses this mm. Otherwise it could simply
> update the page returned by get_user_pages_remote(FOLL_FORCE), just we
> would need to add FOLL_WRITE and if we do this we do not even need SPLIT,
> that is why, say, __access_remote_vm() works without SPLIT.
Will update the comment in next version.
Thanks!
Song
