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.
> +
> + 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.
Oleg.
