On Wed, Jun 12, 2019 at 03:03:17PM -0700, Song Liu wrote:
> This patches introduces a new foll_flag: FOLL_SPLIT_PMD. As the name says
> FOLL_SPLIT_PMD splits huge pmd for given mm_struct, the underlining huge
> page stays as-is.
>
> FOLL_SPLIT_PMD is useful for cases where we need to use regular pages,
> but would switch back to huge page and huge pmd on. One of such example
> is uprobe. The following patches use FOLL_SPLIT_PMD in uprobe.
>
> Signed-off-by: Song Liu <songliubraving@xxxxxx>
> ---
> include/linux/mm.h | 1 +
> mm/gup.c | 38 +++++++++++++++++++++++++++++++++++---
> 2 files changed, 36 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0ab8c7d84cd0..e605acc4fc81 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2642,6 +2642,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
> #define FOLL_COW 0x4000 /* internal GUP flag */
> #define FOLL_ANON 0x8000 /* don't do file mappings */
> #define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */
> +#define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */
>
> /*
> * NOTE on FOLL_LONGTERM:
> diff --git a/mm/gup.c b/mm/gup.c
> index ddde097cf9e4..3d05bddb56c9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -398,7 +398,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> spin_unlock(ptl);
> return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
> }
> - if (flags & FOLL_SPLIT) {
> + if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
> int ret;
> page = pmd_page(*pmd);
> if (is_huge_zero_page(page)) {
> @@ -407,7 +407,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> split_huge_pmd(vma, pmd, address);
> if (pmd_trans_unstable(pmd))
> ret = -EBUSY;
> - } else {
> + } else if (flags & FOLL_SPLIT) {
> if (unlikely(!try_get_page(page))) {
> spin_unlock(ptl);
> return ERR_PTR(-ENOMEM);
> @@ -419,8 +419,40 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> put_page(page);
> if (pmd_none(*pmd))
> return no_page_table(vma, flags);
> - }
> + } else { /* flags & FOLL_SPLIT_PMD */
> + unsigned long addr;
> + pgprot_t prot;
> + pte_t *pte;
> + int i;
> +
> + spin_unlock(ptl);
> + split_huge_pmd(vma, pmd, address);
All the code below is only relevant for file-backed THP. It will break for
anon-THP.
And I'm not convinced that it belongs here at all. User requested PMD
split and it is done after split_huge_pmd(). The rest can be handled by
the caller as needed.
> + lock_page(page);
> + pte = get_locked_pte(mm, address, &ptl);
> + if (!pte) {
> + unlock_page(page);
> + return no_page_table(vma, flags);
Or should it be -ENOMEM?
> + }
>
> + /* get refcount for every small page */
> + page_ref_add(page, HPAGE_PMD_NR);
> +
> + prot = READ_ONCE(vma->vm_page_prot);
> + for (i = 0, addr = address & PMD_MASK;
> + i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
> + struct page *p = page + i;
> +
> + pte = pte_offset_map(pmd, addr);
> + VM_BUG_ON(!pte_none(*pte));
> + set_pte_at(mm, addr, pte, mk_pte(p, prot));
> + page_add_file_rmap(p, false);
> + }
> +
> + spin_unlock(ptl);
> + unlock_page(page);
> + add_mm_counter(mm, mm_counter_file(page), HPAGE_PMD_NR);
> + ret = 0;
> + }
> return ret ? ERR_PTR(ret) :
> follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
> }
> --
> 2.17.1
>
--
Kirill A. Shutemov
