From: Song Liu <songliubraving@xxxxxx> Subject: mm, thp: introduce FOLL_SPLIT_PMD Introduce 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. Link: http://lkml.kernel.org/r/20190815164525.1848545-4-songliubraving@xxxxxx Signed-off-by: Song Liu <songliubraving@xxxxxx> Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/mm.h | 1 + mm/gup.c | 8 ++++++-- 2 files changed, 7 insertions(+), 2 deletions(-) --- a/include/linux/mm.h~mm-thp-introduce-foll_split_pmd +++ a/include/linux/mm.h @@ -2591,6 +2591,7 @@ struct page *follow_page(struct vm_area_ #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: --- a/mm/gup.c~mm-thp-introduce-foll_split_pmd +++ a/mm/gup.c @@ -384,7 +384,7 @@ retry_locked: 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)) { @@ -393,7 +393,7 @@ retry_locked: 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); @@ -405,6 +405,10 @@ retry_locked: put_page(page); if (pmd_none(*pmd)) return no_page_table(vma, flags); + } else { /* flags & FOLL_SPLIT_PMD */ + spin_unlock(ptl); + split_huge_pmd(vma, pmd, address); + ret = pte_alloc(mm, pmd) ? -ENOMEM : 0; } return ret ? ERR_PTR(ret) : _
