Re: [PATCH mm-unstable v1 08/20] mm: extend FAULT_FLAG_UNSHARE support to anything in a COW mapping

Vlastimil Babka
2022-11-22T15:37:02Z

On 11/16/22 11:26, David Hildenbrand wrote:
> Extend FAULT_FLAG_UNSHARE to break COW on anything mapped into a
> COW (i.e., private writable) mapping and adjust the documentation
> accordingly.
>
> FAULT_FLAG_UNSHARE will now also break COW when encountering the shared
> zeropage, a pagecache page, a PFNMAP, ... inside a COW mapping, by
> properly replacing the mapped page/pfn by a private copy (an exclusive
> anonymous page).
>
> Note that only do_wp_page() needs care: hugetlb_wp() already handles
> FAULT_FLAG_UNSHARE correctly. wp_huge_pmd()/wp_huge_pud() also handles it
> correctly, for example, splitting the huge zeropage on FAULT_FLAG_UNSHARE
> such that we can handle FAULT_FLAG_UNSHARE on the PTE level.
>
> This change is a requirement for reliable long-term R/O pinning in
> COW mappings.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> include/linux/mm_types.h | 8 ++++----
> mm/memory.c | 4 ----
> 2 files changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5e7f4fac1e78..5e9aaad8c7b2 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1037,9 +1037,9 @@ typedef struct {
> * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
> * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
> * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
> - * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to unshare (and mark
> - * exclusive) a possibly shared anonymous page that is
> - * mapped R/O.
> + * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to break COW in a
> + * COW mapping, making sure that an exclusive anon page is
> + * mapped after the fault.
> * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
> * We should only access orig_pte if this flag set.
> *
> @@ -1064,7 +1064,7 @@ typedef struct {
> *
> * The combination FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE is illegal.
> * FAULT_FLAG_UNSHARE is ignored and treated like an ordinary read fault when
> - * no existing R/O-mapped anonymous page is encountered.
> + * applied to mappings that are not COW mappings.
> */
> enum fault_flag {
> FAULT_FLAG_WRITE = 1 << 0,
> diff --git a/mm/memory.c b/mm/memory.c
> index d47ad33c6487..56b21ab1e4d2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3432,10 +3432,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> }
> wp_page_reuse(vmf);
> return 0;
> - } else if (unshare) {
> - /* No anonymous page -> nothing to do. */
> - pte_unmap_unlock(vmf->pte, vmf->ptl);
> - return 0;
> }
> copy:
> /*