Re: [PATCH 1/4] mm/rmap: fix comments left over from recent changes

[Yang Shi]

On Wed, Jul 7, 2021 at 1:06 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> Parallel developments in mm/rmap.c have left behind some out-of-date
> comments: try_to_migrate_one() also accepts TTU_SYNC (already commented
> in try_to_migrate() itself), and try_to_migrate() returns nothing at all.
>
> TTU_SPLIT_FREEZE has just been deleted, so reword the comment about it in
> mm/huge_memory.c; and TTU_IGNORE_ACCESS was removed in 5.11, so delete

I just realized this. Currently unmap_page() just unmaps file pages
when splitting THP. But it seems this may cause some trouble for page
cache speculative get for the below case IIUC. Am I missing something?

    CPU A                  CPU B
unmap_page()
...
freeze refcount
                              find_get_page() ->
                                  __page_cache_add_speculative() ->
                                      VM_BUG_ON_PAGE(page_count(page)
== 0, page); //When CONFIG_TINY_RCU is enabled


The race is acceptable, I think we could replace the VM_BUG_ON to
page_ref_add_unless(), just like !CONFIG_TINY_RCU case.


> the "recently referenced" comment from try_to_unmap_one() (once upon a
> time the comment was near the removed codeblock, but they drifted apart).
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 2 +-
>  mm/rmap.c        | 7 +------
>  2 files changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8b731d53e9f4..afff3ac87067 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2331,7 +2331,7 @@ static void remap_page(struct page *page, unsigned int nr)
>  {
>         int i;
>
> -       /* If TTU_SPLIT_FREEZE is ever extended to file, remove this check */
> +       /* If unmap_page() uses try_to_migrate() on file, remove this check */
>         if (!PageAnon(page))
>                 return;
>         if (PageTransHuge(page)) {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 37c24672125c..746013e282c3 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1439,8 +1439,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>         while (page_vma_mapped_walk(&pvmw)) {
>                 /*
>                  * If the page is mlock()d, we cannot swap it out.
> -                * If it's recently referenced (perhaps page_referenced
> -                * skipped over this mm) then we should reactivate it.
>                  */
>                 if (!(flags & TTU_IGNORE_MLOCK)) {
>                         if (vma->vm_flags & VM_LOCKED) {
> @@ -1687,8 +1685,7 @@ void try_to_unmap(struct page *page, enum ttu_flags flags)
>   * @arg: enum ttu_flags will be passed to this argument.
>   *
>   * If TTU_SPLIT_HUGE_PMD is specified any PMD mappings will be split into PTEs
> - * containing migration entries. This and TTU_RMAP_LOCKED are the only supported
> - * flags.
> + * containing migration entries.
>   */
>  static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>                      unsigned long address, void *arg)
> @@ -1928,8 +1925,6 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
>   *
>   * Tries to remove all the page table entries which are mapping this page and
>   * replace them with special swap entries. Caller must hold the page lock.
> - *
> - * If is successful, return true. Otherwise, false.
>   */
>  void try_to_migrate(struct page *page, enum ttu_flags flags)
>  {
> --
> 2.26.2
>