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

[Yang Shi <shy828301@gmail.com>]

On Wed, Jul 7, 2021 at 2:26 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Wed, Jul 7, 2021 at 1:06 PM Hugh Dickins <hughd@google.com> 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 Parallel developments in mm/rmap.c have left behind some out-of-date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.

Please just disregard the above comment, I just found CONFIG_TINY_RCU
is for UP only. Sorry for the noise.

>
>
> > 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@google.com>
> > ---
> >  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
> >