On Fri, Sep 18, 2020 at 09:37:00AM +0200, Michal Hocko wrote:
> On Thu 17-09-20 21:00:40, Yu Zhao wrote:
> > This patch replaces the only open-coded __ClearPageActive() with
> > page_off_lru(). There is no open-coded __ClearPageUnevictable()s.
> >
> > Before this patch, we have:
> > __ClearPageActive()
> > add_page_to_lru_list()
> >
> > After this patch, we have:
> > page_off_lru()
> > if PageUnevictable()
> > __ClearPageUnevictable()
> > else if PageActive()
> > __ClearPageActive()
> > add_page_to_lru_list()
> >
> > Checking PageUnevictable() shouldn't be a problem because these two
> > flags are mutually exclusive. Leaking either will trigger bad_page().
>
> I am sorry but the changelog is really hard to grasp. What are you
> trying to achieve, why and why it is safe. This should be a general
> outline for any patch. I have already commented on the previous patch
> and asked you for the explanation why removing __ClearPageActive from
> this path is desirable and safe. I have specifically asked to clarify
> the compound page situation as that is using its oen destructor in the
> freeing path and that might result in page_off_lru to be not called.
Haven't I explained we are NOT removing __ClearPageActive()? Is my
notion of the code structure above confusing you? Or 'open-coded'
could mean different things?
And I have asked this before: why does 'the compound page situation'
even matter here? Perhaps if you could give a concrete example related
to the code change and help me understand your concern?
> > Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> > ---
> > mm/vmscan.c | 6 +-----
> > 1 file changed, 1 insertion(+), 5 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 503fc5e1fe32..f257d2f61574 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1845,7 +1845,6 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> > int nr_pages, nr_moved = 0;
> > LIST_HEAD(pages_to_free);
> > struct page *page;
> > - enum lru_list lru;
> >
> > while (!list_empty(list)) {
> > page = lru_to_page(list);
> > @@ -1860,14 +1859,11 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
> > lruvec = mem_cgroup_page_lruvec(page, pgdat);
> >
> > SetPageLRU(page);
> > - lru = page_lru(page);
> > -
> > add_page_to_lru_list(page, lruvec, lru);
> >
> > if (put_page_testzero(page)) {
> > __ClearPageLRU(page);
> > - __ClearPageActive(page);
> > - del_page_from_lru_list(page, lruvec, lru);
> > + del_page_from_lru_list(page, lruvec, page_off_lru(page));
> >
> > if (unlikely(PageCompound(page))) {
> > spin_unlock_irq(&pgdat->lru_lock);
> > --
> > 2.28.0.681.g6f77f65b4e-goog
>
> --
> Michal Hocko
> SUSE Labs
