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SubjectRe: [PATCH] mm: page-writeback: simplify memcg handling in test_clear_page_writeback()
On Tue, 9 Feb 2021, Hugh Dickins wrote:
> On Tue, 9 Feb 2021, Johannes Weiner wrote:
>
> > Page writeback doesn't hold a page reference, which allows truncate to
> > free a page the second PageWriteback is cleared. This used to require
> > special attention in test_clear_page_writeback(), where we had to be
> > careful not to rely on the unstable page->memcg binding and look up
> > all the necessary information before clearing the writeback flag.
> >
> > Since commit 073861ed77b6 ("mm: fix VM_BUG_ON(PageTail) and
> > BUG_ON(PageWriteback)") test_clear_page_writeback() is called with an
> > explicit reference on the page, and this dance is no longer needed.
> >
> > Use unlock_page_memcg() and dec_lruvec_page_stat() directly.
>
> s/stat()/state()/
>
> This is a nice cleanup: I hadn't seen that connection at all.
>
> But I think you should take it further:
> __unlock_page_memcg() can then be static in mm/memcontrol.c,
> and its declarations deleted from include/linux/memcontrol.h?

And further: void lock_page_memcg(page), not returning memcg.

>
> And further: delete __dec_lruvec_state() and dec_lruvec_state()
> from include/linux/vmstat.h - unless you feel that every "inc"
> ought to be matched by a "dec", even when unused.
>
> >
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Acked-by: Hugh Dickins <hughd@google.com>
>
> > ---
> > mm/page-writeback.c | 9 +++------
> > 1 file changed, 3 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index eb34d204d4ee..f6c2c3165d4d 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -2722,12 +2722,9 @@ EXPORT_SYMBOL(clear_page_dirty_for_io);
> > int test_clear_page_writeback(struct page *page)
> > {
> > struct address_space *mapping = page_mapping(page);
> > - struct mem_cgroup *memcg;
> > - struct lruvec *lruvec;
> > int ret;
> >
> > - memcg = lock_page_memcg(page);
> > - lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
> > + lock_page_memcg(page);
> > if (mapping && mapping_use_writeback_tags(mapping)) {
> > struct inode *inode = mapping->host;
> > struct backing_dev_info *bdi = inode_to_bdi(inode);
> > @@ -2755,11 +2752,11 @@ int test_clear_page_writeback(struct page *page)
> > ret = TestClearPageWriteback(page);
> > }
> > if (ret) {
> > - dec_lruvec_state(lruvec, NR_WRITEBACK);
> > + dec_lruvec_page_state(page, NR_WRITEBACK);
> > dec_zone_page_state(page, NR_ZONE_WRITE_PENDING);
> > inc_node_page_state(page, NR_WRITTEN);
> > }
> > - __unlock_page_memcg(memcg);
> > + unlock_page_memcg(page);
> > return ret;
> > }
> >
> > --
> > 2.30.0
>

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