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SubjectRe: [PATCH] mm: page-writeback: simplify memcg handling in test_clear_page_writeback()
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: 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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