On 3/13/23 17:08, Yosry Ahmed wrote:
On Mon, Mar 13, 2023 at 12:44 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, 13 Mar 2023 08:34:52 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
From: Hugh Dickins <hughd@xxxxxxxxxx>
In a kernel with added WARN_ON_ONCE(PageTail) in page_memcg_check(), we
observed a warning from page_cgroup_ino() when reading
/proc/kpagecgroup.
If this is the only known situation in which page_memcg_check() is
passed a tail page, why does page_memcg_check() have
if (PageTail(page))
return NULL;
? Can we remove this to simplify, streamline and clarify?
I guess it's a safety check so that we don't end up trying to cast a
tail page to a folio. My opinion is to go one step further and change
page_memcg_check() to do return the memcg of the head page, i.e:
static inline struct mem_cgroup *page_memcg_check(struct page *page)
{
return folio_memcg_check(page_folio(page));
}
This makes it consistent with page_memcg(), and makes sure future
users are getting the "correct" memcg for whatever page they pass in.
I am interested to hear other folks' opinions here.
The only other user today is print_page_owner_memcg(). I am not sure
if it's doing the right thing by explicitly reading page->memcg_data,
but it is already excluding pages that have page->memcg_data == 0,
which should be the case for tail pages.
It is reading memcg_data directly to see if it is slab cache page. It is
currently skipping page that does not have memcg_data set.
Cheers,
Longman
