On 3/14/23 23:10, Yosry Ahmed wrote:
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.
IIUC this skips tail pages, because they should always have
page->memcg_data == 0, even if they are charged to a memcg. To
correctly get their memcg we should read it from the
compound_head()/page_folio().
The purpose of that function is mainly to report pages that have a
reference to a memcg, especially the dead one. So by counting the
occurrence of a particular cgroup name, we can have a rough idea of
that. So only head page has relevance here and we can skip the tail pages.
My 2c, we can check PageSlab() to print the extra message for slab
pages, instead of reading memcg_data directly, which kinda breaks the
abstraction created by the various helpers for reading a page memcg.
Someone can easily change something in how memcg_data is interpreted
in those helpers without realizing that page_owner is also reading it.
You are right. We should be using a helper if available. I will send a
patch to fix that.
Thanks,
Longman
