On Wed, Feb 24, 2021 at 10:22:38PM -0700, Yu Zhao wrote:

On Thu, Feb 25, 2021 at 03:55:53AM +0000, Matthew Wilcox wrote:

quoted

On Wed, Feb 24, 2021 at 04:50:39PM -0700, Yu Zhao wrote:

quoted

Let me work out something *conceptually* smaller first, and if you
think folio is absolutely more suitable even for this specific issue,
I'll go review and test the four patches you listed. Sounds good?

Umm.  It seems to me that no matter what you do, it'll be equivalent to
this, only without the type-safety?

I'm thinking about something trivial but still very effective. So far
I've only tested it with PG_{active,unevictable}, and I'm already
seeing a 4KB gain less the 2KB loss from page_lru().

I didn't go with this at the beginning because it's also time-
consuming. I need to go over every single use of
PG_{active,unevictable,swapbacked,lru}.

Well, yes.  If you went with the folio, it'd also be typesafe.
What you've done here makes it a runtime error, and it's only detected
if you enable CONFIG_DEBUG_VM_PGFLAGS, which people don't do, in general.

... if you went full-on folio in this function, you could also make this
FolioDirty, saving another call to compound_head.

... ditto ...

                md->swapcache += nr_pages;
 
-       if (PageActive(page) || PageUnevictable(page))
+       if (PageActive(head) || PageUnevictable(head))
                md->active += nr_pages;
 
        if (PageWriteback(page))

... ditto...