Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock

[Hugh Dickins]

On Mon, 2 Nov 2020, Johannes Weiner wrote:
> On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote:
> > From: Hugh Dickins <hughd@xxxxxxxxxx>
> > 
> > It is necessary for page_idle_get_page() to recheck PageLRU() after
> > get_page_unless_zero(), but holding lru_lock around that serves no
> > useful purpose, and adds to lru_lock contention: delete it.
> > 
> > See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
> > discussion that led to lru_lock there; but __page_set_anon_rmap() now
> > uses WRITE_ONCE(),
> 
> That doesn't seem to be the case in Linus's or Andrew's tree. Am I
> missing a dependent patch series?

Sorry, I was out of action, then slower than ever, for a while.

Many thanks for calling out my falsehood there, Johannes.

What led me to write that?  It has baffled me, but at last I see:
this patch to page_idle_get_page() was 0002 in my lru_lock patchset
against v5.3 last year, and 0001 was the patch which made it true.
Then when I checked against mainline, I must have got confused by
the similar WRITE_ONCE in page_move_anon_rmap().

Appended below, but not rediffed, and let's not hold up Alex's set
for the rest of it: it is all theoretical until the kernel gets to
be built with a suitably malicious compiler; but I'll follow up
with a fresh version of the below after his set is safely in.