Re: [RFC PATCH] mm/huge_memory: Do not clobber swp_entry_t during THP split

[Mel Gorman]

Adding Brian to cc

On Wed, Oct 19, 2022 at 04:18:10PM -0700, Andrew Morton wrote:
> On Wed, 19 Oct 2022 11:17:14 -0700 Yang Shi <shy828301@xxxxxxxxx> wrote:
> 
> > > The intent of commit b653db77350c patch was to avoid the case where
> > > PG_private is clear but folio->private is not-NULL. However, THP tail
> > > pages uses page->private for "swp_entry_t if folio_test_swapcache()" as
> > > stated in the documentation for struct folio. This patch only clobbers
> > > page->private for tail pages if the head page was not in swapcache and
> > > warns once if page->private had an unexpected value.
> > 
> > It looks like the same issue fixed by
> > https://lore.kernel.org/linux-mm/20220906190602.1626037-1-bfoster@xxxxxxxxxx/
> 
> It is.
> 

Yep, based on Brian's changelog, it was the same workload that triggered
it as it happens to stress the corner case that hits the bug. 

> As I asked earlier this week, what about reverting b653db77350c?  Why
> do we care about the value of ->private for non-PG_private pages?

I don't think we do care but based on the changelog of b653db77350c, it's
part of an effort to either remove the PG_private bit or is a preparation
step for casting page to a meaningful type based on context but only Matthew
can tell us his motivation. There at least is some value to identifying
cases where a referenced page has valid information in page->private that
is not reflected in the flags.

-- 
Mel Gorman
SUSE Labs