Re: [PATCH v2] kpageflags: respect folio head-page flag placement

[Matthew Wilcox <willy@infradead.org>]

On Mon, Oct 30, 2023 at 07:41:23PM -0400, Gregory Price wrote:
> On Mon, Oct 30, 2023 at 11:22:18PM +0000, Matthew Wilcox wrote:
> > On Mon, Oct 30, 2023 at 02:00:05PM -0400, Gregory Price wrote:
> > > kpageflags reads page-flags directly from the page, even when the
> > > respective flag is only updated on the headpage of a folio.
> > > 
> > > Update bitchecks to use PAGEFLAG() interfaces to check folio for the
> > > referenced, dirty, lru, active, and unevictable bits.
> > 
> > But uptodate, writeback and reclaim (amongst others) are also defined
> > only on the head page.
> > 
> 
> Ah yes i was only looking at the things defined w/ PAGEFLAG defines in
> page-flags.h. I'll give it full once over can collect them all, my bad.
> 
> (also i forgot to update my commit message)
> 
> Quick question here since i have your attention: any recommendation on
> what to do for ONLY_HEAD flags?  If the provided page is not the head,
> should the flag report 0... or whatever the head says?

Thinking about it some more, really almost all flags are per-folio, not
per-page.  The only exceptions are HWPoison and AnonExclusive.  So
probably the right way to do this is to make k = folio->flags, and
then just change a few places rather than changing all the places that
test 'k'.