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

[Kefeng Wang]

On 2023/10/31 12:34, Gregory Price wrote:
> On Tue, Oct 31, 2023 at 09:13:44AM +0000, Matthew Wilcox wrote:
> > 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'.
>
> Funny enough that's what i originally did but was confident it was
> correct so walked it back.  I'll take another crack at it.

Hi Gregory, any update?
I changed stable_page_flags[1] when try to remove page idle wrapper,
Matthew pointed it will conflict with this, I could redo my patch
based on your new version:)

[1]https://lore.kernel.org/linux-mm/20231103072906.2000381-5-wangkefeng.wang@xxxxxxxxxx/

Thanks.
> ~Gregory