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

[Gregory Price <gregory.price@memverge.com>]

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?

> >  	u |= kpf_copy_bit(k, KPF_UPTODATE,	PG_uptodate);
> >  	u |= kpf_copy_bit(k, KPF_WRITEBACK,	PG_writeback);
> >  
> >  	u |= kpf_copy_bit(k, KPF_RECLAIM,	PG_reclaim);
> >  
> >  	if (PageSwapCache(page))
> >  		u |= 1 << KPF_SWAPCACHE;
> >  	u |= kpf_copy_bit(k, KPF_SWAPBACKED,	PG_swapbacked);
> >  
> >  	u |= kpf_copy_bit(k, KPF_MLOCKED,	PG_mlocked);
>