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

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Date:   Mon, 30 Oct 2023 19:41:23 -0400
From:   Gregory Price <gregory.price@...verge.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, david@...hat.com, vbabka@...e.cz,
        naoya.horiguchi@...ux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kpageflags: respect folio head-page flag placement

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);
>