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

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

On Tue, Nov 07, 2023 at 09:03:53AM +0800, Kefeng Wang wrote:
> 
> 
> 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@huawei.com/
> 
> Thanks.

looks like we were noticing the same thing.  I haven't done any further
work, got caught up in another project.

Matthew last pointed out:

"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'."

https://lore.kernel.org/linux-mm/ZUDFSEvpxxoGWmdG@casper.infradead.org/

I took a quick look, and the only thing I'm not confident about is that
some flags are stored in the head page, and some are stored on the
second page.

/* Which page is the flag stored in */
#define FOLIO_PF_ANY            0
#define FOLIO_PF_HEAD           0
#define FOLIO_PF_ONLY_HEAD      0
#define FOLIO_PF_NO_TAIL        0
#define FOLIO_PF_NO_COMPOUND    0
#define FOLIO_PF_SECOND         1

There's only a handful, so yeah the best way is probably to go ahead and
swap k = page->flags for k = *folio_flags(folio, 0) and then handle the
couple of outliars.

~Gregory