Re: [RFC PATCH 1/1] mm: mark folio accessed in minor fault

[Yu Zhao]

On Wed, Dec 20, 2023 at 9:09 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Dec 21, 2023 at 09:58:25AM +0800, Zhaoyang Huang wrote:
> > On Wed, Dec 20, 2023 at 10:14 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, Dec 20, 2023 at 06:29:48PM +0800, zhaoyang.huang wrote:
> > > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > > >
> > > > Inactive mapped folio will be promoted to active only when it is
> > > > scanned in shrink_inactive_list, while the vfs folio will do this
> > > > immidiatly when it is accessed. These will introduce two affections:
> > > >
> > > > 1. NR_ACTIVE_FILE is not accurate as expected.
> > > > 2. Low reclaiming efficiency caused by dummy nactive folio which should
> > > >    be kept as earlier as shrink_active_list.
> > > >
> > > > I would like to suggest mark the folio be accessed in minor fault to
> > > > solve this situation.
> > >
> > > This isn't going to be as effective as you imagine.  Almost all file
> > > faults are handled through filemap_map_pages().  So I must ask, what
> > > testing have you done with this patch?
> > >
> > > And while you're gathering data, what effect would this patch have on your
> > > workloads?
> > Thanks for heads-up, I am out of date for readahead mechanism. My goal
>
> It's not a terribly new mechanism ... filemap_map_pages() was added nine
> years ago in 2014 by commit f1820361f83d
>
> > is to have mapped file pages behave like other pages which could be
> > promoted immediately when they are accessed. I will update the patch
> > and provide benchmark data in new patch set.
>
> Understood.  I don't know the history of this, so I'm not sure if the
> decision to not mark folios as accessed here was intentional or not.
> I suspect it's entirely unintentional.

It's intentional. For the active/inactive LRU, all folios start
inactive. The first scan of a folio transfers the A-bit (if it's set
during the initial fault) to PG_referenced; the second scan of this
folio, if the A-bit is set again, moves it to the active list. This
way single-use folios, i.e., folios mapped for file streaming, can be
reclaimed quickly, since they are "demoted" rather than "promoted" on
the second scan. This RFC would regress memory streaming workloads.