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

[Zhaoyang Huang]

On Thu, Dec 21, 2023 at 2:33 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> On Wed, Dec 20, 2023 at 11:28 PM Zhaoyang Huang <huangzhaoyang@xxxxxxxxx> wrote:
> >
> > On Thu, Dec 21, 2023 at 12:53 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> > >
> > > 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 Thanks for heads-up, I am out of date for readahead mechanism.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.
> > Thanks. Please correct me if I am wrong. IMO, there will be no
> > minor-fault for single-use folios
>
> Why not? What prevents a specific *access pattern* from triggering minor faults?
Please find the following chart for mapped page state machine
transfication. We can find that:
1. RFC behaves the same as the mainline in (1)(2)
2. VM_EXEC mapped pages are activated earlier than mainline which help
improve scan efficiency in (3)(4)
3. none VM_EXEC mapped pages are dropped as vfs pages do during 3rd scan.

(1)
                                  1st access
shrink_active_list              1st scan(shink_folio_list)
2nd scan(shrink_folio_list')
mainline                     INA/UNR                        NA
                          INA/REF
DROP
RFC                           INA/UNR                        NA
                           INA/REF
DROP

(2)
                                  1st access                   2nd
access                        shrink_active_list          1st
scan(shink_folio_list)
mainline                     INA/UNR                     INA/UNR
                      NA                                 ACT/REF
RFC                           INA/UNR                     INA/REF
                        NA                                 ACT/REF

(3)
                                  1st access
shrink_active_list        1st scan(shink_folio_list)       2nd access
      2nd scan(shrink_active_list)     3rd scan(shink_folio_list)
mainline                     INA/UNR                        NA
                          INA/REF                           INA/REF
                NA                                     ACT/REF
RFC                           INA/UNR                        NA
                           INA/REF                           ACT/REF
                ACT/REF                           NA
(VM_EXEC)
RFC                           INA/UNR                        NA
                           INA/REF                           ACT/REF
                INA/REF                            DROP
(non VM_EXEC)

(4)
                                  1st access                   2nd
access                         3rd access
shrink_active_list                   shink_folio_list
mainline                     INA/UNR                       INA/UNR
                       INA/UNR                          NA
                           ACT/REF
RFC                           INA/UNR                       INA/REF
                         ACT/REF                         ACT/REF
                       NA
(VM_EXEC)
RFC                           INA/UNR                       INA/REF
                         ACT/REF                         ACT/REF
                       NA
(Non VM_EXEC)
>
> > which means RFC could behave the
> > same as mainline does now? I think it doesn't make sense to have
> > multiple-mapped pages filled in page_list to shrink_page_list since we
> > can distinguish them in advance.