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

[Yu Zhao]

On Fri, Dec 22, 2023 at 2:41 AM 黄朝阳 (Zhaoyang Huang)
<zhaoyang.huang@xxxxxxxxxx> wrote:
>
>
>
> On Fri, Dec 22, 2023 at 2:45 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >
> > On Thu, Dec 21, 2023 at 11:29 PM 黄朝阳 (Zhaoyang Huang)
> > <zhaoyang.huang@xxxxxxxxxx> wrote:
> > >
> > >
> > > On Thu, Dec 21, 2023 at 10:53 PM Zhaoyang Huang <huangzhaoyang@xxxxxxxxx> wrote:
> > > >
> > > > 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 Thanks for heads-up, I am out of date for readahead mechanism.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.
> > >
> > > > I'm not sure what you are asking me to look at -- is the following
> > > > trying to illustrate something related to my question above?
> > >
> > > sorry for my fault on table generation, resend it, I am trying to present how RFC performs in a page's stat transfer
> > >
> > > 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
> >
> > > I don't think this is the case -- with this RFC, *readahead* folios,
> > > which are added into pagecache as INA/UNR, become PG_referenced upon
> > > the initial fault (first access), i.e., INA/REF. The first scan will
> > > actually activate them, i.e., they become ACT/UNR, because they have
> > > both PG_referenced and the A-bit.
> > No,Sorry for the confusion. This RFC actually aims at minor fault of
> > the faulted pages(with one pte setup). In terms of the readahead
> > pages, can we solve it by add one criteria as bellow, which unifies
> > all kinds of mapped pages in RFC.

Again this is still wrong -- how do you know the other process mapping
this folio isn't also streaming the file?

It'd be best to take a step back and think through my original
question: what prevents a specific *access pattern* from triggering
minor faults? The simple answer is that you can't.

> > @@ -3273,6 +3273,12 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> >         */
> >        folio = filemap_get_folio(mapping, index);
> >        if (likely(!IS_ERR(folio))) {
> > +               /*
> > +                * try to promote inactive folio here when it is accessed
> > +                * as minor fault
> > +                */
> > +               if(folio_mapcount(folio))
> > +                       folio_mark_accessed(folio);
> >                /*
> >                 * We found the page, so try async readahead before waiting for
> >                 * the lock.
> >
> Please find bellow for the stat machine table of updated RFC, where RFC behaves same or enhances the scan efficiency by promoting the page in shrink_active_list.
>
> (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
> RA                              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
> RA                              INA/UNR                     INA/REF                       NA                                 ACT/REF
>
> (3)
>                                   1st access        1st scan(shink_folio_list)       2nd access      2nd scan(shrink_active_list)     3rd scan(shrink_folio_list)
> mainline                     INA/UNR                    INA/REF                           INA/REF                NA                                     ACT/REF
> RFC                           INA/UNR                    INA/REF                           ACT/REF                ACT/REF                           NA
> (VM_EXEC)
> RFC                           INA/UNR                    INA/REF                           ACT/REF                INA/REF                            DROP
> (non VM_EXEC)
> RA                              INA/UNR                    INA/REF                           INA/REF                NA                                     ACT/REF
>
> (4)
>                                   1st access                   2nd access                   3rd access          1st scan(shrink_active_list)   2nd scan(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)
> RA                              INA/UNR                       INA/REF                        ACT/REF                   ACT/REF                       NA
> > >
> > > So it doesn't behave the same way the mainline does for the first case
> > > you listed. (I didn't look at the rest of the cases.)