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

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Message-ID: <CAGWkznGPA872eY9VrAEh4wbBRhxtHpUFibxhB6rFaEip899FSw@mail.gmail.com>
Date: Fri, 22 Dec 2023 16:41:20 +0800
From: Zhaoyang Huang <huangzhaoyang@...il.com>
To: Yu Zhao <yuzhao@...gle.com>
Cc: 黄朝阳 (Zhaoyang Huang) <zhaoyang.huang@...soc.com>, 
	Matthew Wilcox <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	康纪滨 (Steve Kang) <Steve.Kang@...soc.com>
Subject: Re: reply: [RFC PATCH 1/1] mm: mark folio accessed in minor fault

On Fri, Dec 22, 2023 at 2:45 PM Yu Zhao <yuzhao@...gle.com> wrote:
>
> On Thu, Dec 21, 2023 at 11:29 PM 黄朝阳 (Zhaoyang Huang)
> <zhaoyang.huang@...soc.com> wrote:
> >
> >
> > On Thu, Dec 21, 2023 at 10:53 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
> > >
> > > On Thu, Dec 21, 2023 at 2:33 PM Yu Zhao <yuzhao@...gle.com> wrote:
> > > >
> > > > On Wed, Dec 20, 2023 at 11:28 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
> > > > >
> > > > > On Thu, Dec 21, 2023 at 12:53 PM Yu Zhao <yuzhao@...gle.com> wrote:
> > > > > >
> > > > > > On Wed, Dec 20, 2023 at 9:09 PM Matthew Wilcox <willy@...radead.org> 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@...radead.org> wrote:
> > > > > > > > >
> > > > > > > > > On Wed, Dec 20, 2023 at 06:29:48PM +0800, zhaoyang.huang wrote:
> > > > > > > > > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > > > > > > >
> > > > > > > > > > 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.

@@ -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.

>
> 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.)