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Date: Thu, 21 Dec 2023 23:14:28 -0700 From: Yu Zhao <yuzhao@...gle.com> To: Zhaoyang Huang <huangzhaoyang@...il.com> Cc: Matthew Wilcox <willy@...radead.org>, "zhaoyang.huang" <zhaoyang.huang@...soc.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, steve.kang@...soc.com Subject: Re: [RFC PATCH 1/1] mm: mark folio accessed in minor fault 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 Thanks for heads-up, I am out of date for readahead mechanism.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? > 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.
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