linux-kernel - Re: [RFC PATCH v12 00/26] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

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Message-ID: <20250220114920.2383-1-hdanton@sina.com>
Date: Thu, 20 Feb 2025 19:49:19 +0800
From: Hillf Danton <hdanton@...a.com>
To: Byungchul Park <byungchul@...com>
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	kernel_team@...ynix.com
Subject: Re: [RFC PATCH v12 00/26] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

On Thu, 20 Feb 2025 20:09:35 +0900 Byungchul Park wrote:
> On Thu, Feb 20, 2025 at 06:32:22PM +0800, Hillf Danton wrote:
> > On Thu, 20 Feb 2025 14:20:01 +0900 Byungchul Park <byungchul@...com>
> > > To check luf's stability, I ran a heavy LLM inference workload consuming
> > > 210GiB over 7 days on a machine with 140GiB memory, and decided it's
> > > stable enough.
> > > 
> > > I'm posting the latest version so that anyone can try luf mechanism if
> > > wanted by any chance.  However, I tagged RFC again because there are
> > > still issues that should be resolved to merge to mainline:
> > > 
> > >    1. Even though system wide total cpu time for TLB shootdown is
> > >       reduced over 95%, page allocation paths should take additional cpu
> > >       time shifted from page reclaim to perform TLB shootdown.
> > > 
> > >    2. We need luf debug feature to detect when luf goes wrong by any
> > >       chance.  I implemented just a draft version that checks the sanity
> > >       on mkwrite(), kmap(), and so on.  I need to gather better ideas
> > >       to improve the debug feature.
> > > 
> > > ---
> > > 
> > > Hi everyone,
> > > 
> > > While I'm working with a tiered memory system e.g. CXL memory, I have
> > > been facing migration overhead esp. tlb shootdown on promotion or
> > > demotion between different tiers.  Yeah..  most tlb shootdowns on
> > > migration through hinting fault can be avoided thanks to Huang Ying's
> > > work, commit 4d4b6d66db ("mm,unmap: avoid flushing tlb in batch if PTE
> > > is inaccessible").
> > > 
> > > However, it's only for migration through hinting fault.  I thought it'd
> > > be much better if we have a general mechanism to reduce all the tlb
> > > numbers that we can apply to any unmap code, that we normally believe
> > > tlb flush should be followed.
> > > 
> > > I'm suggesting a new mechanism, LUF(Lazy Unmap Flush), that defers tlb
> > > flush until folios that have been unmapped and freed, eventually get
> > > allocated again.  It's safe for folios that had been mapped read-only
> > > and were unmapped, as long as the contents of the folios don't change
> > > while staying in pcp or buddy so we can still read the data through the
> > > stale tlb entries.
> > >
> > Given pcp or buddy, you are opening window for use after free which makes
> > no sense in 99% cases.
> 
> Just in case that I don't understand what you meant and for better
> understanding, can you provide a simple and problematic example from
> the u-a-f?
> 
Tell us if it is illegal to commit rape without pregnancy in your home town?

PS defering flushing tlb [1,2] is no go.

Subject: Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs
[1] https://lore.kernel.org/lkml/20250127155146.GB25757@willie-the-truck/
[2] https://lore.kernel.org/lkml/xhsmhwmdwihte.mognet@vschneid-thinkpadt14sgen2i.remote.csb/