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

Hillf Danton
2025-02-20T11:59:10Z

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@sk.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/