linux-kernel - Re: [PATCH v2 0/3] skip redundant TLB sync IPIs

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Message-ID: <f81b98e5-87c0-4c21-9a75-ad5f9b6af6aa@intel.com>
Date: Tue, 30 Dec 2025 20:26:28 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: will@...nel.org, aneesh.kumar@...nel.org, npiggin@...il.com,
 peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, arnd@...db.de,
 david@...nel.org, lorenzo.stoakes@...cle.com, ziy@...dia.com,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org,
 ioworker0@...il.com, shy828301@...il.com, riel@...riel.com,
 jannh@...gle.com, linux-arch@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] skip redundant TLB sync IPIs

On 12/29/25 06:52, Lance Yang wrote:
...
> This series introduces a way for architectures to indicate their TLB flush
> already provides full synchronization, allowing the redundant IPI to be
> skipped. For now, the optimization is implemented for x86 first and applied
> to all page table operations that free or unshare tables.

I really don't like all the complexity here. Even on x86, there are
three or more ways of deriving this. Having the pv_ops check the value
of another pv op is also a bit unsettling.

That said, complexity can be worth it with sufficient demonstrated
gains. But:

> When unsharing hugetlb PMD page tables or collapsing pages in khugepaged,
> we send two IPIs: one for TLB invalidation, and another to synchronize
> with concurrent GUP-fast walkers.

Those aren't exactly hot paths. khugepaged is fundamentally rate
limited. I don't think unsharing hugetlb PMD page tables just is all
that common either.

What kind of end user benefit is there to justify the complexity?