linux-kernel - Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering tlb flush when folios get unmapped

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Date: Fri, 31 May 2024 14:46:23 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Byungchul Park <lkml.byungchul.park@...il.com>
Cc: Byungchul Park <byungchul@...com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, kernel_team@...ynix.com, akpm@...ux-foundation.org,
 ying.huang@...el.com, vernhao@...cent.com, mgorman@...hsingularity.net,
 hughd@...gle.com, willy@...radead.org, david@...hat.com,
 peterz@...radead.org, luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, dave.hansen@...ux.intel.com, rjgolo@...il.com
Subject: Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering
 tlb flush when folios get unmapped

On 5/31/24 11:04, Byungchul Park wrote:
...
> I don't believe you do not agree with the concept itself.  Thing is
> the current version is not good enough.  I will do my best by doing
> what I can do.

More performance is good.  I agree with that.

But it has to be weighed against the risk and the complexity.  The more
I look at this approach, the more I think this is not a good trade off.
There's a lot of risk and a lot of complexity and we haven't seen the
full complexity picture.  The gaps are being fixed by adding complexity
in new subsystems (the VFS in this case).

There are going to be winners and losers, and this version for example
makes file writes lose performance.

Just to be crystal clear: I disagree with the concept of leaving stale
TLB entries in place in an attempt to gain performance.