linux-kernel - Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

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Message-ID: <20240531020654.GA62639@system.software.com>
Date: Fri, 31 May 2024 11:06:54 +0900
From: Byungchul Park <byungchul@...com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: "Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel_team@...ynix.com,
	akpm@...ux-foundation.org, 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 v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers
 over 90%

On Thu, May 30, 2024 at 06:50:48AM -0700, Dave Hansen wrote:
> On 5/30/24 01:41, Byungchul Park wrote:
> > LUF should not optimize tlb flushes for mappings that users explicitly
> > change e.g. through mmap() and munmap().
> 
> We are thoroughly going around in circles at this point.
> 
> I'm not quite sure what to do.  Ying and I see a problem that we've
> tried to explain a couple of times.  We've tried to show the connection
> between a LUF-elided TLB flush and how that could affect a later
> munmap() or mmap(MAP_FIXED).
> 
> But these responses seem to keep going back to the fact that LUF doesn't

I just wanted to understand exactly what Ying meant.  My answer might be
done in a wrong way if I wrongly got him.

> directly affect munmap(), which is true, but quite irrelevant to the
> problem being described.
> 
> So we're at an impasse.
> 
> Byungchul, perhaps you should spin another series and maybe Ying and I

I don't think the current implementation is perfect.  I just wanted to
know what I'm missing now but.. yes.  It would be much better to
communicate with a real bug if existing.

I will respin the next version shortly.

	Byungchul

> have to write up a test case to show the bug that we see.  Or perhaps
> someone else can jump into the thread and bridge the communication gap.