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

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Date: Mon, 27 May 2024 13:19:46 +0900
From: Byungchul Park <byungchul@...com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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 v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers
 over 90%

On Mon, May 27, 2024 at 12:46:14PM +0900, Byungchul Park wrote:
> On Sun, May 26, 2024 at 07:43:10PM -0700, Dave Hansen wrote:
> > On 5/26/24 18:57, Byungchul Park wrote:
> > ...
> > > Plus, I will add another give-up at code changing the permission of vma
> > > to writable.
> > 
> > I suspect you have a much more general problem on your hands. Just
> > tweaking the VFS or mmap() code likely isn't going to cut it.

What a stupid idiot I am.

I already discuss the exact cases with Nadav Amit at the very beginning
around v1.  I didn't remember it when I was answering to you.

mmap() or changing the permission by user already performs TLB flush
needed within that code, which LUF never touch.

Worth noting currently LUF touchs only unmapping during migration or
reclaim.  Other updating mapping would perform TLB flush it needs, as is.
I guess updating page cache is also already perform TLB flush needed.
I need to check it.  Probably, it would already do.

	Byungchul

> LUF is interested in limited folios that are migratable or reclaimable
> in lru for now.  So, IMHO, fixing a few things is going to cut it.
> 
> > I guess we'll see what you come up with next, but this email was really
> > just the result of Vlastimil and I chatting on IRC for five minutes
> > about this set.
> > 
> > It has absolutely not been tested nor reviewed enough.  <fud>I hope the
> > performance gains stick around once more of the bugs are gone.</fud>
> 
> Sure. It should be.
> 
> 	Byungchul