linux-kernel - Re: mm/DAMON: Profiling enhancements for DAMON

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Message-ID: <CAOUHufZW8ZbNDnu3Cfc61oJ4FkXK+AbT90XpS5Ei++1_5mcFFQ@mail.gmail.com>
Date: Fri, 15 Dec 2023 01:33:18 -0700
From: Yu Zhao <yuzhao@...gle.com>
To: Aravinda Prasad <aravinda.prasad@...el.com>
Cc: damon@...ts.linux.dev, linux-mm@...ck.org, sj@...nel.org, 
	linux-kernel@...r.kernel.org, s2322819@...ac.uk, sandeep4.kumar@...el.com, 
	ying.huang@...el.com, dave.hansen@...el.com, dan.j.williams@...el.com, 
	sreenivas.subramoney@...el.com, antti.kervinen@...el.com, 
	alexander.kanevskiy@...el.com, Alan Nair <alan.nair@...el.com>, 
	Juergen Gross <jgross@...e.com>, Ryan Roberts <ryan.roberts@....com>
Subject: Re: mm/DAMON: Profiling enhancements for DAMON

On Fri, Dec 15, 2023 at 12:42 AM Aravinda Prasad
<aravinda.prasad@...el.com> wrote:
...

> This patch proposes profiling different levels of the application’s
> page table tree to detect whether a region is accessed or not. This
> patch is based on the observation that, when the accessed bit for a
> page is set, the accessed bits at the higher levels of the page table
> tree (PMD/PUD/PGD) corresponding to the path of the page table walk
> are also set. Hence, it is efficient to  check the accessed bits at
> the higher levels of the page table tree to detect whether a region
> is accessed or not.

This patch can crash on Xen. See commit 4aaf269c768d("mm: introduce
arch_has_hw_nonleaf_pmd_young()")

MGLRU already does this in the correct way. See mm/vmscan.c.

This patch also can cause USER DATA CORRUPTION. See commit
c11d34fa139e ("mm/damon/ops-common: atomically test and clear young on
ptes and pmds").

The quality of your patch makes me very much doubt the quality of your
paper, especially your results on Google's kstaled and MGLRU in table
6.2.