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From: Mohamed Alzayat <alzayat@mpi-sws.org>
To: linux-mm@kvack.org
Subject: Potential bug in soft-dirty bits (with test case)
Date: Wed, 25 Nov 2020 15:15:17 +0100	[thread overview]
Message-ID: <CA+32v5zzFYJQ7eHfJP-2OHeR+6p5PZsX=RDJNU6vGF3hLO+j-g@mail.gmail.com> (raw)

Hi Everyone,

I have noticed a change in the synchrony of updating the soft-dirty
bits in recent kernel versions (5.6+). More precisely, up to kernel
v5.5, the soft-dirty bits as parsed from /proc/pid/pagemap accurately
capture the dirtied pages. Recently, I started testing on kernels v5.6
- v5.9, and I noticed that the soft-dirty bits are not immediately
updated.

I have prepared a short test that repeatedly causes at least one
memory page to be dirtied, then scans /proc/pid/pagemap counting the
soft-dirty bits. The test fails if this count is zero. In my
observation, this test fails once in every 10-20 trials. The test
defaults to 100 trials and can be found at
https://gitlab.mpi-sws.org/-/snippets/1696

Is this non-synchronous propagation of soft dirty bits intended? If
yes, is there a way to force the soft-dirty bits to be propagated to
the page map entries immediately, or is there an alternative interface
that has the synchronous behavior?

Thanks in advance,
Mohamed Alzayat


             reply	other threads:[~2020-11-25 14:15 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-25 14:15 Mohamed Alzayat [this message]
2020-11-27 16:40 ` Potential bug in soft-dirty bits (with test case) Vlastimil Babka
2020-11-30 10:37   ` Mohamed Alzayat
2020-11-30 11:51     ` Vlastimil Babka
2020-11-30 12:02       ` Peter Zijlstra
2020-11-30 12:50         ` Will Deacon
2021-04-20 18:32           ` Mohamed Alzayat

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