Question: 8324776: How to safely tell if MADV_POPULATE_WRITE is supported

Thu Feb 22 02:54:37 UTC 2024

>> Is this a downstream-only patch specific to the Oracle Linux kernel?
Yes, I think so.
At linux-uek repo [1], it says that the uek6/u2 branch is for kernel v5.4 on Oracle Linux 7 and 8. Looking into the header file, we could see the definitions of 22/23, the commit is [2]. At the same time, Linux mainline’s max mode number was 21 and did not have 22/23. I have an attached PDF describing the diffs with various kernels, mainline vs uek, see [3].

By the way, uek kernel v5.15 used numbers 24/25 [4] for these two customized modes which would generate another conflict with mainline since 5.18-rc1 and 6.1 where the same numbers got used. I would suggest that uek kernel should put customized modes into a separate range, e.g., 100+, to avoid conflict.

[1] https://github.com/oracle/linux-uek/
[2] https://github.com/oracle/linux-uek/commit/a91ae4fa327d8957e2f806420b8b835269b85bd4
mm: introduce MADV_DOEXEC
madvise MADV_DOEXEC preserves a memory range across exec.  Initially
only supported for non-executable, non-stack, anonymous memory.
MADV_DOEXEC is single-use and after exec madvise must done again to
preserve memory for a subsequent exec.
MADV_DONTEXEC reverts the effect of a previous MADV_DOEXEC call and
undoes the preservation of the range.

#define MADV_DOEXEC                 22                           /* do inherit across exec */
#define MADV_DONTEXEC          23                           /* don't inherit across exec */

[3] https://bugs.openjdk.org/secure/attachment/108337/madvise_return_values.pdf, at https://bugs.openjdk.org/browse/JDK-8324776
[4] https://github.com/oracle/linux-uek/commit/4693c5d9d799eb4803c5afc781cc60e2b645e398

Regards, Patrick

From: Thomas Stüfe <thomas.stuefe at gmail.com>
Sent: Thursday, February 22, 2024 0:56
To: Patrick Zhang OS <patrick at os.amperecomputing.com>
Cc: hotspot-dev developers <hotspot-dev at openjdk.java.net>; hotspot-gc-dev at openjdk.java.net
Subject: Re: Question: 8324776: How to safely tell if MADV_POPULATE_WRITE is supported

Hi,

I am trying to understand this better.

MADV_POPULATE_READ and MADV_POPULATE_WRITE were added to mainline with

```
 commit 4ca9b3859dac14bbef0c27d00667bb5b10917adb
 Author: David Hildenbrand <david at redhat.com<mailto:david at redhat.com>>
 Date:   Wed Jun 30 18:52:28 2021 -0700
     mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables
```

and they used 22/23 as numerical identifier:

```
+#define MADV_POPULATE_READ     22      /* populate (prefault) page tables readable */
+#define MADV_POPULATE_WRITE    23      /* populate (prefault) page tables writable */
```

But I don't see a patch in mainline for MADV_DOEXEC. I do see a very lengthy RFR thread that seemed to end in patent disputes (?):
https://lore.kernel.org/all/1595869887-23307-1-git-send-email-anthony.yznaga@oracle.com/T/#u

Is this a downstream-only patch specific to the Oracle Linux kernel?

Cheers, Thomas

On Wed, Feb 21, 2024 at 4:25 PM Patrick Zhang OS <patrick at os.amperecomputing.com<mailto:patrick at os.amperecomputing.com>> wrote:
Hi,

I have a question regarding the regression reported by [1] which was caused by the commit [2].

The root cause can be briefly described as: MADV_POPULATE_WRITE has been supported since Linux 5.14. It is suggested to call madvise(0, 0, advice) to tell if this madvise mode is valid, which “will return zero iff advice is supported by the kernel and can be relied on to probe for support” [3]. However Oracle UEKR6 5.4.17 has a duplicate definition of the number 23: MADV_DONTEXEC [4]. As a result, the test passes on Linux 5.4 mainline, while fails on UEK 5.4.17, because madvise(0, 0, 23) gives different return values. See details at [5].

It is a dilemma that how to fix so inside JVM. Thanks for any advice.

[1] https://bugs.openjdk.org/browse/JDK-8324776 runtime/os/TestTransparentHugePageUsage.java fails with The usage of THP is not enough
[2] https://bugs.openjdk.org/browse/JDK-8315923 pretouch_memory by atomic-add-0 fragments huge pages unexpectedly
[3] https://www.man7.org/linux/man-pages/man2/madvise.2.html
[4] https://yum.oracle.com/repo/OracleLinux/OL8/developer/UEKR6/aarch64/getPackageSource/kernel-uek-5.4.17-2136.327.2.el8uek.src.rpm, linux-5.4.17/include/uapi/asm-generic/mman-common.h#L75
[5] https://bugs.openjdk.org/browse/JDK-8324776?focusedId=14651275&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14651275.

Regards
Patrick
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