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Description of problem:
In RHEL8, numa_police_memory(3) required both read and write access to the pages it is to fault in. Prior to RHEL8 (include RHEL8-Beta), only read access was required.
The bug manifests itself as a SIGSEGV violation.
Version-Release number of selected component (if applicable):
How reproducible:
Every time.
Steps to Reproduce:
1.
2.
3.
Actual results:
Expected results:
Additional info:
Here is the function definition, extracted from -lnuma via gdb, on RHEL-7.5, which requires only read access, and RHEL-8, which requires both read and write access. Please go back to the 7.5 version.
7.5 version (-lnuma):
847 void numa_police_memory(void *mem, size_t size)
848 {
849 int pagesize = numa_pagesize_int();
850 unsigned long i;
851 for (i = 0; i < size; i += pagesize)
852 asm volatile("" :: "r" (((volatile unsigned char *)mem)[i]));
853 }
8.0 version (-lnuma):
58 void numa_police_memory(void *mem, size_t size)
859 {
860 int pagesize = numa_pagesize_int();
861 unsigned long i;
862 for (i = 0; i < size; i += pagesize)
863 ((volatile char*)mem)[i] = ((volatile char*)mem)[i];
864 }
I might add that the RHEL8 version is unsafe as it modifies the shared memory area which might interleave with other store operations, performed by other processes or threads, at the same time as numa_police_memory is executing.
This is introduced by:
commit aeff67cc502b192a267f53259a355d9d2696178e
Author: Krzysztof Kulakowski <krzysztof.kulakowski>
Date: Thu May 25 10:10:29 2017 -0400
Fix for numa_police_memory()
Implementation of this function was based on outdated assumption
that memory read causes page fault (and allocation of physical memory).
This assumption is not true for Linux since 2.6.32 (a13ea5b); Uninitialized
memory is backed by zero_page until time of first write.
This patch makes numa_policy_memory() works as expected, by rewriting
(instead of just reading) first byte of every page.
(In reply to Joe Korty from comment #1)
> I might add that the RHEL8 version is unsafe as it modifies the shared
> memory area which might interleave with other store operations, performed by
> other processes or threads, at the same time as numa_police_memory is
> executing.
Yes, this is looks really, really problematic. Is there a system call that could be used to get the desired effect in a safe manner?
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (numactl bug fix and enhancement update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2021:4406
Description of problem: In RHEL8, numa_police_memory(3) required both read and write access to the pages it is to fault in. Prior to RHEL8 (include RHEL8-Beta), only read access was required. The bug manifests itself as a SIGSEGV violation. Version-Release number of selected component (if applicable): How reproducible: Every time. Steps to Reproduce: 1. 2. 3. Actual results: Expected results: Additional info: Here is the function definition, extracted from -lnuma via gdb, on RHEL-7.5, which requires only read access, and RHEL-8, which requires both read and write access. Please go back to the 7.5 version. 7.5 version (-lnuma): 847 void numa_police_memory(void *mem, size_t size) 848 { 849 int pagesize = numa_pagesize_int(); 850 unsigned long i; 851 for (i = 0; i < size; i += pagesize) 852 asm volatile("" :: "r" (((volatile unsigned char *)mem)[i])); 853 } 8.0 version (-lnuma): 58 void numa_police_memory(void *mem, size_t size) 859 { 860 int pagesize = numa_pagesize_int(); 861 unsigned long i; 862 for (i = 0; i < size; i += pagesize) 863 ((volatile char*)mem)[i] = ((volatile char*)mem)[i]; 864 }