Revision e2093926a098a8ccf0f1d10f6df8dad452cb28d3 authored by Ross Zwisler on 02 June 2017, 21:46:37 UTC, committed by Linus Torvalds on 02 June 2017, 22:07:37 UTC
We currently have two related PMD vs PTE races in the DAX code. These can both be easily triggered by having two threads reading and writing simultaneously to the same private mapping, with the key being that private mapping reads can be handled with PMDs but private mapping writes are always handled with PTEs so that we can COW. Here is the first race: CPU 0 CPU 1 (private mapping write) __handle_mm_fault() create_huge_pmd() - FALLBACK handle_pte_fault() passes check for pmd_devmap() (private mapping read) __handle_mm_fault() create_huge_pmd() dax_iomap_pmd_fault() inserts PMD dax_iomap_pte_fault() does a PTE fault, but we already have a DAX PMD installed in our page tables at this spot. Here's the second race: CPU 0 CPU 1 (private mapping read) __handle_mm_fault() passes check for pmd_none() create_huge_pmd() dax_iomap_pmd_fault() inserts PMD (private mapping write) __handle_mm_fault() create_huge_pmd() - FALLBACK (private mapping read) __handle_mm_fault() passes check for pmd_none() create_huge_pmd() handle_pte_fault() dax_iomap_pte_fault() inserts PTE dax_iomap_pmd_fault() inserts PMD, but we already have a PTE at this spot. The core of the issue is that while there is isolation between faults to the same range in the DAX fault handlers via our DAX entry locking, there is no isolation between faults in the code in mm/memory.c. This means for instance that this code in __handle_mm_fault() can run: if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) { ret = create_huge_pmd(&vmf); But by the time we actually get to run the fault handler called by create_huge_pmd(), the PMD is no longer pmd_none() because a racing PTE fault has installed a normal PMD here as a parent. This is the cause of the 2nd race. The first race is similar - there is the following check in handle_pte_fault(): } else { /* See comment in pte_alloc_one_map() */ if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd)) return 0; So if a pmd_devmap() PMD (a DAX PMD) has been installed at vmf->pmd, we will bail and retry the fault. This is correct, but there is nothing preventing the PMD from being installed after this check but before we actually get to the DAX PTE fault handlers. In my testing these races result in the following types of errors: BUG: Bad rss-counter state mm:ffff8800a817d280 idx:1 val:1 BUG: non-zero nr_ptes on freeing mm: 15 Fix this issue by having the DAX fault handlers verify that it is safe to continue their fault after they have taken an entry lock to block other racing faults. [ross.zwisler@linux.intel.com: improve fix for colliding PMD & PTE entries] Link: http://lkml.kernel.org/r/20170526195932.32178-1-ross.zwisler@linux.intel.com Link: http://lkml.kernel.org/r/20170522215749.23516-2-ross.zwisler@linux.intel.com Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reported-by: Pawel Lebioda <pawel.lebioda@intel.com> Reviewed-by: Jan Kara <jack@suse.cz> Cc: "Darrick J. Wong" <darrick.wong@oracle.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Christoph Hellwig <hch@lst.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Matthew Wilcox <mawilcox@microsoft.com> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com> Cc: Pawel Lebioda <pawel.lebioda@intel.com> Cc: Dave Jiang <dave.jiang@intel.com> Cc: Xiong Zhou <xzhou@redhat.com> Cc: Eryu Guan <eguan@redhat.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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export_report.pl
#!/usr/bin/perl -w
#
# (C) Copyright IBM Corporation 2006.
# Released under GPL v2.
# Author : Ram Pai (linuxram@us.ibm.com)
#
# Usage: export_report.pl -k Module.symvers [-o report_file ] -f *.mod.c
#
use Getopt::Std;
use strict;
sub numerically {
my $no1 = (split /\s+/, $a)[1];
my $no2 = (split /\s+/, $b)[1];
return $no1 <=> $no2;
}
sub alphabetically {
my ($module1, $value1) = @{$a};
my ($module2, $value2) = @{$b};
return $value1 <=> $value2 || $module2 cmp $module1;
}
sub print_depends_on {
my ($href) = @_;
print "\n";
for my $mod (sort keys %$href) {
my $list = $href->{$mod};
print "\t$mod:\n";
foreach my $sym (sort numerically @{$list}) {
my ($symbol, $no) = split /\s+/, $sym;
printf("\t\t%-25s\n", $symbol);
}
print "\n";
}
print "\n";
print "~"x80 , "\n";
}
sub usage {
print "Usage: @_ -h -k Module.symvers [ -o outputfile ] \n",
"\t-f: treat all the non-option argument as .mod.c files. ",
"Recommend using this as the last option\n",
"\t-h: print detailed help\n",
"\t-k: the path to Module.symvers file. By default uses ",
"the file from the current directory\n",
"\t-o outputfile: output the report to outputfile\n";
exit 0;
}
sub collectcfiles {
my @file;
while (<.tmp_versions/*.mod>) {
open my $fh, '<', $_ or die "cannot open $_: $!\n";
push (@file,
grep s/\.ko/.mod.c/, # change the suffix
grep m/.+\.ko/, # find the .ko path
<$fh>); # lines in opened file
}
chomp @file;
return @file;
}
my (%SYMBOL, %MODULE, %opt, @allcfiles);
if (not getopts('hk:o:f',\%opt) or defined $opt{'h'}) {
usage($0);
}
if (defined $opt{'f'}) {
@allcfiles = @ARGV;
} else {
@allcfiles = collectcfiles();
}
if (not defined $opt{'k'}) {
$opt{'k'} = "Module.symvers";
}
open (my $module_symvers, '<', $opt{'k'})
or die "Sorry, cannot open $opt{'k'}: $!\n";
if (defined $opt{'o'}) {
open (my $out, '>', $opt{'o'})
or die "Sorry, cannot open $opt{'o'} $!\n";
select $out;
}
#
# collect all the symbols and their attributes from the
# Module.symvers file
#
while ( <$module_symvers> ) {
chomp;
my (undef, $symbol, $module, $gpl) = split;
$SYMBOL { $symbol } = [ $module , "0" , $symbol, $gpl];
}
close($module_symvers);
#
# collect the usage count of each symbol.
#
my $modversion_warnings = 0;
foreach my $thismod (@allcfiles) {
my $module;
unless (open ($module, '<', $thismod)) {
warn "Sorry, cannot open $thismod: $!\n";
next;
}
my $state=0;
while ( <$module> ) {
chomp;
if ($state == 0) {
$state = 1 if ($_ =~ /static const struct modversion_info/);
next;
}
if ($state == 1) {
$state = 2 if ($_ =~ /__attribute__\(\(section\("__versions"\)\)\)/);
next;
}
if ($state == 2) {
if ( $_ !~ /0x[0-9a-f]+,/ ) {
next;
}
my $sym = (split /([,"])/,)[4];
my ($module, $value, $symbol, $gpl) = @{$SYMBOL{$sym}};
$SYMBOL{ $sym } = [ $module, $value+1, $symbol, $gpl];
push(@{$MODULE{$thismod}} , $sym);
}
}
if ($state != 2) {
warn "WARNING:$thismod is not built with CONFIG_MODVERSIONS enabled\n";
$modversion_warnings++;
}
close($module);
}
print "\tThis file reports the exported symbols usage patterns by in-tree\n",
"\t\t\t\tmodules\n";
printf("%s\n\n\n","x"x80);
printf("\t\t\t\tINDEX\n\n\n");
printf("SECTION 1: Usage counts of all exported symbols\n");
printf("SECTION 2: List of modules and the exported symbols they use\n");
printf("%s\n\n\n","x"x80);
printf("SECTION 1:\tThe exported symbols and their usage count\n\n");
printf("%-25s\t%-25s\t%-5s\t%-25s\n", "Symbol", "Module", "Usage count",
"export type");
#
# print the list of unused exported symbols
#
foreach my $list (sort alphabetically values(%SYMBOL)) {
my ($module, $value, $symbol, $gpl) = @{$list};
printf("%-25s\t%-25s\t%-10s\t", $symbol, $module, $value);
if (defined $gpl) {
printf("%-25s\n",$gpl);
} else {
printf("\n");
}
}
printf("%s\n\n\n","x"x80);
printf("SECTION 2:\n\tThis section reports export-symbol-usage of in-kernel
modules. Each module lists the modules, and the symbols from that module that
it uses. Each listed symbol reports the number of modules using it\n");
print "\nNOTE: Got $modversion_warnings CONFIG_MODVERSIONS warnings\n\n"
if $modversion_warnings;
print "~"x80 , "\n";
for my $thismod (sort keys %MODULE) {
my $list = $MODULE{$thismod};
my %depends;
$thismod =~ s/\.mod\.c/.ko/;
print "\t\t\t$thismod\n";
foreach my $symbol (@{$list}) {
my ($module, $value, undef, $gpl) = @{$SYMBOL{$symbol}};
push (@{$depends{"$module"}}, "$symbol $value");
}
print_depends_on(\%depends);
}
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