The block numbers reported on dmesg when vm.block_dump=1 are sector numbers - 512 bytes per block - but the block numbers that the debugfs command expects for its icheck command are usually 4096 bytes per block for ext4, so all that is required is a division by 8.
While block size is usually 4096 bytes, you have to be sure, so get the block size like this:
$ sudo blockdev --getbsz /dev/xvda3
4096
While this value is usually 4096, it can be different in certain cases like eg. mkfs.ext4 with -b can create an ext4 filesystem with a different block size; vfat has 512 byte block size.
To get the divider for the division, from the block size, all in one go (in bash):
$ echo $(( `sudo blockdev --getbsz /dev/xvda3` / 512 ))
8
Let's use this line as input:
[ 3140.736022] sleep(13522): READ block 5379184 on xvda3 (24 sectors)
Get the block number for icheck by dividing the above by 8:
$ bc -l
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
5379184/8
672398.00000000000000000000
I used bc -l to make sure that I didn't typo the number(if it end with .00000000000000000000 I most likely didn't)
Get the path name by using debugfs:
$ sudo debugfs
debugfs 1.44.2 (14-May-2018)
debugfs: open /dev/xvda3
debugfs: icheck 672398
Block Inode number
672398 134571
debugfs: ncheck 134571
Inode Pathname
134571 /usr/bin/sleep
debugfs: close
debugfs: quit
I am unsure if the block numbers reported in dmesg can ever be multiples of values other than 512 bytes per sector. For example, are they still 512 bytes per block if the underlying disk has 4096 bytes per sector physically? If I were to guess, I'd say it's safe to assume they're always 512 bytes per block.
Here's the script version of the above:
#!/bin/bash
#./showblock rev.02 rewritten for question/answer from: https://stackoverflow.com/q/52058914/10239615
#----
bytes_per_sector=512 #assumed that dmesg block numbers are 512 bytes each (ie. 512 bytes per sector; aka block size is 512)!
#----
#use `sudo` only when not already root
if test "`id -u`" != "0"; then
sudo='sudo'
else
sudo=''
fi
if ! test "$#" -ge "2"; then
echo "Usage: $0 <device> <dmesgblocknumber> [dmesgblocknumber ...]"
echo "Examples:"
echo "$0 /dev/xvda3 5379184"
echo "$0 /dev/xvda3 5379184 5129952 7420192"
exit 1
fi
within_exit() {
echo -e "\nSkipped current instruction within on_exit()"
}
on_exit() {
#trap - EXIT SIGINT SIGQUIT SIGHUP #will exit by skipping the rest of all instrunction from on_exit() eg. if C-c
trap within_exit EXIT SIGINT SIGQUIT SIGHUP #skip only current instruction from on_exit() eg. when C-c is pressed
#echo "first sleep"
#sleep 10
#echo "second sleep"
#sleep 10
if test "${#remaining_args[@]}" -gt 0; then
echo -n "WARNING: There are '${#remaining_args[@]}' remaining args not processed, they are: " >&2
for i in `seq 0 1 "$(( "${#remaining_args[@]}" - 1 ))"`; do #seq is part of coreutils package
echo -n "'${remaining_args[${i}]}' " >&2
done
echo >&2
fi
}
trap on_exit EXIT SIGINT SIGQUIT SIGHUP
dev="$1"
shift 1
if test -z "$dev" -o ! -b "$dev"; then
echo "Bad device name or not a device: '$dev'" >&2
exit 1
fi
blocksize="`$sudo blockdev --getbsz "$dev"`"
if test "${blocksize:-0}" -le "0"; then #handles empty arg too
echo "Failed getting block size for '$dev', got '$blocksize'" >&2
exit 1
fi
#TODO: check and fail if not a multiplier
divider="$(( $blocksize / $bytes_per_sector ))"
if ! test "${divider:-0}" -gt "0"; then
echo "Failed computing divider from: '$blocksize' / '$bytes_per_sector', got '$divider'" >&2
exit 1
fi
# for each passed-in dmesg block number do
while test "$#" -gt "0"; do
dmesgblock="$1"
shift
remaining_args=("$@") #for on_exit() above
echo '--------'
echo "Passed-in dmesg block($bytes_per_sector bytes per block) number: '$dmesgblock'"
#have to handle the case when $dmesgblock is empty and when it's negative eg. "-1" so using a default value(of 0) when unset in the below 'if' block will help not break the 'test' expecting an integer while also allowing negative numbers ("0$dmesgblock" would otherwise yield "0-1" a non-integer):
if test "${dmesgblock:-0}" -le "0"; then
echo "Bad passed-in dmesg block number: '$dmesgblock'" >&2
exit 1
fi
#TODO: check and fail(or warn? nah, it should be fail!) if not a multiplier (eg. use modullo? is it "%" ?)
block=$(( $dmesgblock / 8 ))
if ! test "${block:--1}" -ge "0"; then
echo "Badly computed device block number: '$block'" >&2
exit 1
fi
echo "Actual block number(of $blocksize bytes per block): $block"
inode="$(echo "open ${dev}"$'\n'"icheck ${block}"$'\n'"close" | $sudo debugfs -f - 2>/dev/null | tail -n2|head -1|cut -f2 -d$'\t')"
if test "<block not found>" == "$inode"; then
echo "No inode was found for the provided dmesg block number '$dmesgblock' which mapped to dev block number '$block'" >&2
exit 1
else
#assuming number TODO: check for this!
echo "Found inode: $inode"
fpath="$(echo "open ${dev}"$'\n'"ncheck ${inode}"$'\n'"close" | $sudo debugfs -f - 2>/dev/null | tail -n2|head -1|cut -f2- -d$'\t')"
#fpath always begins with '/' right?
if test "$fpath" != "Pathname"; then
echo "Found path : $fpath"
else
echo "not found"
fi
fi
done
Sample usage:
[user@dev01-w-s-f-fdr28 ~]$ ./showblock /dev/xvda3 5379184 5129952 "" -1 "" 1200 ; echo "exit code: $?"
--------
Passed-in dmesg block(512 bytes per block) number: '5379184'
Actual block number(of 4096 bytes per block): 672398
Found inode: 134571
Found path : /usr/bin/sleep
--------
Passed-in dmesg block(512 bytes per block) number: '5129952'
Actual block number(of 4096 bytes per block): 641244
Found inode: 141497
Found path : /usr/lib64/libIlmImf-2_2.so.22.0.0
--------
Passed-in dmesg block(512 bytes per block) number: ''
Bad passed-in dmesg block number: ''
WARNING: There are '3' remaining args not processed, they are: '-1' '' '1200'
exit code: 1
[user@dev01-w-s-f-fdr28 ~]$ ./showblock /dev/xvda3 5379184 5129952 -1 "" "" 1200 ; echo "exit code: $?"
--------
Passed-in dmesg block(512 bytes per block) number: '5379184'
Actual block number(of 4096 bytes per block): 672398
Found inode: 134571
Found path : /usr/bin/sleep
--------
Passed-in dmesg block(512 bytes per block) number: '5129952'
Actual block number(of 4096 bytes per block): 641244
Found inode: 141497
Found path : /usr/lib64/libIlmImf-2_2.so.22.0.0
--------
Passed-in dmesg block(512 bytes per block) number: '-1'
Bad passed-in dmesg block number: '-1'
WARNING: There are '3' remaining args not processed, they are: '' '' '1200'
exit code: 1
[user@dev01-w-s-f-fdr28 ~]$ ./showblock /dev/xvda3 5379184 5129952 0 ; echo "exit code: $?"
--------
Passed-in dmesg block(512 bytes per block) number: '5379184'
Actual block number(of 4096 bytes per block): 672398
Found inode: 134571
Found path : /usr/bin/sleep
--------
Passed-in dmesg block(512 bytes per block) number: '5129952'
Actual block number(of 4096 bytes per block): 641244
Found inode: 141497
Found path : /usr/lib64/libIlmImf-2_2.so.22.0.0
--------
Passed-in dmesg block(512 bytes per block) number: '0'
Bad passed-in dmesg block number: '0'
exit code: 1
[user@dev01-w-s-f-fdr28 ~]$ ./showblock /dev/xvda3 5379184 5129952 3 ; echo "exit code: $?"
--------
Passed-in dmesg block(512 bytes per block) number: '5379184'
Actual block number(of 4096 bytes per block): 672398
Found inode: 134571
Found path : /usr/bin/sleep
--------
Passed-in dmesg block(512 bytes per block) number: '5129952'
Actual block number(of 4096 bytes per block): 641244
Found inode: 141497
Found path : /usr/lib64/libIlmImf-2_2.so.22.0.0
--------
Passed-in dmesg block(512 bytes per block) number: '3'
Actual block number(of 4096 bytes per block): 0
No inode was found for the provided dmesg block number '3' which mapped to dev block number '0'
exit code: 1
[user@dev01-w-s-f-fdr28 ~]$ ./showblock /dev/xvda3 5379184 5129952 ; echo "exit code: $?"
--------
Passed-in dmesg block(512 bytes per block) number: '5379184'
Actual block number(of 4096 bytes per block): 672398
Found inode: 134571
Found path : /usr/bin/sleep
--------
Passed-in dmesg block(512 bytes per block) number: '5129952'
Actual block number(of 4096 bytes per block): 641244
Found inode: 141497
Found path : /usr/lib64/libIlmImf-2_2.so.22.0.0
exit code: 0
Here's a script to process an entire dmesg log:
#!/bin/bash
#./showallblocks rev.01 rewritten for question/answer from: https://stackoverflow.com/q/52058914/10239615
if test "`id -u`" != "0"; then
sudo='sudo'
else
sudo=''
fi
dmesglog="$1"
if test -z "$dmesglog"; then
echo "Usage: '$0' <dmesglogfile>"
echo "Examples:"
echo "sudo dmesg > dmesg1.log && '$0' dmesg1.log"
echo "'$0' <(sudo dmesg)"
#Note: '$0' used for the case when $0 has spaces or other things in its path names, and user wants to copy paste, for whatever reason, the output of the above into the command line.
exit 1
fi
#(optional) Stop logging if already in progress:
$sudo sysctl -w vm.block_dump=0
#Using the answer from here(thanks to glenn jackman): https://unix.stackexchange.com/a/467377/306023
#grep --color=never -E -- 'READ block [0-9]+ on xvda3' "$dmesglog" |
#cat "$dmesglog" |
$sudo perl -pe '
if (! /READ block [0-9]+ on [A-Za-z0-9]+ .*$/) {
s{.*}{}s
}
s{(READ block) (\d+) (on) ([A-Za-z0-9]+) ([^\$]*)\n$}
{join " ",$1, $2, $3, $4, $5, qx(./showblock "/dev/$4" "$2" | grep -F -- "Found path :" | cut -f4- -d" ")}es
' -- "$dmesglog"
#Note: the output of qx(...) above is purposely allowed to have trailing newline! (I did wonder if purposely is correct here or it should be purposefully, https://www.merriam-webster.com/words-at-play/purposely-purposefully-usage )
#To find out what "}es"(above) is, see perlre modifiers: https://perldoc.perl.org/perlre.html#Modifiers
#FIXME: noobish try to exclude lines not matching the lines that need to be replaced, from output! used 'if' above
Sample output:
[user@dev01-w-s-f-fdr28 ~]$ ./showallblocks sample.dmesg.log
vm.block_dump = 0
[ 6031.953619] sysctl(20774): READ block 5285528 on xvda3 (32 sectors) /usr/lib64/libgpg-error.so.0.24.2
[ 6031.954317] sysctl(20774): READ block 5285768 on xvda3 (8 sectors) /usr/lib64/libgpg-error.so.0.24.2
[ 6031.954598] sysctl(20774): READ block 5285648 on xvda3 (120 sectors) /usr/lib64/libgpg-error.so.0.24.2
[ 6031.954617] sysctl(20774): READ block 5285776 on xvda3 (24 sectors) /usr/lib64/libgpg-error.so.0.24.2
[ 6031.955482] sysctl(20774): READ block 5285560 on xvda3 (88 sectors) /usr/lib64/libgpg-error.so.0.24.2
[ 6031.955699] sysctl(20774): READ block 4473568 on xvda3 (8 sectors) /usr/lib64/libuuid.so.1.3.0
[ 6031.955730] sysctl(20774): READ block 4473584 on xvda3 (16 sectors) /usr/lib64/libuuid.so.1.3.0
[ 6031.955787] sysctl(20774): READ block 4749496 on xvda3 (224 sectors) /usr/lib64/libblkid.so.1.1.0
[ 6030.638492] bash(20757): READ block 4543096 on xvda3 (32 sectors) /usr/bin/gawk
[ 6030.639133] awk(20757): READ block 4544280 on xvda3 (176 sectors) /usr/bin/gawk