Re: libblkid: Idea to force given cached entry to be invalidated?

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

-------- Original Message --------
Subject: Re: libblkid: Idea to force given cached entry to be invalidated?
From: Karel Zak <kzak@redhat.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Date: 2014=E5=B9=B404=E6=9C=8815=E6=97=A5 19:21
> On Tue, Apr 15, 2014 at 10:15:01AM +0800, Qu Wenruo wrote:
>> When using the blkid_get_cache(), things will be cached to reduce the pr=
obe
>> frequency.
> The question is if you really want to use the cache at all :-)
>
> It's pretty common that we have udevd that maintain all necessary
> information about devices, including information from libblkid. The
> udev db is the primary source for many system actions and tools.
>
> All you need is to use libudev udev_enumerate API to scan for all
> block devices and udev_device_get_property_value(ID_FS_TYPE). Maybe
> you can add a new method BTRFS_SCAN_UDEV and use it as default. It
> would be better to keep libblkid as a fallback solution only. (For
> very unusual situations where there is no udev or so.)
Thanks for the suggestion to use libudev.
It sounds solid and I'll try to use it instead.
>
>> But filesystem which supports device management like btrfs can remove de=
vice
>> and if fast enough, libblkid will not invalidate the old cache and cause=
 to get
>> the wrong filesystem type.
> Well, since commit 6c2f2b9d (year 2010, v2.18), the libblkid uses
> microsecond resolution to check for device modifications.
>
> So the problem is not that btrfs is fast enough ;-)
According to your explaination, it seems that if there is any=20
modification to the block device, libblkid will rescan it and not
to use the old cache, do I understand it right?
>
>> The bug is describe in the following btrfs patch:
>> https://patchwork.kernel.org/patch/3960191/
> The patch does not make sense at all, blkid without a cache scans
> /proc/partitions -- so it's almost the same like BTRFS_SCAN_PROC
> btrfs_scan_block_devices().
>
>
>> So is there any idea about forcing given cached entry invalidated?
> It seems you're trying workaround the *btrfs problem*. The command
>
>    btrfs device delete
>
> does not open and modify the device! It uses btrfs ioctl and kernel
> does the changes to the device. The problem is that kernel does not
> update atime and mtime of the device. So the changes are completely
> invisible for libblkid (or another mtime based solutions). See
> my example below.
Thanks for pointing out the problem a lot !

>
> IMHO it's strange manner to modify files in kernel, but no update
> mtime.
>
> Fortunately kernel at least sends event to udev.
>
> Maybe possible workaround is to add utime() call to "btrfs device
> delete". Something like:
This sounds good, much better than my poor patch.
I will also try to fix it in kernel codes if possible.
>
> diff --git a/cmds-device.c b/cmds-device.c
> index 309b49e..5c438bf 100644
> --- a/cmds-device.c
> +++ b/cmds-device.c
> @@ -20,6 +20,8 @@
>   #include <unistd.h>
>   #include <fcntl.h>
>   #include <sys/ioctl.h>
> +#include <sys/types.h>
> +#include <utime.h>
>   #include <errno.h>
>   #include <sys/stat.h>
>   #include <getopt.h>
> @@ -179,7 +181,8 @@ static int cmd_rm_dev(int argc, char **argv)
>   				"ERROR: error removing the device '%s' - %s\n",
>   				argv[i], strerror(e));
>   			ret++;
> -		}
> +		} else
> +			utime(argv[i], NULL);
>   	}
>  =20
>   	close_file_or_dir(fdmnt, dirstream);
>
>
> BTW, it would be nice to have --verbose option for btrfs-scan, now it
> seems like black box.
I'd like to add it, but as you know, 'btrfs dev scan' just calls a btrfs=20
ioctl, so even '--verbose' is added,
nothing useful will be printed out. :(

>
>      Karel
>
>
> See mtime:
>
>
> # stat /dev/sdb2
>    File: =E2=80=98/dev/sdb2=E2=80=99
>    Size: 0               Blocks: 0          IO Block: 4096   block specia=
l file
> Device: 5h/5d   Inode: 392701      Links: 1     Device type: 8,12
> Access: (0660/brw-rw----)  Uid: (    0/    root)   Gid: (    6/    disk)
> Access: 2014-04-15 11:58:58.338758155 +0200
> Modify: 2014-04-15 11:58:58.338758155 +0200
> Change: 2014-04-15 11:58:58.338758155 +0200
>   Birth: -
>
> # btrfs device add /dev/sdb2 /mnt/test
> SMALL VOLUME: forcing mixed metadata/data groups
>
> # stat /dev/sdb2
>    File: =E2=80=98/dev/sdb2=E2=80=99
>    Size: 0               Blocks: 0          IO Block: 4096   block specia=
l file
> Device: 5h/5d   Inode: 392701      Links: 1     Device type: 8,12
> Access: (0660/brw-rw----)  Uid: (    0/    root)   Gid: (    6/    disk)
> Access: 2014-04-15 11:59:34.930143021 +0200
> Modify: 2014-04-15 11:59:34.930143021 +0200
> Change: 2014-04-15 11:59:34.930143021 +0200
>                              ^^^^^^^^
>
> # btrfs device delete /dev/sdb2 /mnt/test
>
> # stat /dev/sdb2
>    File: =E2=80=98/dev/sdb2=E2=80=99
>    Size: 0               Blocks: 0          IO Block: 4096   block specia=
l file
> Device: 5h/5d   Inode: 392701      Links: 1     Device type: 8,12
> Access: (0660/brw-rw----)  Uid: (    0/    root)   Gid: (    6/    disk)
> Access: 2014-04-15 11:59:34.930143021 +0200
> Modify: 2014-04-15 11:59:34.930143021 +0200
> Change: 2014-04-15 11:59:34.930143021 +0200
>                              ^^^^^^^^^
>
>
Great thanks for all your advise! It helps a lot!!

Thanks,
Qu