btrfs-progs: Force 'btrfs dev scan' not using old libblkid cache

Commit Message

Qu Wenruo April 10, 2014, 7:30 a.m. UTC

'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
cache to reduce the probe.

But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
'btrfs scan' will still use the uncorrect cache and scan on the deleted device.
0. /dev/sda[1-4] mounted on /mnt using single data/metadata
1. btrfs dev scan
2. btrfs dev del /dev/sda3 /mnt
3. btrfs dev scan

Since the cache made by step 1 is still validated, step 3 will use the
cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
But the superblock(at least the first one) is wiped and failed to scan,
a error message, which can be avoided and is unneeded, is output.

This patch will force scan_for_btrfs() not to use cache to avoid the
problem.

Reported-by: Tsutomu Itoh <[e-mail]>
Singed-off-by: Qu Wenruo <[e-mail]>
---
 utils.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Comments

Anand Jain April 10, 2014, 7:37 a.m. UTC | #1

On 04/10/2014 03:30 PM, Qu Wenruo wrote:
> 'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
> cache to reduce the probe.
>
> But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
> 'btrfs scan' will still use the uncorrect cache and scan on the deleted device.

  Great thanks for looking into this.

Anand


> 0. /dev/sda[1-4] mounted on /mnt using single data/metadata
> 1. btrfs dev scan
> 2. btrfs dev del /dev/sda3 /mnt
> 3. btrfs dev scan

> Since the cache made by step 1 is still validated, step 3 will use the
> cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
> But the superblock(at least the first one) is wiped and failed to scan,
> a error message, which can be avoided and is unneeded, is output.
>
> This patch will force scan_for_btrfs() not to use cache to avoid the
> problem.
>
> Reported-by: Tsutomu Itoh <[e-mail]>
> Singed-off-by: Qu Wenruo <[e-mail]>
> ---
>   utils.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/utils.c b/utils.c
> index 0bfb9d9..1cd1d28 100644
> --- a/utils.c
> +++ b/utils.c
> @@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
>   	blkid_cache cache = NULL;
>   	char path[PATH_MAX];
>
> -	if (blkid_get_cache(&cache, 0) < 0) {
> +	/* No to use libblkid cache to avoid old data */
> +	if (blkid_get_cache(&cache, "/dev/null") < 0) {
>   		printf("ERROR: lblkid cache get failed\n");
>   		return 1;
>   	}
>
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David Sterba April 14, 2014, 4:29 p.m. UTC | #2

On Thu, Apr 10, 2014 at 03:30:21PM +0800, Qu Wenruo wrote:
> 'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
> cache to reduce the probe.
> 
> But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
> 'btrfs scan' will still use the uncorrect cache and scan on the deleted device.
> 0. /dev/sda[1-4] mounted on /mnt using single data/metadata
> 1. btrfs dev scan
> 2. btrfs dev del /dev/sda3 /mnt
> 3. btrfs dev scan
> 
> Since the cache made by step 1 is still validated, step 3 will use the
> cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
> But the superblock(at least the first one) is wiped and failed to scan,
> a error message, which can be avoided and is unneeded, is output.
> 
> This patch will force scan_for_btrfs() not to use cache to avoid the
> problem.

> --- a/utils.c
> +++ b/utils.c
> @@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
>  	blkid_cache cache = NULL;
>  	char path[PATH_MAX];
>  
> -	if (blkid_get_cache(&cache, 0) < 0) {
> +	/* No to use libblkid cache to avoid old data */
> +	if (blkid_get_cache(&cache, "/dev/null") < 0) {

This effectively avoid the blkid cache for all devices but the point of
blkid was to use it so repeated probes are not done.

I think it's not right to skip the whole cache because one entry may be
stale, more that we know which one and when.

We should rather explicitly invalidate the removed device after delete,
I don't what's the right way to do that. Maybe blkid_do_probe() or
blkid_gc_cache()
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Qu Wenruo April 15, 2014, 1:17 a.m. UTC | #3

-------- Original Message --------
Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old 
libblkid cache
From: David Sterba <[e-mail]>
To: Qu Wenruo <[e-mail]>
Date: 2014?04?15? 00:29
> On Thu, Apr 10, 2014 at 03:30:21PM +0800, Qu Wenruo wrote:
>> 'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
>> cache to reduce the probe.
>>
>> But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
>> 'btrfs scan' will still use the uncorrect cache and scan on the deleted device.
>> 0. /dev/sda[1-4] mounted on /mnt using single data/metadata
>> 1. btrfs dev scan
>> 2. btrfs dev del /dev/sda3 /mnt
>> 3. btrfs dev scan
>>
>> Since the cache made by step 1 is still validated, step 3 will use the
>> cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
>> But the superblock(at least the first one) is wiped and failed to scan,
>> a error message, which can be avoided and is unneeded, is output.
>>
>> This patch will force scan_for_btrfs() not to use cache to avoid the
>> problem.
>> --- a/utils.c
>> +++ b/utils.c
>> @@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
>>   	blkid_cache cache = NULL;
>>   	char path[PATH_MAX];
>>   
>> -	if (blkid_get_cache(&cache, 0) < 0) {
>> +	/* No to use libblkid cache to avoid old data */
>> +	if (blkid_get_cache(&cache, "/dev/null") < 0) {
> This effectively avoid the blkid cache for all devices but the point of
> blkid was to use it so repeated probes are not done.
>
> I think it's not right to skip the whole cache because one entry may be
> stale, more that we know which one and when.
I totally agree with that.
>
> We should rather explicitly invalidate the removed device after delete,
> I don't what's the right way to do that. Maybe blkid_do_probe() or
> blkid_gc_cache()
But the problem is, libblkid does not provide such API to invalidate one 
entry of cache.
blkid_gc_cache() will only remove non-exsist device cache, and 
blkid_do_probe() seems not using/update the cache,
so I used "/dev/null" as cache to avoid the cache completly.

I'll try to dig the libblkid codes deeper to find a method to invalidate 
an entry.

Thanks,
Qu.
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Qu Wenruo April 16, 2014, 1:24 a.m. UTC | #4

-------- Original Message --------
Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old 
libblkid cache
From: David Sterba <[e-mail]>
To: Qu Wenruo <[e-mail]>
Date: 2014?04?15? 00:29
> On Thu, Apr 10, 2014 at 03:30:21PM +0800, Qu Wenruo wrote:
>> 'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
>> cache to reduce the probe.
>>
>> But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
>> 'btrfs scan' will still use the uncorrect cache and scan on the deleted device.
>> 0. /dev/sda[1-4] mounted on /mnt using single data/metadata
>> 1. btrfs dev scan
>> 2. btrfs dev del /dev/sda3 /mnt
>> 3. btrfs dev scan
>>
>> Since the cache made by step 1 is still validated, step 3 will use the
>> cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
>> But the superblock(at least the first one) is wiped and failed to scan,
>> a error message, which can be avoided and is unneeded, is output.
>>
>> This patch will force scan_for_btrfs() not to use cache to avoid the
>> problem.
>> --- a/utils.c
>> +++ b/utils.c
>> @@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
>>   	blkid_cache cache = NULL;
>>   	char path[PATH_MAX];
>>   
>> -	if (blkid_get_cache(&cache, 0) < 0) {
>> +	/* No to use libblkid cache to avoid old data */
>> +	if (blkid_get_cache(&cache, "/dev/null") < 0) {
> This effectively avoid the blkid cache for all devices but the point of
> blkid was to use it so repeated probes are not done.
>
> I think it's not right to skip the whole cache because one entry may be
> stale, more that we know which one and when.
>
> We should rather explicitly invalidate the removed device after delete,
> I don't what's the right way to do that. Maybe blkid_do_probe() or
> blkid_gc_cache()
Please ignore this patch, since after consulting with libblkid mail 
list, the real problem is that btrfs ioctl for remove device
does not update the mtime/ctime for the device.

Libblkid will only use cache when the device's mtime/ctime stay 
unchanged, if mtime/ctime changed libblkid will do the lowprobe.

I'll try to fix it in kernel space.

Thanks,
Qu

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Qu Wenruo April 16, 2014, 2:09 a.m. UTC | #5

-------- Original Message --------
Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old 
libblkid cache
From: Qu Wenruo <[e-mail]>
To: [e-mail], [e-mail]
Date: 2014?04?16? 09:24
>
> -------- Original Message --------
> Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old 
> libblkid cache
> From: David Sterba <[e-mail]>
> To: Qu Wenruo <[e-mail]>
> Date: 2014?04?15? 00:29
>> On Thu, Apr 10, 2014 at 03:30:21PM +0800, Qu Wenruo wrote:
>>> 'btrfs scan' uses libblkid to scan devices by default, and libblkid 
>>> uses
>>> cache to reduce the probe.
>>>
>>> But if operations below is done in less than 2 
>>> seconds(BLKID_PROBE_MIN),
>>> 'btrfs scan' will still use the uncorrect cache and scan on the 
>>> deleted device.
>>> 0. /dev/sda[1-4] mounted on /mnt using single data/metadata
>>> 1. btrfs dev scan
>>> 2. btrfs dev del /dev/sda3 /mnt
>>> 3. btrfs dev scan
>>>
>>> Since the cache made by step 1 is still validated, step 3 will use the
>>> cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
>>> But the superblock(at least the first one) is wiped and failed to scan,
>>> a error message, which can be avoided and is unneeded, is output.
>>>
>>> This patch will force scan_for_btrfs() not to use cache to avoid the
>>> problem.
>>> --- a/utils.c
>>> +++ b/utils.c
>>> @@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
>>>       blkid_cache cache = NULL;
>>>       char path[PATH_MAX];
>>>   -    if (blkid_get_cache(&cache, 0) < 0) {
>>> +    /* No to use libblkid cache to avoid old data */
>>> +    if (blkid_get_cache(&cache, "/dev/null") < 0) {
>> This effectively avoid the blkid cache for all devices but the point of
>> blkid was to use it so repeated probes are not done.
>>
>> I think it's not right to skip the whole cache because one entry may be
>> stale, more that we know which one and when.
>>
>> We should rather explicitly invalidate the removed device after delete,
>> I don't what's the right way to do that. Maybe blkid_do_probe() or
>> blkid_gc_cache()
> Please ignore this patch, since after consulting with libblkid mail 
> list, the real problem is that btrfs ioctl for remove device
> does not update the mtime/ctime for the device.
I'm sorry that kernel updates the mtime/ctime, but it seems there is 
some small latency, which caused libblkd fail to detect the
modification and use old cache.

Will go on investigation.
Thanks,
Qu
>
> Libblkid will only use cache when the device's mtime/ctime stay 
> unchanged, if mtime/ctime changed libblkid will do the lowprobe.
>
> I'll try to fix it in kernel space.
>
> Thanks,
> Qu
>
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Patch

diff --git a/utils.c b/utils.c
index 0bfb9d9..1cd1d28 100644
--- a/utils.c
+++ b/utils.c
@@ -2057,7 +2057,8 @@  int btrfs_scan_lblkid(int update_kernel)
 	blkid_cache cache = NULL;
 	char path[PATH_MAX];
 
-	if (blkid_get_cache(&cache, 0) < 0) {
+	/* No to use libblkid cache to avoid old data */
+	if (blkid_get_cache(&cache, "/dev/null") < 0) {
 		printf("ERROR: lblkid cache get failed\n");
 		return 1;
 	}