[PATCH 1/5] btrfs: nodatasum drop compress

[PATCH 1/5] btrfs: nodatasum drop compress

[Satoru Takeuchi]

From: Naohiro Aota <naota@elisp.net>

We can enable both compress option and nodatasum option at the same time
with the following command.

===
$ sudo mount -o remount,compress,nodatasum /
$ mount | grep btrfs
/dev/vda2 on / type btrfs (rw,relatime,seclabel,nodatasum,compress=zlib,space_cache)
===

This behavior collides with Documentation/filesystems/btrfs.txt:

===
...
  If compression is enabled, nodatacow and nodatasum are disabled.
...
===

This patch drops the compress flags upon nodatasum flag.

Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
---
 fs/btrfs/super.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c4124de..d1c5b6d 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -440,8 +440,15 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 			 */
 			break;
 		case Opt_nodatasum:
-			btrfs_set_and_info(root, NODATASUM,
-					   "setting nodatasum");
+			if (!btrfs_test_opt(root, NODATASUM)) {
+				if (btrfs_test_opt(root, COMPRESS))
+					btrfs_info(root->fs_info, "setting nodatasum, compression disabled");
+				else
+					btrfs_info(root->fs_info, "setting nodatasum");
+			}
+			btrfs_clear_opt(info->mount_opt, COMPRESS);
+			btrfs_clear_opt(info->mount_opt, FORCE_COMPRESS);
+			btrfs_set_opt(info->mount_opt, NODATASUM);
 			break;
 		case Opt_datasum:
 			if (btrfs_test_opt(root, NODATASUM)) {
-- 1.8.3.1 

[PATCH 2/5] btrfs: correct a message on setting nodatacow

[Satoru Takeuchi]

From: Naohiro Aota <naota@elisp.net>

If we set nodatacow mount option after compress-force option,
we don't get compression disabling message.

===
$ sudo mount -o remount,compress-force,nodatacow /; dmesg|tail -n 3
[ 3845.719047] BTRFS info (device vda2): force zlib compression
[ 3845.719052] BTRFS info (device vda2): setting nodatacow
[ 3845.719055] BTRFS info (device vda2): disk space caching is enabled
===

Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
---
 fs/btrfs/super.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index d1c5b6d..d131098 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -462,8 +462,7 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 			break;
 		case Opt_nodatacow:
 			if (!btrfs_test_opt(root, NODATACOW)) {
-				if (!btrfs_test_opt(root, COMPRESS) ||
-				    !btrfs_test_opt(root, FORCE_COMPRESS)) {
+				if (btrfs_test_opt(root, COMPRESS)) {
 					btrfs_info(root->fs_info,
 						   "setting nodatacow, compression disabled");
 				} else {
-- 1.8.3.1 

[PATCH 3/5] btrfs: notice nodatasum is also enabled with nodatacow

[Satoru Takeuchi]

From: Naohiro Aota <naota@elisp.net>

Documentation/filesystems/btrfs.txt states the following:

===
...
  Nodatacow implies nodatasum, and disables all compression.
...
===

However the current message only mentions nodatacow. It's better to
also mention nodatasum.

Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
---
 fs/btrfs/super.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index d131098..7ad4293 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -464,9 +464,9 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 			if (!btrfs_test_opt(root, NODATACOW)) {
 				if (btrfs_test_opt(root, COMPRESS)) {
 					btrfs_info(root->fs_info,
-						   "setting nodatacow, compression disabled");
+						   "setting nodatacow and nodatasum, compression disabled");
 				} else {
-					btrfs_info(root->fs_info, "setting nodatacow");
+					btrfs_info(root->fs_info, "setting nodatacow and nodatasum");
 				}
 			}
 			btrfs_clear_opt(info->mount_opt, COMPRESS);
-- 1.8.3.1 

[PATCH 4/5] btrfs: suppress a verbose message about enabling space cache on remounting

[Satoru Takeuchi]

From: Naohiro Aota <naota@elisp.net>

Remount operation always tells you it enabled space cache.
I consider it's a bit verbose and is better not to show
this message on remounting.

Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
---
 fs/btrfs/super.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 7ad4293..03131c5 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -404,7 +404,8 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 
 	cache_gen = btrfs_super_cache_generation(root->fs_info->super_copy);
 	if (cache_gen)
-		btrfs_set_opt(info->mount_opt, SPACE_CACHE);
+		btrfs_set_and_info(root, SPACE_CACHE,
+				   "disk space caching is enabled");
 
 	if (!options)
 		goto out;
@@ -758,8 +759,6 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 		}
 	}
 out:
-	if (!ret && btrfs_test_opt(root, SPACE_CACHE))
-		btrfs_info(root->fs_info, "disk space caching is enabled");
 	kfree(orig);
 	return ret;
 }
-- 1.8.3.1 

[PATCH 5/5] btrfs: rework compression related options processing

[Satoru Takeuchi]

From: Naohiro Aota <naota@elisp.net>

There are mutual exclusive mount options corresponding to
compression options. If we set these options at once, we get
improper messages and states, and a verbose message.

 * Improper states
  - We can't disable "compress forcing" with compress={zlib,lzo}
    mount option.

    ====
    # mount -o remount,compress-force=zlib,compress=lzo
    [58852.210209] BTRFS info (device vda2): force zlib compression
    (We should see "use lzo compression" here)
    # mount | grep btrfs
    /dev/vda2 on / type btrfs (rw,relatime,seclabel,compress-force=lzo,space_cache)
    (We should also change "compress-force=lzo" to "compress=lzo")
    ====

 * Improper messages
  - We don't see compression enabled message when we enables it
    after disabling it.

    ====
    # mount -o remount,compress=no,compress=zlib; dmesg -c
    [ 4077.130625] BTRFS info (device vda2): btrfs: use no compression
    (We should see "use zlib compression" here)
    ====

  - We don't see any message when we change compression type.

    ====
    # mount -o remount,compress=zlib,compress=lzo
    (Since "BTRFS info" is at the beginning of this message, "btrfs: " is verbose
     and we don't need it.)
    ====

  - We don't see datacow and/or datasum enabling message when we
    enable compression.

    ====
    # mount -o remount,nodatacow,nodatasum,compress
    [58913.704536] BTRFS info (device vda2): setting nodatacow
    (We should see "use zlib compression, setting datacow and datasum" here)
    ====

    ====
    # mount -o remount,nodatasum,compress
    [58950.529392] BTRFS info (device vda2): setting nodatasum
    (We should see "use zlib compression, setting datasum" here)
    ====

 * A verbose message
  - We see verbose "btrfs: " prefix on disabling compression.

    ====
    # mount -o remount,compress=zlib,compress=no
    [58790.727788] BTRFS info (device vda2): btrfs: use no compression
    ("btrfs: " is verbose here. We don't need it, since we already
    state this is btrfs with "BTRFS info")
    ====

This commit solves all these problems.

I separated the code in the following two parts:

  a) Parse the options.
  b) Set and clear flags of mount_opt and show proper messages.

I've confirmed that this patch doesn't cause any regression
by running the following script, which tests all combinations
of corresponding mount options (compress, compress-force,
datacow, and datasum).

=====================
#!/bin/bash

BTRFS_ROOT=/

states=(
  compress=zlib,datacow,datasum
  compress=lzo,datacow,datasum
  compress-force=zlib,datacow,datasum
  compress-force=lzo,datacow,datasum
  compress=no,nodatacow,nodatasum
  compress=no,datacow,nodatasum
  compress=no,datacow,datasum
)

options=(
  compress
  compress=zlib
  compress=lzo
  compress=no
  compress=badopt
  compress-force
  compress-force=zlib
  compress-force=lzo
  compress-force=no
  compress-force=badopt
  nodatacow
  datacow
  nodatasum
  datasum
)

for s in ${states[@]}; do
  for o in ${options[@]}; do
    # set initial state
    echo initial: mount -o remount,$s
    mount -o remount,$s ${BTRFS_ROOT}
    # mount with each option
    echo mount -o remount,$o
    dmesg -C
    mount -o remount,$o ${BTRFS_ROOT}
    echo dmesg:
    dmesg -t | perl -ne 'if(/^BTRFS info \(device .+?\): (.+)$/){print "\t$1\n"}'
    echo -n 'mount options: '
    grep -v rootfs /proc/mounts | \
      awk "{if(\$2==\"${BTRFS_ROOT}\"){print \$4}}" | \
      perl -ne '@a=split(/,/); for(@a){print "$_ " if(/^compress(-force)?=|nodata(cow|sum)$/)}'
    echo
    echo
  done
done
=====================

Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
---
 fs/btrfs/super.c | 57 +++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 03131c5..83433e15 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -401,6 +401,7 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 	char *compress_type;
 	bool compress_force = false;
 	bool compress = false;
+	unsigned long old_compress_type;
 
 	cache_gen = btrfs_super_cache_generation(root->fs_info->super_copy);
 	if (cache_gen)
@@ -486,40 +487,62 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 		case Opt_compress:
 		case Opt_compress_type:
 			compress = true;
+			old_compress_type = info->compress_type;
 			if (token == Opt_compress ||
 			    token == Opt_compress_force ||
 			    strcmp(args[0].from, "zlib") == 0) {
 				compress_type = "zlib";
 				info->compress_type = BTRFS_COMPRESS_ZLIB;
-				btrfs_set_opt(info->mount_opt, COMPRESS);
-				btrfs_clear_opt(info->mount_opt, NODATACOW);
-				btrfs_clear_opt(info->mount_opt, NODATASUM);
 			} else if (strcmp(args[0].from, "lzo") == 0) {
 				compress_type = "lzo";
 				info->compress_type = BTRFS_COMPRESS_LZO;
-				btrfs_set_opt(info->mount_opt, COMPRESS);
-				btrfs_clear_opt(info->mount_opt, NODATACOW);
-				btrfs_clear_opt(info->mount_opt, NODATASUM);
 				btrfs_set_fs_incompat(info, COMPRESS_LZO);
 			} else if (strncmp(args[0].from, "no", 2) == 0) {
 				compress_type = "no";
-				btrfs_clear_opt(info->mount_opt, COMPRESS);
-				btrfs_clear_opt(info->mount_opt, FORCE_COMPRESS);
-				compress_force = false;
+				compress = compress_force = false;
 			} else {
 				ret = -EINVAL;
 				goto out;
 			}
 
-			if (compress_force) {
-				btrfs_set_and_info(root, FORCE_COMPRESS,
-						   "force %s compression",
-						   compress_type);
-			} else if (compress) {
-				if (!btrfs_test_opt(root, COMPRESS))
+			if (compress_force || compress) {
+				const char *method = "", *setting = "";
+
+				if ((info->compress_type != old_compress_type) ||
+				    (compress_force &&
+				     !btrfs_test_opt(root, FORCE_COMPRESS)) ||
+				    (!compress_force &&
+				     btrfs_test_opt(root, FORCE_COMPRESS)) ||
+				    (compress && !btrfs_test_opt(root, COMPRESS))) {
+					method = compress_force?"force":"use";
+				}
+
+				if (btrfs_test_opt(root, NODATACOW))
+					setting = ", setting datacow and datasum";
+				else if (btrfs_test_opt(root, NODATASUM))
+					setting = ", setting datasum";
+
+				if (strcmp(method, "") != 0) {
 					btrfs_info(root->fs_info,
-						   "btrfs: use %s compression",
-						   compress_type);
+						   "%s %s compression%s",
+						   method, compress_type,
+						   setting);
+				}
+				if (compress_force) {
+					btrfs_set_opt(info->mount_opt,
+						      FORCE_COMPRESS);
+				} else {
+					btrfs_clear_opt(info->mount_opt,
+							FORCE_COMPRESS);
+				}
+				btrfs_set_opt(info->mount_opt, COMPRESS);
+				btrfs_clear_opt(info->mount_opt, NODATACOW);
+				btrfs_clear_opt(info->mount_opt, NODATASUM);
+			} else {
+				btrfs_clear_and_info(root, COMPRESS,
+						     "use no compression");
+				btrfs_clear_opt(info->mount_opt,
+						FORCE_COMPRESS);
 			}
 			break;
 		case Opt_ssd:
-- 1.8.3.1 

Re: [PATCH 2/5] btrfs: correct a message on setting nodatacow

[Qu Wenruo]

-------- Original Message --------
Subject: [PATCH 2/5] btrfs: correct a message on setting nodatacow
From: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
To: linux-btrfs@vger.kernel.org <linux-btrfs@vger.kernel.org>
Date: 2014年09月18日 16:28
> From: Naohiro Aota <naota@elisp.net>
>
> If we set nodatacow mount option after compress-force option,
> we don't get compression disabling message.
>
> ===
> $ sudo mount -o remount,compress-force,nodatacow /; dmesg|tail -n 3
> [ 3845.719047] BTRFS info (device vda2): force zlib compression
> [ 3845.719052] BTRFS info (device vda2): setting nodatacow
> [ 3845.719055] BTRFS info (device vda2): disk space caching is enabled
> ===
>
> Signed-off-by: Naohiro Aota <naota@elisp.net>
> Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
> ---
>   fs/btrfs/super.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index d1c5b6d..d131098 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -462,8 +462,7 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
>   			break;
>   		case Opt_nodatacow:
>   			if (!btrfs_test_opt(root, NODATACOW)) {
> -				if (!btrfs_test_opt(root, COMPRESS) ||
> -				    !btrfs_test_opt(root, FORCE_COMPRESS)) {
> +				if (btrfs_test_opt(root, COMPRESS)) {
>   					btrfs_info(root->fs_info,
>   						   "setting nodatacow, compression disabled");
>   				} else {
> -- 1.8.3.1
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
Although the patch makes the output ok, the core problem is missing 
conflict options check.

compress-force mount options implies datacow and datasum, but following 
nodatasum will disable datasum and compress, in fact they are 
conflicting mount option...

Even the current behavior(later mount option will override previous 
ones) provides great tolerance,
IMO there should better be some conflicting check for mount options.

For example, we first save all the mount options passed in into a 
temporary bitmaps to finds out the conflicting
and only when they contains no conflicts, set the mount options to fs_info.
(Maybe bitmap is not enough for this case, since we can't distinguish 
default value and value to be set?)

What do you think about this idea ?

Thanks,
Qu

Re: [PATCH 2/5] btrfs: correct a message on setting nodatacow

[Satoru Takeuchi]

Hi Qu,

Thank you for your comment.

(2014/09/19 11:03), Qu Wenruo wrote:
>
> -------- Original Message --------
> Subject: [PATCH 2/5] btrfs: correct a message on setting nodatacow
> From: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
> To: linux-btrfs@vger.kernel.org <linux-btrfs@vger.kernel.org>
> Date: 2014年09月18日 16:28
>> From: Naohiro Aota <naota@elisp.net>
>>
>> If we set nodatacow mount option after compress-force option,
>> we don't get compression disabling message.
>>
>> ===
>> $ sudo mount -o remount,compress-force,nodatacow /; dmesg|tail -n 3
>> [ 3845.719047] BTRFS info (device vda2): force zlib compression
>> [ 3845.719052] BTRFS info (device vda2): setting nodatacow
>> [ 3845.719055] BTRFS info (device vda2): disk space caching is enabled
>> ===
>>
>> Signed-off-by: Naohiro Aota <naota@elisp.net>
>> Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
>> ---
>>   fs/btrfs/super.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>> index d1c5b6d..d131098 100644
>> --- a/fs/btrfs/super.c
>> +++ b/fs/btrfs/super.c
>> @@ -462,8 +462,7 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
>>               break;
>>           case Opt_nodatacow:
>>               if (!btrfs_test_opt(root, NODATACOW)) {
>> -                if (!btrfs_test_opt(root, COMPRESS) ||
>> -                    !btrfs_test_opt(root, FORCE_COMPRESS)) {
>> +                if (btrfs_test_opt(root, COMPRESS)) {
>>                       btrfs_info(root->fs_info,
>>                              "setting nodatacow, compression disabled");
>>                   } else {
>> -- 1.8.3.1
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Although the patch makes the output ok, the core problem is missing conflict options check.
>
> compress-force mount options implies datacow and datasum, but following nodatasum will disable datasum and compress, in fact they are conflicting mount option...
>
> Even the current behavior(later mount option will override previous ones) provides great tolerance,
> IMO there should better be some conflicting check for mount options.
>
> For example, we first save all the mount options passed in into a temporary bitmaps to finds out the conflicting
> and only when they contains no conflicts, set the mount options to fs_info.
> (Maybe bitmap is not enough for this case, since we can't distinguish default value and value to be set?)
>
> What do you think about this idea ?

I'm against your idea for two reasons and it's better to
stay in current behavior though it's a bit complex.

First, the rule "last one wins" is not only a conventional rule,
but also is what mount(8) says.

https://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/sys-utils/mount.8#n253

======
The usual behavior is that the last option wins if there are conflicting
ones.
======

Second, if we change the behavior, we would break existing
systems. At worst case, users would fail to boot their system
after updating kernel, because of the failure of mounting
Btrfs at the init process.

Thanks,
Satoru

>
> Thanks,
> Qu
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH 2/5] btrfs: correct a message on setting nodatacow

[Qu Wenruo]

-------- Original Message --------
Subject: Re: [PATCH 2/5] btrfs: correct a message on setting nodatacow
From: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>, linux-btrfs@vger.kernel.org 
<linux-btrfs@vger.kernel.org>
Date: 2014年09月19日 14:45
> Hi Qu,
>
> Thank you for your comment.
>
> (2014/09/19 11:03), Qu Wenruo wrote:
>>
>> -------- Original Message --------
>> Subject: [PATCH 2/5] btrfs: correct a message on setting nodatacow
>> From: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
>> To: linux-btrfs@vger.kernel.org <linux-btrfs@vger.kernel.org>
>> Date: 2014年09月18日 16:28
>>> From: Naohiro Aota <naota@elisp.net>
>>>
>>> If we set nodatacow mount option after compress-force option,
>>> we don't get compression disabling message.
>>>
>>> ===
>>> $ sudo mount -o remount,compress-force,nodatacow /; dmesg|tail -n 3
>>> [ 3845.719047] BTRFS info (device vda2): force zlib compression
>>> [ 3845.719052] BTRFS info (device vda2): setting nodatacow
>>> [ 3845.719055] BTRFS info (device vda2): disk space caching is enabled
>>> ===
>>>
>>> Signed-off-by: Naohiro Aota <naota@elisp.net>
>>> Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
>>> ---
>>>   fs/btrfs/super.c | 3 +--
>>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
>>> index d1c5b6d..d131098 100644
>>> --- a/fs/btrfs/super.c
>>> +++ b/fs/btrfs/super.c
>>> @@ -462,8 +462,7 @@ int btrfs_parse_options(struct btrfs_root *root, 
>>> char *options)
>>>               break;
>>>           case Opt_nodatacow:
>>>               if (!btrfs_test_opt(root, NODATACOW)) {
>>> -                if (!btrfs_test_opt(root, COMPRESS) ||
>>> -                    !btrfs_test_opt(root, FORCE_COMPRESS)) {
>>> +                if (btrfs_test_opt(root, COMPRESS)) {
>>>                       btrfs_info(root->fs_info,
>>>                              "setting nodatacow, compression 
>>> disabled");
>>>                   } else {
>>> -- 1.8.3.1
>>>
>>> -- 
>>> To unsubscribe from this list: send the line "unsubscribe 
>>> linux-btrfs" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>> Although the patch makes the output ok, the core problem is missing 
>> conflict options check.
>>
>> compress-force mount options implies datacow and datasum, but 
>> following nodatasum will disable datasum and compress, in fact they 
>> are conflicting mount option...
>>
>> Even the current behavior(later mount option will override previous 
>> ones) provides great tolerance,
>> IMO there should better be some conflicting check for mount options.
>>
>> For example, we first save all the mount options passed in into a 
>> temporary bitmaps to finds out the conflicting
>> and only when they contains no conflicts, set the mount options to 
>> fs_info.
>> (Maybe bitmap is not enough for this case, since we can't distinguish 
>> default value and value to be set?)
>>
>> What do you think about this idea ?
>
> I'm against your idea for two reasons and it's better to
> stay in current behavior though it's a bit complex.
>
> First, the rule "last one wins" is not only a conventional rule,
> but also is what mount(8) says.
>
> https://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/sys-utils/mount.8#n253 
>
>
> ======
> The usual behavior is that the last option wins if there are conflicting
> ones.
> ======
>
> Second, if we change the behavior, we would break existing
> systems. At worst case, users would fail to boot their system
> after updating kernel, because of the failure of mounting
> Btrfs at the init process.
>
> Thanks,
> Satoru
>
It really makes sense.

So I'm OK to keep things and it's true that the conflict check is 
somewhat overkilled.

Thanks,
Qu
>>
>> Thanks,
>> Qu
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>