[PATCH 0/1] Reduce unnecessary opens of raid members

[PATCH 0/1] Reduce unnecessary opens of raid members

[Jes.Sorensen]

From: Jes Sorensen <Jes.Sorensen@redhat.com>

Hi,

I am posting this on Harald's request. This avoids unncessary double
opens of raid members and also has the side effect of not triggering
udev notify twice.

Jes


Harald Hoyer (1):
  prevent double open(O_RDWR) on raid creation

 Kill.c        | 28 +++++++++++++++++-----------
 Manage.c      |  2 +-
 mdadm.c       |  4 ++--
 mdadm.h       |  2 +-
 super-ddf.c   |  2 +-
 super-intel.c |  2 +-
 super0.c      |  2 +-
 super1.c      |  2 +-
 8 files changed, 25 insertions(+), 19 deletions(-)

-- 
1.8.1.4

[PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Jes.Sorensen]

From: Harald Hoyer <harald@redhat.com>

This does not trigger the udev inotify twice and saves a lot of blk I/O
for the raid members.

Also fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=947815

Signed-off-by: Harald Hoyer <harald@redhat.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
---
 Kill.c        | 28 +++++++++++++++++-----------
 Manage.c      |  2 +-
 mdadm.c       |  4 ++--
 mdadm.h       |  2 +-
 super-ddf.c   |  2 +-
 super-intel.c |  2 +-
 super0.c      |  2 +-
 super1.c      |  2 +-
 8 files changed, 25 insertions(+), 19 deletions(-)

diff --git a/Kill.c b/Kill.c
index f2fdb85..b5c93ae 100644
--- a/Kill.c
+++ b/Kill.c
@@ -29,7 +29,7 @@
 #include	"md_u.h"
 #include	"md_p.h"
 
-int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl)
+int Kill(char *dev, int oldfd, struct supertype *st, int force, int verbose, int noexcl)
 {
 	/*
 	 * Nothing fancy about Kill.  It just zeroes out a superblock
@@ -42,21 +42,26 @@ int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl)
 
 	int fd, rv = 0;
 
-	if (force)
-		noexcl = 1;
-	fd = open(dev, O_RDWR|(noexcl ? 0 : O_EXCL));
-	if (fd < 0) {
-		if (verbose >= 0)
-			pr_err("Couldn't open %s for write - not zeroing\n",
-				dev);
-		return 2;
+	if (oldfd != -1) {
+		fd = oldfd;
+	} else {
+		if (force)
+			noexcl = 1;
+		fd = open(dev, O_RDWR|(noexcl ? 0 : O_EXCL));
+		if (fd < 0) {
+			if (verbose >= 0)
+				pr_err("Couldn't open %s for write - not zeroing\n",
+					dev);
+			return 2;
+		}
 	}
 	if (st == NULL)
 		st = guess_super(fd);
 	if (st == NULL || st->ss->init_super == NULL) {
 		if (verbose >= 0)
 			pr_err("Unrecognised md component device - %s\n", dev);
-		close(fd);
+		if (oldfd == -1)
+			close(fd);
 		return 2;
 	}
 	st->ignore_hw_compat = 1;
@@ -76,7 +81,8 @@ int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl)
 			rv = 0;
 		}
 	}
-	close(fd);
+	if (oldfd == -1)
+		close(fd);
 	return rv;
 }
 
diff --git a/Manage.c b/Manage.c
index 6267c0c..509e921 100644
--- a/Manage.c
+++ b/Manage.c
@@ -785,7 +785,7 @@ int Manage_add(int fd, int tfd, struct mddev_dev *dv,
 			return -1;
 		}
 
-		Kill(dv->devname, NULL, 0, -1, 0);
+		Kill(dv->devname, -1, NULL, 0, -1, 0);
 		dfd = dev_open(dv->devname, O_RDWR | O_EXCL|O_DIRECT);
 		if (mdmon_running(tst->container_devnm))
 			tst->update_tail = &tst->updates;
diff --git a/mdadm.c b/mdadm.c
index 214afa3..d68ee96 100644
--- a/mdadm.c
+++ b/mdadm.c
@@ -1739,11 +1739,11 @@ static int misc_list(struct mddev_dev *devlist,
 			continue;
 		case KillOpt: /* Zero superblock */
 			if (ss)
-				rv |= Kill(dv->devname, ss, c->force, c->verbose,0);
+				rv |= Kill(dv->devname, -1, ss, c->force, c->verbose, 0);
 			else {
 				int v = c->verbose;
 				do {
-					rv |= Kill(dv->devname, NULL, c->force, v, 0);
+					rv |= Kill(dv->devname, -1, NULL, c->force, v, 0);
 					v = -1;
 				} while (rv == 0);
 				rv &= ~2;
diff --git a/mdadm.h b/mdadm.h
index c7b5183..e55dec5 100644
--- a/mdadm.h
+++ b/mdadm.h
@@ -1156,7 +1156,7 @@ extern int Monitor(struct mddev_dev *devlist,
 		   int dosyslog, char *pidfile, int increments,
 		   int share);
 
-extern int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl);
+extern int Kill(char *dev, int oldfd, struct supertype *st, int force, int verbose, int noexcl);
 extern int Kill_subarray(char *dev, char *subarray, int verbose);
 extern int Update_subarray(char *dev, char *subarray, char *update, struct mddev_ident *ident, int quiet);
 extern int Wait(char *dev);
diff --git a/super-ddf.c b/super-ddf.c
index c5f6f5c..a88699c 100644
--- a/super-ddf.c
+++ b/super-ddf.c
@@ -2597,7 +2597,7 @@ static int write_init_super_ddf(struct supertype *st)
 	} else {
 		struct dl *d;
 		for (d = ddf->dlist; d; d=d->next)
-			while (Kill(d->devname, NULL, 0, -1, 1) == 0);
+			while (Kill(d->devname, d->fd, NULL, 0, -1, 1) == 0);
 		return __write_init_super_ddf(st);
 	}
 }
diff --git a/super-intel.c b/super-intel.c
index 24016b7..743a7fc 100644
--- a/super-intel.c
+++ b/super-intel.c
@@ -5221,7 +5221,7 @@ static int write_init_super_imsm(struct supertype *st)
 	} else {
 		struct dl *d;
 		for (d = super->disks; d; d = d->next)
-			Kill(d->devname, NULL, 0, -1, 1);
+			Kill(d->devname, d->fd, NULL, 0, -1, 1);
 		return write_super_imsm(st, 1);
 	}
 }
diff --git a/super0.c b/super0.c
index 1f4dc75..57b549e 100644
--- a/super0.c
+++ b/super0.c
@@ -779,7 +779,7 @@ static int write_init_super0(struct supertype *st)
 			continue;
 		if (di->fd == -1)
 			continue;
-		while (Kill(di->devname, NULL, 0, -1, 1) == 0)
+		while (Kill(di->devname, di->fd, NULL, 0, -1, 1) == 0)
 			;
 
 		sb->disks[di->disk.number].state &= ~(1<<MD_DISK_FAULTY);
diff --git a/super1.c b/super1.c
index d0f1d5f..e3eeb80 100644
--- a/super1.c
+++ b/super1.c
@@ -1333,7 +1333,7 @@ static int write_init_super1(struct supertype *st)
 		if (di->fd < 0)
 			continue;
 
-		while (Kill(di->devname, NULL, 0, -1, 1) == 0)
+		while (Kill(di->devname, di->fd, NULL, 0, -1, 1) == 0)
 			;
 
 		sb->dev_number = __cpu_to_le32(di->disk.number);
-- 
1.8.1.4

Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[NeilBrown]

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On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:

> From: Harald Hoyer <harald@redhat.com>
> 
> This does not trigger the udev inotify twice and saves a lot of blk I/O
> for the raid members.
> 
> Also fixes:
> https://bugzilla.redhat.com/show_bug.cgi?id=947815
> 
> Signed-off-by: Harald Hoyer <harald@redhat.com>
> Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>

(Sorry for delays.  Thanks for reminders).

That patch seems to make sense, but the description above is awfully thin.

Why is double-open a problem exactly?  What does it make udev do?  And how
does that related to ID_FS_TYPE being wrong as mentioned in the bugzilla
entry.

NeilBrown


> ---
>  Kill.c        | 28 +++++++++++++++++-----------
>  Manage.c      |  2 +-
>  mdadm.c       |  4 ++--
>  mdadm.h       |  2 +-
>  super-ddf.c   |  2 +-
>  super-intel.c |  2 +-
>  super0.c      |  2 +-
>  super1.c      |  2 +-
>  8 files changed, 25 insertions(+), 19 deletions(-)
> 
> diff --git a/Kill.c b/Kill.c
> index f2fdb85..b5c93ae 100644
> --- a/Kill.c
> +++ b/Kill.c
> @@ -29,7 +29,7 @@
>  #include	"md_u.h"
>  #include	"md_p.h"
>  
> -int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl)
> +int Kill(char *dev, int oldfd, struct supertype *st, int force, int verbose, int noexcl)
>  {
>  	/*
>  	 * Nothing fancy about Kill.  It just zeroes out a superblock
> @@ -42,21 +42,26 @@ int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl)
>  
>  	int fd, rv = 0;
>  
> -	if (force)
> -		noexcl = 1;
> -	fd = open(dev, O_RDWR|(noexcl ? 0 : O_EXCL));
> -	if (fd < 0) {
> -		if (verbose >= 0)
> -			pr_err("Couldn't open %s for write - not zeroing\n",
> -				dev);
> -		return 2;
> +	if (oldfd != -1) {
> +		fd = oldfd;
> +	} else {
> +		if (force)
> +			noexcl = 1;
> +		fd = open(dev, O_RDWR|(noexcl ? 0 : O_EXCL));
> +		if (fd < 0) {
> +			if (verbose >= 0)
> +				pr_err("Couldn't open %s for write - not zeroing\n",
> +					dev);
> +			return 2;
> +		}
>  	}
>  	if (st == NULL)
>  		st = guess_super(fd);
>  	if (st == NULL || st->ss->init_super == NULL) {
>  		if (verbose >= 0)
>  			pr_err("Unrecognised md component device - %s\n", dev);
> -		close(fd);
> +		if (oldfd == -1)
> +			close(fd);
>  		return 2;
>  	}
>  	st->ignore_hw_compat = 1;
> @@ -76,7 +81,8 @@ int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl)
>  			rv = 0;
>  		}
>  	}
> -	close(fd);
> +	if (oldfd == -1)
> +		close(fd);
>  	return rv;
>  }
>  
> diff --git a/Manage.c b/Manage.c
> index 6267c0c..509e921 100644
> --- a/Manage.c
> +++ b/Manage.c
> @@ -785,7 +785,7 @@ int Manage_add(int fd, int tfd, struct mddev_dev *dv,
>  			return -1;
>  		}
>  
> -		Kill(dv->devname, NULL, 0, -1, 0);
> +		Kill(dv->devname, -1, NULL, 0, -1, 0);
>  		dfd = dev_open(dv->devname, O_RDWR | O_EXCL|O_DIRECT);
>  		if (mdmon_running(tst->container_devnm))
>  			tst->update_tail = &tst->updates;
> diff --git a/mdadm.c b/mdadm.c
> index 214afa3..d68ee96 100644
> --- a/mdadm.c
> +++ b/mdadm.c
> @@ -1739,11 +1739,11 @@ static int misc_list(struct mddev_dev *devlist,
>  			continue;
>  		case KillOpt: /* Zero superblock */
>  			if (ss)
> -				rv |= Kill(dv->devname, ss, c->force, c->verbose,0);
> +				rv |= Kill(dv->devname, -1, ss, c->force, c->verbose, 0);
>  			else {
>  				int v = c->verbose;
>  				do {
> -					rv |= Kill(dv->devname, NULL, c->force, v, 0);
> +					rv |= Kill(dv->devname, -1, NULL, c->force, v, 0);
>  					v = -1;
>  				} while (rv == 0);
>  				rv &= ~2;
> diff --git a/mdadm.h b/mdadm.h
> index c7b5183..e55dec5 100644
> --- a/mdadm.h
> +++ b/mdadm.h
> @@ -1156,7 +1156,7 @@ extern int Monitor(struct mddev_dev *devlist,
>  		   int dosyslog, char *pidfile, int increments,
>  		   int share);
>  
> -extern int Kill(char *dev, struct supertype *st, int force, int verbose, int noexcl);
> +extern int Kill(char *dev, int oldfd, struct supertype *st, int force, int verbose, int noexcl);
>  extern int Kill_subarray(char *dev, char *subarray, int verbose);
>  extern int Update_subarray(char *dev, char *subarray, char *update, struct mddev_ident *ident, int quiet);
>  extern int Wait(char *dev);
> diff --git a/super-ddf.c b/super-ddf.c
> index c5f6f5c..a88699c 100644
> --- a/super-ddf.c
> +++ b/super-ddf.c
> @@ -2597,7 +2597,7 @@ static int write_init_super_ddf(struct supertype *st)
>  	} else {
>  		struct dl *d;
>  		for (d = ddf->dlist; d; d=d->next)
> -			while (Kill(d->devname, NULL, 0, -1, 1) == 0);
> +			while (Kill(d->devname, d->fd, NULL, 0, -1, 1) == 0);
>  		return __write_init_super_ddf(st);
>  	}
>  }
> diff --git a/super-intel.c b/super-intel.c
> index 24016b7..743a7fc 100644
> --- a/super-intel.c
> +++ b/super-intel.c
> @@ -5221,7 +5221,7 @@ static int write_init_super_imsm(struct supertype *st)
>  	} else {
>  		struct dl *d;
>  		for (d = super->disks; d; d = d->next)
> -			Kill(d->devname, NULL, 0, -1, 1);
> +			Kill(d->devname, d->fd, NULL, 0, -1, 1);
>  		return write_super_imsm(st, 1);
>  	}
>  }
> diff --git a/super0.c b/super0.c
> index 1f4dc75..57b549e 100644
> --- a/super0.c
> +++ b/super0.c
> @@ -779,7 +779,7 @@ static int write_init_super0(struct supertype *st)
>  			continue;
>  		if (di->fd == -1)
>  			continue;
> -		while (Kill(di->devname, NULL, 0, -1, 1) == 0)
> +		while (Kill(di->devname, di->fd, NULL, 0, -1, 1) == 0)
>  			;
>  
>  		sb->disks[di->disk.number].state &= ~(1<<MD_DISK_FAULTY);
> diff --git a/super1.c b/super1.c
> index d0f1d5f..e3eeb80 100644
> --- a/super1.c
> +++ b/super1.c
> @@ -1333,7 +1333,7 @@ static int write_init_super1(struct supertype *st)
>  		if (di->fd < 0)
>  			continue;
>  
> -		while (Kill(di->devname, NULL, 0, -1, 1) == 0)
> +		while (Kill(di->devname, di->fd, NULL, 0, -1, 1) == 0)
>  			;
>  
>  		sb->dev_number = __cpu_to_le32(di->disk.number);


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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 04/29/2013 02:57 AM, NeilBrown wrote:
> On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
> 
>> From: Harald Hoyer <harald@redhat.com>
>> 
>> This does not trigger the udev inotify twice and saves a lot of blk I/O 
>> for the raid members.
>> 
>> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
>> 
>> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by: Jes
>> Sorensen <Jes.Sorensen@redhat.com>
> 
> (Sorry for delays.  Thanks for reminders).
> 
> That patch seems to make sense, but the description above is awfully thin.
> 
> Why is double-open a problem exactly?  What does it make udev do?  And how 
> does that related to ID_FS_TYPE being wrong as mentioned in the bugzilla 
> entry.
> 
> NeilBrown
> 

udevd with watch enabled (inotify on /dev/sd*) gets triggered on close(), when
you opened it writeable. So, if you double open() and udev wakes up from the
first close(), not all information are written to disk yet, it will not get
the ID_FS_TYPE.

Seems like the second close() does not trigger an inotify sometimes, so it is
missing afterwards all the time.

Watch via inotify is just a lazy workaround, so we don't have to modify every
tool to emit a "change" uevent, after they changed the disk.
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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[NeilBrown]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Mon, 29 Apr 2013 07:33:21 +0200 Harald Hoyer <harald@redhat.com> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 04/29/2013 02:57 AM, NeilBrown wrote:
> > On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
> > 
> >> From: Harald Hoyer <harald@redhat.com>
> >> 
> >> This does not trigger the udev inotify twice and saves a lot of blk I/O 
> >> for the raid members.
> >> 
> >> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
> >> 
> >> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by: Jes
> >> Sorensen <Jes.Sorensen@redhat.com>
> > 
> > (Sorry for delays.  Thanks for reminders).
> > 
> > That patch seems to make sense, but the description above is awfully thin.
> > 
> > Why is double-open a problem exactly?  What does it make udev do?  And how 
> > does that related to ID_FS_TYPE being wrong as mentioned in the bugzilla 
> > entry.
> > 
> > NeilBrown
> > 
> 
> udevd with watch enabled (inotify on /dev/sd*) gets triggered on close(), when
> you opened it writeable. So, if you double open() and udev wakes up from the
> first close(), not all information are written to disk yet, it will not get
> the ID_FS_TYPE.
> 
> Seems like the second close() does not trigger an inotify sometimes, so it is
> missing afterwards all the time.
> 
> Watch via inotify is just a lazy workaround, so we don't have to modify every
> tool to emit a "change" uevent, after they changed the disk.

So udev have a "lazy workaround" so that other programs don't need to trigger
a change, and as a result, I need to add some special code to mdadm.
Doesn't seem like I'm getting any advantage out of this laziness.

How about when udev gets an inotify for a block device, it first checks
that it can open it O_EXCL.  If not, it doesn't generate the change event.
That seems like the laziest option to me :-)

NeilBrown


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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 04/29/2013 08:11 AM, NeilBrown wrote:
> On Mon, 29 Apr 2013 07:33:21 +0200 Harald Hoyer <harald@redhat.com> wrote:
> 
>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> 
>> On 04/29/2013 02:57 AM, NeilBrown wrote:
>>> On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
>>> 
>>>> From: Harald Hoyer <harald@redhat.com>
>>>> 
>>>> This does not trigger the udev inotify twice and saves a lot of blk
>>>> I/O for the raid members.
>>>> 
>>>> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
>>>> 
>>>> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by: Jes 
>>>> Sorensen <Jes.Sorensen@redhat.com>
>>> 
>>> (Sorry for delays.  Thanks for reminders).
>>> 
>>> That patch seems to make sense, but the description above is awfully
>>> thin.
>>> 
>>> Why is double-open a problem exactly?  What does it make udev do?  And
>>> how does that related to ID_FS_TYPE being wrong as mentioned in the
>>> bugzilla entry.
>>> 
>>> NeilBrown
>>> 
> 
>> udevd with watch enabled (inotify on /dev/sd*) gets triggered on close(),
>> when you opened it writeable. So, if you double open() and udev wakes up
>> from the first close(), not all information are written to disk yet, it
>> will not get the ID_FS_TYPE.
> 
>> Seems like the second close() does not trigger an inotify sometimes, so
>> it is missing afterwards all the time.
> 
>> Watch via inotify is just a lazy workaround, so we don't have to modify
>> every tool to emit a "change" uevent, after they changed the disk.
> 
> So udev have a "lazy workaround" so that other programs don't need to
> trigger a change, and as a result, I need to add some special code to
> mdadm. Doesn't seem like I'm getting any advantage out of this laziness.
> 
> How about when udev gets an inotify for a block device, it first checks 
> that it can open it O_EXCL.  If not, it doesn't generate the change event. 
> That seems like the laziest option to me :-)

We cannot open with O_EXCL, because the device can be mounted, and O_EXCL
would fail there.

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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[NeilBrown]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Mon, 29 Apr 2013 08:32:31 +0200 Harald Hoyer <harald@redhat.com> wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 04/29/2013 08:11 AM, NeilBrown wrote:
> > On Mon, 29 Apr 2013 07:33:21 +0200 Harald Hoyer <harald@redhat.com> wrote:
> > 
> >> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> > 
> >> On 04/29/2013 02:57 AM, NeilBrown wrote:
> >>> On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
> >>> 
> >>>> From: Harald Hoyer <harald@redhat.com>
> >>>> 
> >>>> This does not trigger the udev inotify twice and saves a lot of blk
> >>>> I/O for the raid members.
> >>>> 
> >>>> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
> >>>> 
> >>>> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by: Jes 
> >>>> Sorensen <Jes.Sorensen@redhat.com>
> >>> 
> >>> (Sorry for delays.  Thanks for reminders).
> >>> 
> >>> That patch seems to make sense, but the description above is awfully
> >>> thin.
> >>> 
> >>> Why is double-open a problem exactly?  What does it make udev do?  And
> >>> how does that related to ID_FS_TYPE being wrong as mentioned in the
> >>> bugzilla entry.
> >>> 
> >>> NeilBrown
> >>> 
> > 
> >> udevd with watch enabled (inotify on /dev/sd*) gets triggered on close(),
> >> when you opened it writeable. So, if you double open() and udev wakes up
> >> from the first close(), not all information are written to disk yet, it
> >> will not get the ID_FS_TYPE.
> > 
> >> Seems like the second close() does not trigger an inotify sometimes, so
> >> it is missing afterwards all the time.
> > 
> >> Watch via inotify is just a lazy workaround, so we don't have to modify
> >> every tool to emit a "change" uevent, after they changed the disk.
> > 
> > So udev have a "lazy workaround" so that other programs don't need to
> > trigger a change, and as a result, I need to add some special code to
> > mdadm. Doesn't seem like I'm getting any advantage out of this laziness.
> > 
> > How about when udev gets an inotify for a block device, it first checks 
> > that it can open it O_EXCL.  If not, it doesn't generate the change event. 
> > That seems like the laziest option to me :-)
> 
> We cannot open with O_EXCL, because the device can be mounted, and O_EXCL
> would fail there.
> 

If the device is mounted, why would you want udev to be doing anything to it?

I assumed this was for things like "mkfs" so that as soon as you mkfs a
filesystem udev could tell udisks to immediately mount it...  though I'm not
sure this is a good idea.

I'm probably missing something important: what is the particular use case for
udev mapping a close-after-write to a change event?

Thanks,
NeilBrown
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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 04/29/2013 08:53 AM, NeilBrown wrote:
> On Mon, 29 Apr 2013 08:32:31 +0200 Harald Hoyer <harald@redhat.com> wrote:
> 
>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> 
>> On 04/29/2013 08:11 AM, NeilBrown wrote:
>>> On Mon, 29 Apr 2013 07:33:21 +0200 Harald Hoyer <harald@redhat.com>
>>> wrote:
>>> 
>>>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
>>> 
>>>> On 04/29/2013 02:57 AM, NeilBrown wrote:
>>>>> On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
>>>>> 
>>>>>> From: Harald Hoyer <harald@redhat.com>
>>>>>> 
>>>>>> This does not trigger the udev inotify twice and saves a lot of
>>>>>> blk I/O for the raid members.
>>>>>> 
>>>>>> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
>>>>>> 
>>>>>> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by:
>>>>>> Jes Sorensen <Jes.Sorensen@redhat.com>
>>>>> 
>>>>> (Sorry for delays.  Thanks for reminders).
>>>>> 
>>>>> That patch seems to make sense, but the description above is
>>>>> awfully thin.
>>>>> 
>>>>> Why is double-open a problem exactly?  What does it make udev do?
>>>>> And how does that related to ID_FS_TYPE being wrong as mentioned in
>>>>> the bugzilla entry.
>>>>> 
>>>>> NeilBrown
>>>>> 
>>> 
>>>> udevd with watch enabled (inotify on /dev/sd*) gets triggered on
>>>> close(), when you opened it writeable. So, if you double open() and
>>>> udev wakes up from the first close(), not all information are written
>>>> to disk yet, it will not get the ID_FS_TYPE.
>>> 
>>>> Seems like the second close() does not trigger an inotify sometimes,
>>>> so it is missing afterwards all the time.
>>> 
>>>> Watch via inotify is just a lazy workaround, so we don't have to
>>>> modify every tool to emit a "change" uevent, after they changed the
>>>> disk.
>>> 
>>> So udev have a "lazy workaround" so that other programs don't need to 
>>> trigger a change, and as a result, I need to add some special code to 
>>> mdadm. Doesn't seem like I'm getting any advantage out of this
>>> laziness.
>>> 
>>> How about when udev gets an inotify for a block device, it first
>>> checks that it can open it O_EXCL.  If not, it doesn't generate the
>>> change event. That seems like the laziest option to me :-)
> 
>> We cannot open with O_EXCL, because the device can be mounted, and
>> O_EXCL would fail there.
> 
> 
> If the device is mounted, why would you want udev to be doing anything to
> it?
> 
> I assumed this was for things like "mkfs" so that as soon as you mkfs a 
> filesystem udev could tell udisks to immediately mount it...  though I'm
> not sure this is a good idea.
> 
> I'm probably missing something important: what is the particular use case
> for udev mapping a close-after-write to a change event?
> 
> Thanks, NeilBrown

Well, you would not recognize LABEL or UUID or MD UUID changes, if you don't
call blkid after a mkfs or mdadm or lvm action.

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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 04/29/2013 08:53 AM, NeilBrown wrote:
> On Mon, 29 Apr 2013 08:32:31 +0200 Harald Hoyer <harald@redhat.com> wrote:
> 
>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
> 
>> On 04/29/2013 08:11 AM, NeilBrown wrote:
>>> On Mon, 29 Apr 2013 07:33:21 +0200 Harald Hoyer <harald@redhat.com>
>>> wrote:
>>> 
>>>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1
>>> 
>>>> On 04/29/2013 02:57 AM, NeilBrown wrote:
>>>>> On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
>>>>> 
>>>>>> From: Harald Hoyer <harald@redhat.com>
>>>>>> 
>>>>>> This does not trigger the udev inotify twice and saves a lot of
>>>>>> blk I/O for the raid members.
>>>>>> 
>>>>>> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
>>>>>> 
>>>>>> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by:
>>>>>> Jes Sorensen <Jes.Sorensen@redhat.com>
>>>>> 
>>>>> (Sorry for delays.  Thanks for reminders).
>>>>> 
>>>>> That patch seems to make sense, but the description above is
>>>>> awfully thin.
>>>>> 
>>>>> Why is double-open a problem exactly?  What does it make udev do?
>>>>> And how does that related to ID_FS_TYPE being wrong as mentioned in
>>>>> the bugzilla entry.
>>>>> 
>>>>> NeilBrown
>>>>> 
>>> 
>>>> udevd with watch enabled (inotify on /dev/sd*) gets triggered on
>>>> close(), when you opened it writeable. So, if you double open() and
>>>> udev wakes up from the first close(), not all information are written
>>>> to disk yet, it will not get the ID_FS_TYPE.
>>> 
>>>> Seems like the second close() does not trigger an inotify sometimes,
>>>> so it is missing afterwards all the time.
>>> 
>>>> Watch via inotify is just a lazy workaround, so we don't have to
>>>> modify every tool to emit a "change" uevent, after they changed the
>>>> disk.
>>> 
>>> So udev have a "lazy workaround" so that other programs don't need to 
>>> trigger a change, and as a result, I need to add some special code to 
>>> mdadm. Doesn't seem like I'm getting any advantage out of this
>>> laziness.
>>> 
>>> How about when udev gets an inotify for a block device, it first
>>> checks that it can open it O_EXCL.  If not, it doesn't generate the
>>> change event. That seems like the laziest option to me :-)
> 
>> We cannot open with O_EXCL, because the device can be mounted, and
>> O_EXCL would fail there.
> 
> 
> If the device is mounted, why would you want udev to be doing anything to
> it?
> 
> I assumed this was for things like "mkfs" so that as soon as you mkfs a 
> filesystem udev could tell udisks to immediately mount it...  though I'm
> not sure this is a good idea.
> 
> I'm probably missing something important: what is the particular use case
> for udev mapping a close-after-write to a change event?
> 
> Thanks, NeilBrown
> 


Anyway, if you don't want to play nicely with the inotify mechanism of udev,
you have to inject the "change" uevent manually for every device mdadm changes.
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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 04/29/2013 10:40 AM, Harald Hoyer wrote:
> On 04/29/2013 08:53 AM, NeilBrown wrote:
>> On Mon, 29 Apr 2013 08:32:31 +0200 Harald Hoyer <harald@redhat.com>
>> wrote:
>>> We cannot open with O_EXCL, because the device can be mounted, and 
>>> O_EXCL would fail there.
> 
> 
>> If the device is mounted, why would you want udev to be doing anything
>> to it?
> 
>> I assumed this was for things like "mkfs" so that as soon as you mkfs a 
>> filesystem udev could tell udisks to immediately mount it...  though I'm 
>> not sure this is a good idea.
> 
>> I'm probably missing something important: what is the particular use
>> case for udev mapping a close-after-write to a change event?
> 
>> Thanks, NeilBrown
> 
> 
> 
> Anyway, if you don't want to play nicely with the inotify mechanism of
> udev, you have to inject the "change" uevent manually for every device
> mdadm changes.
> 
*rant mode on*

I think we should drop the inotify mechanism from udev and say "f*** you" to
the tools... If they change s.th. on the disk, they can also emit a "change"
uevent to the kernel themselves.
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Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 04/29/2013 10:45 AM, Harald Hoyer wrote:
> On 04/29/2013 10:40 AM, Harald Hoyer wrote:
>> On 04/29/2013 08:53 AM, NeilBrown wrote:
>>> On Mon, 29 Apr 2013 08:32:31 +0200 Harald Hoyer <harald@redhat.com> 
>>> wrote:
>>>> We cannot open with O_EXCL, because the device can be mounted, and 
>>>> O_EXCL would fail there.
> 
> 
>>> If the device is mounted, why would you want udev to be doing anything 
>>> to it?
> 
>>> I assumed this was for things like "mkfs" so that as soon as you mkfs a
>>>  filesystem udev could tell udisks to immediately mount it...  though
>>> I'm not sure this is a good idea.
> 
>>> I'm probably missing something important: what is the particular use 
>>> case for udev mapping a close-after-write to a change event?
> 
>>> Thanks, NeilBrown
> 
> 
> 
>> Anyway, if you don't want to play nicely with the inotify mechanism of 
>> udev, you have to inject the "change" uevent manually for every device 
>> mdadm changes.
> 
> *rant mode on*
> 
> I think we should drop the inotify mechanism from udev and say "f*** you"
> to the tools... If they change s.th. on the disk, they can also emit a
> "change" uevent to the kernel themselves.
> 

Oh, and the tools then also have wait for udev to settle..

Imagine a script doing:

# mdadm -A --uuid=<my-UUID> ....
# mkfs  -L mylabel /dev/disk/by-uuid/<my-MD-UUID>
# mount /dev/disk/by-label/mylabel /mnt

This is all broken by design today:
- - there is no guaranteed change event
- - there is no automatic udev settle
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