[PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Carlo Marcelo Arenas Belon]

revert a merge conflict from 075da586c92f09bd9a7401f1e80d72fde27c173 that
redefined sector as an array of pointers to char, instead of a statically
allocated buffer of chars, that was triggering the following warnings :

block.c: In function `bdrv_commit':
block.c:480: warning: passing arg 3 of `bdrv_read' from incompatible pointer type
block.c:484: warning: passing arg 3 of `bdrv_write' from incompatible pointer type

Signed-off-by: Carlo Marcelo Arenas Belon <carenas-kLeDWSohozoJb6fo7hG9ng@public.gmane.org>
---
 qemu/block.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/qemu/block.c b/qemu/block.c
index 519be24..492a4d3 100644
--- a/qemu/block.c
+++ b/qemu/block.c
@@ -460,7 +460,7 @@ int bdrv_commit(BlockDriverState *bs)
     BlockDriver *drv = bs->drv;
     int64_t i, total_sectors;
     int n, j;
-    unsigned char *sector[512];
+    unsigned char sector[512];
 
     if (!drv)
         return -ENOMEDIUM;
-- 
1.5.3.7


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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Avi Kivity]

Carlo Marcelo Arenas Belon wrote:
> revert a merge conflict from 075da586c92f09bd9a7401f1e80d72fde27c173 that
> redefined sector as an array of pointers to char, instead of a statically
> allocated buffer of chars, that was triggering the following warnings :
>
> block.c: In function `bdrv_commit':
> block.c:480: warning: passing arg 3 of `bdrv_read' from incompatible pointer type
> block.c:484: warning: passing arg 3 of `bdrv_write' from incompatible pointer type
>
> Signed-off-by: Carlo Marcelo Arenas Belon <carenas-kLeDWSohozoJb6fo7hG9ng@public.gmane.org>
>   

Doesn't the cache=off option warrant an allocation here to ensure 
alignment (or perhaps a 1K stack buffer with runtime adjustment)?

-- 
error compiling committee.c: too many arguments to function


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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Laurent Vivier]

[-- Attachment #1.1: Type: text/plain, Size: 2483 bytes --]

Le lundi 07 janvier 2008 à 11:27 +0200, Avi Kivity a écrit :
> Carlo Marcelo Arenas Belon wrote:
> > revert a merge conflict from 075da586c92f09bd9a7401f1e80d72fde27c173 that
> > redefined sector as an array of pointers to char, instead of a statically
> > allocated buffer of chars, that was triggering the following warnings :
> >
> > block.c: In function `bdrv_commit':
> > block.c:480: warning: passing arg 3 of `bdrv_read' from incompatible pointer type
> > block.c:484: warning: passing arg 3 of `bdrv_write' from incompatible pointer type
> >
> > Signed-off-by: Carlo Marcelo Arenas Belon <carenas-kLeDWSohozoJb6fo7hG9ng@public.gmane.org>
> >   
> 
> Doesn't the cache=off option warrant an allocation here to ensure 
> alignment (or perhaps a 1K stack buffer with runtime adjustment)?

You're right, a good patch should be something like this (it is not
tested or even compiled) :

Signed-off-by: Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org>
diff --git a/qemu/block.c b/qemu/block.c
index 519be24..c198659 100644
--- a/qemu/block.c
+++ b/qemu/block.c
@@ -460,7 +460,7 @@ int bdrv_commit(BlockDriverState *bs)
     BlockDriver *drv = bs->drv;
     int64_t i, total_sectors;
     int n, j;
-    unsigned char *sector[512];
+    unsigned char *sector;
 
     if (!drv)
         return -ENOMEDIUM;
@@ -473,15 +473,21 @@ int bdrv_commit(BlockDriverState *bs)
        return -ENOTSUP;
     }
 
+    sector = qemu_memalign(512,512);
+    if (sector == NULL)
+        return -ENOMEM;
+
     total_sectors = bdrv_getlength(bs) >> SECTOR_BITS;
     for (i = 0; i < total_sectors;) {
         if (drv->bdrv_is_allocated(bs, i, 65536, &n)) {
             for(j = 0; j < n; j++) {
                 if (bdrv_read(bs, i, sector, 1) != 0) {
+                    qemu_free(sector);
                     return -EIO;
                 }
 
                 if (bdrv_write(bs->backing_hd, i, sector, 1) != 0) {
+                    qemu_free(sector);
                     return -EIO;
                 }
                 i++;
@@ -491,6 +497,7 @@ int bdrv_commit(BlockDriverState *bs)
         }
     }
 
+    qemu_free(sector);
     if (drv->bdrv_make_empty)
        return drv->bdrv_make_empty(bs);

-- 
----------------- Laurent.Vivier-6ktuUTfB/bM@public.gmane.org  ------------------
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ajouter mais quand il ne reste rien à enlever." Saint Exupéry

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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Avi Kivity]

Laurent Vivier wrote:
> Le lundi 07 janvier 2008 à 11:27 +0200, Avi Kivity a écrit :
>   
>> Carlo Marcelo Arenas Belon wrote:
>>     
>>> revert a merge conflict from 075da586c92f09bd9a7401f1e80d72fde27c173 that
>>> redefined sector as an array of pointers to char, instead of a statically
>>> allocated buffer of chars, that was triggering the following warnings :
>>>
>>> block.c: In function `bdrv_commit':
>>> block.c:480: warning: passing arg 3 of `bdrv_read' from incompatible pointer type
>>> block.c:484: warning: passing arg 3 of `bdrv_write' from incompatible pointer type
>>>
>>> Signed-off-by: Carlo Marcelo Arenas Belon <carenas@sajinet.com.pe>
>>>   
>>>       
>> Doesn't the cache=off option warrant an allocation here to ensure 
>> alignment (or perhaps a 1K stack buffer with runtime adjustment)?
>>     
>
> You're right, a good patch should be something like this (it is not
> tested or even compiled) :
>   

Looks good, but patch is corrupted by mail client.

-- 
error compiling committee.c: too many arguments to function


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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Laurent Vivier]

[-- Attachment #1.1: Type: text/plain, Size: 1436 bytes --]

Le lundi 07 janvier 2008 à 12:47 +0200, Avi Kivity a écrit :
> Laurent Vivier wrote:
> > Le lundi 07 janvier 2008 à 11:27 +0200, Avi Kivity a écrit :
> >   
> >> Carlo Marcelo Arenas Belon wrote:
> >>     
> >>> revert a merge conflict from 075da586c92f09bd9a7401f1e80d72fde27c173 that
> >>> redefined sector as an array of pointers to char, instead of a statically
> >>> allocated buffer of chars, that was triggering the following warnings :
> >>>
> >>> block.c: In function `bdrv_commit':
> >>> block.c:480: warning: passing arg 3 of `bdrv_read' from incompatible pointer type
> >>> block.c:484: warning: passing arg 3 of `bdrv_write' from incompatible pointer type
> >>>
> >>> Signed-off-by: Carlo Marcelo Arenas Belon <carenas-kLeDWSohozoJb6fo7hG9ng@public.gmane.org>
> >>>   
> >>>       
> >> Doesn't the cache=off option warrant an allocation here to ensure 
> >> alignment (or perhaps a 1K stack buffer with runtime adjustment)?
> >>     
> >
> > You're right, a good patch should be something like this (it is not
> > tested or even compiled) :
> >   
> 
> Looks good, but patch is corrupted by mail client.

OK, I'll resend it later (I'd like to compile it at least).

Laurent
-- 
----------------- Laurent.Vivier-6ktuUTfB/bM@public.gmane.org  ------------------
  "La perfection est atteinte non quand il ne reste rien à
ajouter mais quand il ne reste rien à enlever." Saint Exupéry

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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Laurent Vivier]

[-- Attachment #1.1: Type: text/plain, Size: 1599 bytes --]

Le lundi 07 janvier 2008 à 12:47 +0200, Avi Kivity a écrit :
> Laurent Vivier wrote:
> > Le lundi 07 janvier 2008 à 11:27 +0200, Avi Kivity a écrit :
> >   
> >> Carlo Marcelo Arenas Belon wrote:
> >>     
> >>> revert a merge conflict from 075da586c92f09bd9a7401f1e80d72fde27c173 that
> >>> redefined sector as an array of pointers to char, instead of a statically
> >>> allocated buffer of chars, that was triggering the following warnings :
> >>>
> >>> block.c: In function `bdrv_commit':
> >>> block.c:480: warning: passing arg 3 of `bdrv_read' from incompatible pointer type
> >>> block.c:484: warning: passing arg 3 of `bdrv_write' from incompatible pointer type
> >>>
> >>> Signed-off-by: Carlo Marcelo Arenas Belon <carenas-kLeDWSohozoJb6fo7hG9ng@public.gmane.org>
> >>>   
> >>>       
> >> Doesn't the cache=off option warrant an allocation here to ensure 
> >> alignment (or perhaps a 1K stack buffer with runtime adjustment)?
> >>     
> >
> > You're right, a good patch should be something like this (it is not
> > tested or even compiled) :
> >   
> 
> Looks good, but patch is corrupted by mail client.

What I'm wondering now is: is it really useful to have "cache=off" and
"snapshot=on" at the same time ?
If not, the patch of Carlo is good, otherwise there is more
modifications to do (in other parts of qemu).

Regards,
Laurent
-- 
----------------- Laurent.Vivier-6ktuUTfB/bM@public.gmane.org  ------------------
  "La perfection est atteinte non quand il ne reste rien à
ajouter mais quand il ne reste rien à enlever." Saint Exupéry

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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Javier Guerra]

On 1/7/08, Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org> wrote:
> What I'm wondering now is: is it really useful to have "cache=off" and
> "snapshot=on" at the same time ?

does "cache=off" means disk cache? if so, it might be useful to test
clustering filesystems.

so far, the only way is to setup a network block device (iSCSI, AoE,
nbd). i'd like to simply specify the same backing file for two
instances' hdb parameter.

and snapshots help a lot to go back after blowing up the on-disk structures


-- 
Javier

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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Laurent Vivier]

[-- Attachment #1.1: Type: text/plain, Size: 1182 bytes --]

Le lundi 07 janvier 2008 à 10:34 -0500, Javier Guerra a écrit :
> On 1/7/08, Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org> wrote:
> > What I'm wondering now is: is it really useful to have "cache=off" and
> > "snapshot=on" at the same time ?
> 
> does "cache=off" means disk cache? if so, it might be useful to test
> clustering filesystems.

"cache=off" means files is opened with "O_DIRECT" and thus there is no
cache in the kernel memory on the host side.
IMO, "cache=off" and "snapshot=on" are incompatible because a snapshot
can be seen like a cache.

> so far, the only way is to setup a network block device (iSCSI, AoE,
> nbd). i'd like to simply specify the same backing file for two
> instances' hdb parameter.

I'm sorry but I don't understand this part.

> and snapshots help a lot to go back after blowing up the on-disk structures

But I think if you use a snapshot there is no reason to use "cache=off"

Laurent
-- 
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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Javier Guerra]

On 1/7/08, Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org> wrote:
> "cache=off" means files is opened with "O_DIRECT" and thus there is no
> cache in the kernel memory on the host side.
> IMO, "cache=off" and "snapshot=on" are incompatible because a snapshot
> can be seen like a cache.
>
> > so far, the only way is to setup a network block device (iSCSI, AoE,
> > nbd). i'd like to simply specify the same backing file for two
> > instances' hdb parameter.
>
> I'm sorry but I don't understand this part.

to test a cluster filesystem, i need two (virtual) machines with some
shared storage.  i tried long ago something like this (with (k)qemu):

- create a disk image, call it hda-1.img
- boot and install linux on it, shutdown
- copy to hda-2.img
- boot it (with new MAC) and change IP, hostname, little things, shutdown
- boot both with the same bridge, check that network works between them
- create a new disk image, call id hdb-shr.img
- boot both VMs, sharing hdb-shr.img:

qemu -hda=hda-1.img -hdb-shr.img
qemu -hda=hda-2.img -hdb-shr.img

- try to setup a cluster filesystem on hdb

it almost worked... but some writes didn't propagate to the other
until some extra writes to hdb; so i guessed that each qemu instance
had some caching on file I/O

hopefully, it would now work with "-cache=off", don't you think?

> > and snapshots help a lot to go back after blowing up the on-disk structures
>
> But I think if you use a snapshot there is no reason to use "cache=off"

in the above case, if both KVM instances share the snapshot without
cacheing it, the cluster should still work, and have some rollback
capability at the same time

or is it too much wishful thinking?

-- 
Javier

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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Laurent Vivier]

[-- Attachment #1.1: Type: text/plain, Size: 2660 bytes --]

Le lundi 07 janvier 2008 à 11:03 -0500, Javier Guerra a écrit :
> On 1/7/08, Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org> wrote:
> > "cache=off" means files is opened with "O_DIRECT" and thus there is no
> > cache in the kernel memory on the host side.
> > IMO, "cache=off" and "snapshot=on" are incompatible because a snapshot
> > can be seen like a cache.
> >
> > > so far, the only way is to setup a network block device (iSCSI, AoE,
> > > nbd). i'd like to simply specify the same backing file for two
> > > instances' hdb parameter.
> >
> > I'm sorry but I don't understand this part.
> 
> to test a cluster filesystem, i need two (virtual) machines with some
> shared storage.  i tried long ago something like this (with (k)qemu):
> 
> - create a disk image, call it hda-1.img
> - boot and install linux on it, shutdown
> - copy to hda-2.img
> - boot it (with new MAC) and change IP, hostname, little things, shutdown
> - boot both with the same bridge, check that network works between them
> - create a new disk image, call id hdb-shr.img
> - boot both VMs, sharing hdb-shr.img:
> 
> qemu -hda=hda-1.img -hdb-shr.img
> qemu -hda=hda-2.img -hdb-shr.img
> 
> - try to setup a cluster filesystem on hdb
> 
> it almost worked... but some writes didn't propagate to the other
> until some extra writes to hdb; so i guessed that each qemu instance
> had some caching on file I/O
> 
> hopefully, it would now work with "-cache=off", don't you think?

Well, I don't think the problem is at the host level but at the guest
level, because both instances of qemu share the host cache and thus
first instance should see changes made by the second instance (and
vice-versa).
There are also some caches at qemu level to emulate DMA, for instance in
hw/ide.c it is MAX_MULT_SECTORS (16) which is 8 kB buffer, perhaps your
problem is here but "cache=off" doesn't remove this.
Did you try to change MAX_MULT_SECTORS to 1 ?

What do you call a "cluster filesystem" ?
 
> > > and snapshots help a lot to go back after blowing up the on-disk structures
> >
> > But I think if you use a snapshot there is no reason to use "cache=off"
> 
> in the above case, if both KVM instances share the snapshot without
> cacheing it, the cluster should still work, and have some rollback
> capability at the same time
> 
> or is it too much wishful thinking?

Unfortunately I guess...

Laurent
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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Javier Guerra]

On 1/7/08, Laurent Vivier <Laurent.Vivier@bull.net> wrote:
> Le lundi 07 janvier 2008 à 11:03 -0500, Javier Guerra a écrit :
> > hopefully, it would now work with "-cache=off", don't you think?
>
> Well, I don't think the problem is at the host level but at the guest
> level, because both instances of qemu share the host cache and thus
> first instance should see changes made by the second instance (and
> vice-versa).

that's what a cluster filesystem is designed to cope with (and in fact expects)

> There are also some caches at qemu level to emulate DMA, for instance in
> hw/ide.c it is MAX_MULT_SECTORS (16) which is 8 kB buffer, perhaps your
> problem is here but "cache=off" doesn't remove this.
> Did you try to change MAX_MULT_SECTORS to 1 ?

nope, don't know enough of qemu internals...

but if those caches can be flushed from the guest, the filesystem
should do that when writing its metadata

> What do you call a "cluster filesystem" ?

GFS, OCFS2, etc.   that is, filesystems that are designed to run on
more than one host with shared block storage.  usually that means
FibreChannel or iSCSI, but on VMs, a common backing could work too (at
least in theory).

Xen manages it, at least on paravirtualized guests; hadn't tried on
HVM guests. maybe the IDE-like emulation is too poor an interface to
handle it.

-- 
Javier
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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Laurent Vivier]

[-- Attachment #1.1: Type: text/plain, Size: 2000 bytes --]

Well, in fact, I think you can't use snapshot with cluster filesystem:
as each qemu instance will write in its own snapshot and will not see
modifications made by other, and I don't think there is currently a way
to share snapshot between qemu instances.

Laurent

Le lundi 07 janvier 2008 à 11:42 -0500, Javier Guerra a écrit :
> On 1/7/08, Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org> wrote:
> > Le lundi 07 janvier 2008 à 11:03 -0500, Javier Guerra a écrit :
> > > hopefully, it would now work with "-cache=off", don't you think?
> >
> > Well, I don't think the problem is at the host level but at the guest
> > level, because both instances of qemu share the host cache and thus
> > first instance should see changes made by the second instance (and
> > vice-versa).
> 
> that's what a cluster filesystem is designed to cope with (and in fact expects)
> 
> > There are also some caches at qemu level to emulate DMA, for instance in
> > hw/ide.c it is MAX_MULT_SECTORS (16) which is 8 kB buffer, perhaps your
> > problem is here but "cache=off" doesn't remove this.
> > Did you try to change MAX_MULT_SECTORS to 1 ?
> 
> nope, don't know enough of qemu internals...
> 
> but if those caches can be flushed from the guest, the filesystem
> should do that when writing its metadata
> 
> > What do you call a "cluster filesystem" ?
> 
> GFS, OCFS2, etc.   that is, filesystems that are designed to run on
> more than one host with shared block storage.  usually that means
> FibreChannel or iSCSI, but on VMs, a common backing could work too (at
> least in theory).
> 
> Xen manages it, at least on paravirtualized guests; hadn't tried on
> HVM guests. maybe the IDE-like emulation is too poor an interface to
> handle it.
> 
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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Avi Kivity]

Laurent Vivier wrote:
> What I'm wondering now is: is it really useful to have "cache=off" and
> "snapshot=on" at the same time ?
>   

No idea, but wouldn't want to rule it out.

> If not, the patch of Carlo is good, otherwise there is more
> modifications to do (in other parts of qemu).
>   

If other modifications are needed, perhaps do them in the I/O submission 
path (if unaligned and cache=off, force alignment).  Since most of the 
data will be aligned by the guest, the overhead will only be incurred by 
the rare metadata accesses.


-- 
error compiling committee.c: too many arguments to function


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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Avi Kivity]

Javier Guerra wrote:
> On 1/7/08, Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org> wrote:
>   
>> What I'm wondering now is: is it really useful to have "cache=off" and
>> "snapshot=on" at the same time ?
>>     
>
> does "cache=off" means disk cache? if so, it might be useful to test
> clustering filesystems.
>
> so far, the only way is to setup a network block device (iSCSI, AoE,
> nbd). i'd like to simply specify the same backing file for two
> instances' hdb parameter.
>   

For that to work you need to use the raw format (and then, cache=off 
doesn't matter, as a clustering filesystem ought to have a coherent cache).


-- 
error compiling committee.c: too many arguments to function


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Re: [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Avi Kivity]

Javier Guerra wrote:
> to test a cluster filesystem, i need two (virtual) machines with some
> shared storage.  i tried long ago something like this (with (k)qemu):
>
> - create a disk image, call it hda-1.img
> - boot and install linux on it, shutdown
> - copy to hda-2.img
> - boot it (with new MAC) and change IP, hostname, little things, shutdown
> - boot both with the same bridge, check that network works between them
> - create a new disk image, call id hdb-shr.img
> - boot both VMs, sharing hdb-shr.img:
>
> qemu -hda=hda-1.img -hdb-shr.img
> qemu -hda=hda-2.img -hdb-shr.img
>
> - try to setup a cluster filesystem on hdb
>
> it almost worked... but some writes didn't propagate to the other
> until some extra writes to hdb; so i guessed that each qemu instance
> had some caching on file I/O
>
> hopefully, it would now work with "-cache=off", don't you think?
>   
>

cache=off is not necessary.  However you need raw format images (not 
qcow/vmdk/etc).


-- 
error compiling committee.c: too many arguments to function


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