[PATCH] remove BUG() in possible but rare condition

[PATCH] remove BUG() in possible but rare condition

[Glauber Costa]

While stressing the kernel with with failing allocations today,
I hit the following chain of events:

alloc_page_buffers():

	bh = alloc_buffer_head(GFP_NOFS);
	if (!bh)
		goto no_grow; <= path taken

grow_dev_page():
        bh = alloc_page_buffers(page, size, 0);
        if (!bh)
                goto failed;  <= taken, consequence of the above

and then the failed path BUG()s the kernel.

The failure is inserted a litte bit artificially, but even then,
I see no reason why it should be deemed impossible in a real box.

Even though this is not a condition that we expect to see
around every time, failed allocations are expected to be handled,
and BUG() sounds just too much. As a matter of fact, grow_dev_page()
can return NULL just fine in other circumstances, so I propose we just
remove it, then.

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Andrew Morton <akpm@linux-foundation.org>
---
 fs/buffer.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 36d6665..351e18e 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -985,7 +985,6 @@ grow_dev_page(struct block_device *bdev, sector_t block,
 	return page;
 
 failed:
-	BUG();
 	unlock_page(page);
 	page_cache_release(page);
 	return NULL;
-- 
1.7.7.6

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Re: [PATCH] remove BUG() in possible but rare condition

[Michal Hocko]

On Wed 11-04-12 15:10:24, Glauber Costa wrote:
> While stressing the kernel with with failing allocations today,
> I hit the following chain of events:
> 
> alloc_page_buffers():
> 
> 	bh = alloc_buffer_head(GFP_NOFS);
> 	if (!bh)
> 		goto no_grow; <= path taken
> 
> grow_dev_page():
>         bh = alloc_page_buffers(page, size, 0);
>         if (!bh)
>                 goto failed;  <= taken, consequence of the above
> 
> and then the failed path BUG()s the kernel.
> 
> The failure is inserted a litte bit artificially, but even then,
> I see no reason why it should be deemed impossible in a real box.
> 
> Even though this is not a condition that we expect to see
> around every time, failed allocations are expected to be handled,
> and BUG() sounds just too much. As a matter of fact, grow_dev_page()
> can return NULL just fine in other circumstances, so I propose we just
> remove it, then.

I am not familiar with the code much but a trivial call chain walk up to
write_dev_supers (in btrfs) shows that we do not check for the return value
from __getblk so we would nullptr and there might be more. 
I guess these need some treat before the BUG might be removed, right?

> 
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Andrew Morton <akpm@linux-foundation.org>
> ---
>  fs/buffer.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 36d6665..351e18e 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -985,7 +985,6 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>  	return page;
>  
>  failed:
> -	BUG();
>  	unlock_page(page);
>  	page_cache_release(page);
>  	return NULL;
> -- 
> 1.7.7.6
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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Re: [PATCH] remove BUG() in possible but rare condition

[Linus Torvalds]

On Wed, Apr 11, 2012 at 11:48 AM, Michal Hocko <mhocko@suse.cz> wrote:
>
> I am not familiar with the code much but a trivial call chain walk up to
> write_dev_supers (in btrfs) shows that we do not check for the return value
> from __getblk so we would nullptr and there might be more.
> I guess these need some treat before the BUG might be removed, right?

Well, realistically, isn't BUG() as bad as a NULL pointer dereference?

Do you care about the exact message on the screen when your machine dies?

                     Linus

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Re: [PATCH] remove BUG() in possible but rare condition

[Glauber Costa]

On 04/11/2012 03:57 PM, Linus Torvalds wrote:
> On Wed, Apr 11, 2012 at 11:48 AM, Michal Hocko<mhocko@suse.cz>  wrote:
>>
>> I am not familiar with the code much but a trivial call chain walk up to
>> write_dev_supers (in btrfs) shows that we do not check for the return value
>> from __getblk so we would nullptr and there might be more.
>> I guess these need some treat before the BUG might be removed, right?
>
> Well, realistically, isn't BUG() as bad as a NULL pointer dereference?
>
> Do you care about the exact message on the screen when your machine dies?
Not particular, but I don't see why (I might be wrong) it would 
necessarily lead to a NULL pointer dereference.

At least in my test cases, after turning this to a WARN (to make sure it 
was still being hit), the machine could go on just fine.

I was running this in a container system, with restricted memory. After
killing the container - at least in my ext4 system - everything seemed 
as happy as ever.

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Re: [PATCH] remove BUG() in possible but rare condition

[Michal Hocko]

On Wed 11-04-12 16:02:19, Glauber Costa wrote:
> On 04/11/2012 03:57 PM, Linus Torvalds wrote:
> >On Wed, Apr 11, 2012 at 11:48 AM, Michal Hocko<mhocko@suse.cz>  wrote:
> >>
> >>I am not familiar with the code much but a trivial call chain walk up to
> >>write_dev_supers (in btrfs) shows that we do not check for the return value
> >>from __getblk so we would nullptr and there might be more.
> >>I guess these need some treat before the BUG might be removed, right?
> >
> >Well, realistically, isn't BUG() as bad as a NULL pointer dereference?
> >
> >Do you care about the exact message on the screen when your machine dies?
> Not particular, but I don't see why (I might be wrong) it would
> necessarily lead to a NULL pointer dereference.

Ahh, OK scratch that. I have misread __getblk_slow which returns NULL
only if grow_buffers returned with < 0 which doesn't happen for the
allocation failure.

Sorry about noise
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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Re: [PATCH] remove BUG() in possible but rare condition

[Michal Hocko]

On Wed 11-04-12 11:57:56, Linus Torvalds wrote:
> On Wed, Apr 11, 2012 at 11:48 AM, Michal Hocko <mhocko@suse.cz> wrote:
> >
> > I am not familiar with the code much but a trivial call chain walk up to
> > write_dev_supers (in btrfs) shows that we do not check for the return value
> > from __getblk so we would nullptr and there might be more.
> > I guess these need some treat before the BUG might be removed, right?
> 
> Well, realistically, isn't BUG() as bad as a NULL pointer dereference?
> 
> Do you care about the exact message on the screen when your machine dies?

I personally do not care as I do not allow anything to map at that area.

It just seems that there are some callers who do not expect that the
allocation fails. BUG at the allocation failure which dates back when it
replaced buffer_error might have let to some assumptions (not good of
course but we should better fix them.

That being said I am not against the patch. BUG on an allocation failure
just doesn't feel right...

> 
>                      Linus
> 
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SUSE Labs
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Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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Re: [PATCH] remove BUG() in possible but rare condition

[Don Morris]

On 04/11/2012 12:20 PM, Michal Hocko wrote:
> On Wed 11-04-12 11:57:56, Linus Torvalds wrote:
>> On Wed, Apr 11, 2012 at 11:48 AM, Michal Hocko <mhocko@suse.cz> wrote:
>>>
>>> I am not familiar with the code much but a trivial call chain walk up to
>>> write_dev_supers (in btrfs) shows that we do not check for the return value
>>> from __getblk so we would nullptr and there might be more.
>>> I guess these need some treat before the BUG might be removed, right?
>>
>> Well, realistically, isn't BUG() as bad as a NULL pointer dereference?
>>
>> Do you care about the exact message on the screen when your machine dies?
> 
> I personally do not care as I do not allow anything to map at that area.
> 
> It just seems that there are some callers who do not expect that the
> allocation fails. BUG at the allocation failure which dates back when it
> replaced buffer_error might have let to some assumptions (not good of
> course but we should better fix them.
> 
> That being said I am not against the patch. BUG on an allocation failure
> just doesn't feel right...

Apologies in advance for the relatively wide distribution for what may
be an obvious/stupid question, but if this is in a Documentation/vm
file, I don't see it.

Unless I'm really missing something, the allocation in question uses
GFP_NOFS flags, resulting in a "Wait Ok" request to the underlying
allocators. Other than gotchas like force failures returning a fail
even for Wait-safe allocations... is it actually expected for Wait-safe
kernel allocations to fail? Certainly page requests for user memory
can run afoul of OOM or other mechanisms, and certainly any request
for special/hard memory could be failed... but wouldn't a general small
kernel allocation should generally sleep until satisfied?

That's what the current code looks like to me -- but I could easily
be missing something in the reading, so I'm mainly asking if there's
a general policy to the API here. If it is that non-special kernel
allocations wait until they're satisfied... I'm wondering, why the
test/force mode that seems guaranteed to light off BUG() statements
such as this which are simply validating the allocation contract?

Don Morris

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Re: [PATCH] remove BUG() in possible but rare condition

[Jiri Kosina]

On Wed, 11 Apr 2012, Linus Torvalds wrote:

> > I am not familiar with the code much but a trivial call chain walk up to
> > write_dev_supers (in btrfs) shows that we do not check for the return value
> > from __getblk so we would nullptr and there might be more.
> > I guess these need some treat before the BUG might be removed, right?
> 
> Well, realistically, isn't BUG() as bad as a NULL pointer dereference?

Well, there still could be weirdos out there not setting 
sys.vm.mmap_min_addr to something sane. For those, NULL pointer 
dereference could have worse consequences than BUG (unlikely in this 
particular case, yes).

-- 
Jiri Kosina
SUSE Labs

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Re: [PATCH] remove BUG() in possible but rare condition

[Glauber Costa]

On 04/11/2012 03:48 PM, Michal Hocko wrote:
> On Wed 11-04-12 15:10:24, Glauber Costa wrote:
>> While stressing the kernel with with failing allocations today,
>> I hit the following chain of events:
>>
>> alloc_page_buffers():
>>
>> 	bh = alloc_buffer_head(GFP_NOFS);
>> 	if (!bh)
>> 		goto no_grow;<= path taken
>>
>> grow_dev_page():
>>          bh = alloc_page_buffers(page, size, 0);
>>          if (!bh)
>>                  goto failed;<= taken, consequence of the above
>>
>> and then the failed path BUG()s the kernel.
>>
>> The failure is inserted a litte bit artificially, but even then,
>> I see no reason why it should be deemed impossible in a real box.
>>
>> Even though this is not a condition that we expect to see
>> around every time, failed allocations are expected to be handled,
>> and BUG() sounds just too much. As a matter of fact, grow_dev_page()
>> can return NULL just fine in other circumstances, so I propose we just
>> remove it, then.
>
> I am not familiar with the code much but a trivial call chain walk up to
> write_dev_supers (in btrfs) shows that we do not check for the return value
> from __getblk so we would nullptr and there might be more.
> I guess these need some treat before the BUG might be removed, right?

You might very well be right, but if this is the case, this function is 
probably wrong already.

find_or_create_page() failing will make it return NULL as well, and that 
won't trigger the BUG() path.

At least in ext4 in my test case, the filesystem seems consistent after 
a couple of runs
triggering this

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Re: [PATCH] remove BUG() in possible but rare condition

[Andrew Morton]

On Wed, 11 Apr 2012 15:10:24 -0300
Glauber Costa <glommer@parallels.com> wrote:

> While stressing the kernel with with failing allocations today,
> I hit the following chain of events:
> 
> alloc_page_buffers():
> 
> 	bh = alloc_buffer_head(GFP_NOFS);
> 	if (!bh)
> 		goto no_grow; <= path taken
> 
> grow_dev_page():
>         bh = alloc_page_buffers(page, size, 0);
>         if (!bh)
>                 goto failed;  <= taken, consequence of the above
> 
> and then the failed path BUG()s the kernel.
> 
> The failure is inserted a litte bit artificially, but even then,
> I see no reason why it should be deemed impossible in a real box.
> 
> Even though this is not a condition that we expect to see
> around every time, failed allocations are expected to be handled,
> and BUG() sounds just too much. As a matter of fact, grow_dev_page()
> can return NULL just fine in other circumstances, so I propose we just
> remove it, then.
> 
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Andrew Morton <akpm@linux-foundation.org>
> ---
>  fs/buffer.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 36d6665..351e18e 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -985,7 +985,6 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>  	return page;
>  
>  failed:
> -	BUG();
>  	unlock_page(page);
>  	page_cache_release(page);
>  	return NULL;

Cute.

AFAICT what happened was that in my April 2002 rewrite of this code I
put a non-fatal buffer_error() warning in that case to tell us that
something bad happened.

Years later we removed the temporary buffer_error() and mistakenly
replaced that warning with a BUG().  Only it *can* happen.

We can remove the BUG() and fix up callers, or we can pass retry=1 into
alloc_page_buffers(), so grow_dev_page() "cannot fail".  Immortal
functions are a silly fiction, so we should remove the BUG() and fix up
callers.

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Re: [PATCH] remove BUG() in possible but rare condition

[Glauber Costa]

On 04/11/2012 05:26 PM, Andrew Morton wrote:
>>
>> >    failed:
>> >  -	BUG();
>> >    	unlock_page(page);
>> >    	page_cache_release(page);
>> >    	return NULL;
> Cute.
>
> AFAICT what happened was that in my April 2002 rewrite of this code I
> put a non-fatal buffer_error() warning in that case to tell us that
> something bad happened.
>
> Years later we removed the temporary buffer_error() and mistakenly
> replaced that warning with a BUG().  Only it*can*  happen.
>
> We can remove the BUG() and fix up callers, or we can pass retry=1 into
> alloc_page_buffers(), so grow_dev_page() "cannot fail".  Immortal
> functions are a silly fiction, so we should remove the BUG() and fix up
> callers.
>
Any particular caller you are concerned with ?

As I mentioned, this function already returns NULL for other reason - 
that seem even more probable than this specific failure. So whoever is
not checking this return value, is already broken without this patch as 
well.


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Re: [PATCH] remove BUG() in possible but rare condition

[Andrew Morton]

On Wed, 11 Apr 2012 17:51:57 -0300
Glauber Costa <glommer@parallels.com> wrote:

> On 04/11/2012 05:26 PM, Andrew Morton wrote:
> >>
> >> >    failed:
> >> >  -	BUG();
> >> >    	unlock_page(page);
> >> >    	page_cache_release(page);
> >> >    	return NULL;
> > Cute.
> >
> > AFAICT what happened was that in my April 2002 rewrite of this code I
> > put a non-fatal buffer_error() warning in that case to tell us that
> > something bad happened.
> >
> > Years later we removed the temporary buffer_error() and mistakenly
> > replaced that warning with a BUG().  Only it*can*  happen.
> >
> > We can remove the BUG() and fix up callers, or we can pass retry=1 into
> > alloc_page_buffers(), so grow_dev_page() "cannot fail".  Immortal
> > functions are a silly fiction, so we should remove the BUG() and fix up
> > callers.
> >
> Any particular caller you are concerned with ?

Didn't someone see a buggy caller in btrfs?

I'm thinking that we should retain some sort of assertion (a WARN_ON)
if the try_to_free_buffers() failed.  This is a weird case which I
assume handles the situation where a blockdev's blocksize has changed. 
The code tries to throw away the old wrongly-sized buffer_heads and to
then add new correctly-sized ones.  If that discarding of buffers
fails then the kernel is in rather a mess.

It's quite possible that this code is never executed - we _should_ have
invalidated all the pagecache for that device when changing blocksize. 
Or maybe it *is* executed, I dunno.  It's one of those things which has
hung around for decades as code in other places has vastly changed.

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Re: [PATCH] remove BUG() in possible but rare condition

[Michal Hocko]

On Wed 11-04-12 14:12:44, Andrew Morton wrote:
> On Wed, 11 Apr 2012 17:51:57 -0300
> Glauber Costa <glommer@parallels.com> wrote:
> 
> > On 04/11/2012 05:26 PM, Andrew Morton wrote:
> > >>
> > >> >    failed:
> > >> >  -	BUG();
> > >> >    	unlock_page(page);
> > >> >    	page_cache_release(page);
> > >> >    	return NULL;
> > > Cute.
> > >
> > > AFAICT what happened was that in my April 2002 rewrite of this code I
> > > put a non-fatal buffer_error() warning in that case to tell us that
> > > something bad happened.
> > >
> > > Years later we removed the temporary buffer_error() and mistakenly
> > > replaced that warning with a BUG().  Only it*can*  happen.
> > >
> > > We can remove the BUG() and fix up callers, or we can pass retry=1 into
> > > alloc_page_buffers(), so grow_dev_page() "cannot fail".  Immortal
> > > functions are a silly fiction, so we should remove the BUG() and fix up
> > > callers.
> > >
> > Any particular caller you are concerned with ?
> 
> Didn't someone see a buggy caller in btrfs?

No I missed that __getblk (__getblk_slow) returns NULL only if
grow_buffers < 0 while it returns 0 for the allocation failure.

Sorry for confusion.
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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Re: [PATCH] remove BUG() in possible but rare condition

[Michal Hocko]

On Wed 11-04-12 15:10:24, Glauber Costa wrote:
> While stressing the kernel with with failing allocations today,
> I hit the following chain of events:
> 
> alloc_page_buffers():
> 
> 	bh = alloc_buffer_head(GFP_NOFS);
> 	if (!bh)
> 		goto no_grow; <= path taken
> 
> grow_dev_page():
>         bh = alloc_page_buffers(page, size, 0);
>         if (!bh)
>                 goto failed;  <= taken, consequence of the above
> 
> and then the failed path BUG()s the kernel.
> 
> The failure is inserted a litte bit artificially, but even then,
> I see no reason why it should be deemed impossible in a real box.
> 
> Even though this is not a condition that we expect to see
> around every time, failed allocations are expected to be handled,
> and BUG() sounds just too much. As a matter of fact, grow_dev_page()
> can return NULL just fine in other circumstances, so I propose we just
> remove it, then.

I had to be blind yesterday. I have double checked the call chain and
this is safe already because __getblk_slow tries to free some memory and
then retry the allocation if it gets allocation failure from
grow_buffers (grow_dev_page).

> 
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Andrew Morton <akpm@linux-foundation.org>

Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  fs/buffer.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 36d6665..351e18e 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -985,7 +985,6 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>  	return page;
>  
>  failed:
> -	BUG();
>  	unlock_page(page);
>  	page_cache_release(page);
>  	return NULL;
> -- 
> 1.7.7.6
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>