[PATCH] drop_buffers() shouldn't de-ref page->mapping if its NULL

[PATCH] drop_buffers() shouldn't de-ref page->mapping if its NULL

[Badari Pulavarty]

[-- Attachment #1: Type: text/plain, Size: 2599 bytes --]

Hi,

I answered my own question. It looks like we could have pages
with buffers without page->mapping. In such cases, we shouldn't
de-ref page->mapping in drop_buffers(). Here is the trivial
patch to fix it.

Thanks,
Badari

On Wed, 2005-04-27 at 16:38, Badari Pulavarty wrote:
> Hi Andrew,
> 
> We ran into a panic in drop_buffers() while running some networking
> tests and I am wondering if this a valid case. try_to_free_buffers()
> seems to call drop_buffers() even if the mapping is NULL. drop_buffers()
> seems to de-ref the mapping. This is causing NULL pointer deref.
> 
> But, is "mapping == NULL" still valid case here ? Can we be in the
> code to drop buffers and have mapping NULL ? We would be in this
> code only if PagePrivate() is set. Can we have page private with
> out a valid mapping ?
> 
> Thanks,
> Badari
> 
> int try_to_free_buffers(struct page *page)
> {
>         struct address_space * const mapping = page->mapping;
>         ....
>                                                                                                                        
>         if (mapping == NULL) {          /* can this still happen? */
>                 ret = drop_buffers(page, &buffers_to_free);
>                 goto out;
>         }
> }
> 
> drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
> {
>         ....
>                 if (buffer_write_io_error(bh))
>                         set_bit(AS_EIO, &page->mapping->flags); <<<<<<
> 	...
> }
> 
> 1:mon> e
> cpu 0x1: Vector: 300 (Data Access) at [c00000007ff4b620]
>     pc: c0000000000bd524: .drop_buffers+0x40/0xcc
>     lr: c0000000000bd614: .try_to_free_buffers+0x64/0xf4
>     sp: c00000007ff4b8a0
>    msr: 8000000000009032
>    dar: 60
>  dsisr: 40000000
>   current = 0xc00000000fe7e040
>   paca    = 0xc0000000003da800
>     pid   = 40, comm = kswapd1
> 
> 1:mon> t
> [c00000007ff4b920] c0000000000bd614 .try_to_free_buffers+0x64/0xf4
> [c00000007ff4b9c0] c0000000000baadc .try_to_release_page+0x88/0x9c
> [c00000007ff4ba40] c000000000099418 .shrink_list+0x3a0/0x608
> [c00000007ff4bb90] c000000000099a04 .shrink_cache+0x384/0x610
> [c00000007ff4bcd0] c00000000009a4d4 .shrink_zone+0x104/0x140
> [c00000007ff4bd70] c00000000009aaf0 .balance_pgdat+0x270/0x448
> [c00000007ff4be90] c00000000009ade4 .kswapd+0x11c/0x120
> [c00000007ff4bf90] c000000000018ad0 .kernel_thread+0x4c/0x6c
> 
> 
> 
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[-- Attachment #2: drop_buffer_fix.patch --]
[-- Type: text/plain, Size: 450 bytes --]

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
--- linux-2.6.12-rc2.org/fs/buffer.c	2005-04-27 07:19:44.000000000 -0700
+++ linux-2.6.12-rc2/fs/buffer.c	2005-04-27 07:20:34.000000000 -0700
@@ -2917,7 +2917,7 @@ drop_buffers(struct page *page, struct b
 
 	bh = head;
 	do {
-		if (buffer_write_io_error(bh))
+		if (buffer_write_io_error(bh) && page->mapping)
 			set_bit(AS_EIO, &page->mapping->flags);
 		if (buffer_busy(bh))
 			goto failed;

Re: [PATCH] drop_buffers() shouldn't de-ref page->mapping if its NULL

[OGAWA Hirofumi]

Badari Pulavarty <pbadari@us.ibm.com> writes:

> Hi,
>
> I answered my own question. It looks like we could have pages
> with buffers without page->mapping. In such cases, we shouldn't
> de-ref page->mapping in drop_buffers(). Here is the trivial
> patch to fix it.
>
> Thanks,
> Badari

[...]

>
> Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
> --- linux-2.6.12-rc2.org/fs/buffer.c	2005-04-27 07:19:44.000000000 -0700
> +++ linux-2.6.12-rc2/fs/buffer.c	2005-04-27 07:20:34.000000000 -0700
> @@ -2917,7 +2917,7 @@ drop_buffers(struct page *page, struct b
>  
>  	bh = head;
>  	do {
> -		if (buffer_write_io_error(bh))
> +		if (buffer_write_io_error(bh) && page->mapping)
>  			set_bit(AS_EIO, &page->mapping->flags);
>  		if (buffer_busy(bh))
>  			goto failed;

On my experience, this happened the bh leak case only.

If you are not sure whether this is valid state or not, I worry this
patch hides real bug.  How about adding the warning, not just remove
de-ref?

Thanks.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Re: [PATCH] drop_buffers() shouldn't de-ref page->mapping if its NULL

[Badari Pulavarty]

On Wed, 2005-04-27 at 20:46, OGAWA Hirofumi wrote:
> Badari Pulavarty <pbadari@us.ibm.com> writes:
> 
> > Hi,
> >
> > I answered my own question. It looks like we could have pages
> > with buffers without page->mapping. In such cases, we shouldn't
> > de-ref page->mapping in drop_buffers(). Here is the trivial
> > patch to fix it.
> >
> > Thanks,
> > Badari
> 
> [...]
> 
> >
> > Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
> > --- linux-2.6.12-rc2.org/fs/buffer.c	2005-04-27 07:19:44.000000000 -0700
> > +++ linux-2.6.12-rc2/fs/buffer.c	2005-04-27 07:20:34.000000000 -0700
> > @@ -2917,7 +2917,7 @@ drop_buffers(struct page *page, struct b
> >  
> >  	bh = head;
> >  	do {
> > -		if (buffer_write_io_error(bh))
> > +		if (buffer_write_io_error(bh) && page->mapping)
> >  			set_bit(AS_EIO, &page->mapping->flags);
> >  		if (buffer_busy(bh))
> >  			goto failed;
> 
> On my experience, this happened the bh leak case only.


Could you explain more on bh leak ? Is there one in the current code ?

> 
> If you are not sure whether this is valid state or not, I worry this
> patch hides real bug.  How about adding the warning, not just remove
> de-ref?

Andrew confirmed that this is a valid case.

I don't understand what you want to do here ? If the mapping is NULL,
we can't de-ref it.  Whats the point in putting a warning and de-refing
it. Its going to cause NULL pointer de-ref anyway.

Thanks,
Badari

Re: [PATCH] drop_buffers() shouldn't de-ref page->mapping if its NULL

[OGAWA Hirofumi]

Badari Pulavarty <pbadari@us.ibm.com> writes:

> Andrew confirmed that this is a valid case.
>
> I don't understand what you want to do here ? If the mapping is NULL,
> we can't de-ref it.  Whats the point in putting a warning and de-refing
> it. Its going to cause NULL pointer de-ref anyway.

I meant your patch + warning. If it is just bh leak, not valid state,
I thought we can notice the leak of bh by warning.

I wanted above things. If it's valid state, of course warning is just
crap.

Sorry for noise.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>