linux-kernel - Re: [PATCH] parisc: fix module loading failure of large kernel modules (take 2)

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Message-Id: <200812310915.41693.rusty@rustcorp.com.au>
Date:	Wed, 31 Dec 2008 09:15:40 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Helge Deller <deller@....de>
Cc:	"linux-parisc" <linux-parisc@...r.kernel.org>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	Kyle McMartin <kyle@...artin.ca>,
	Randolph Chung <randolph@...sq.org>,
	Linus <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	John David Anglin <dave@...uly1.hia.nrc.ca>
Subject: Re: [PATCH] parisc: fix module loading failure of large kernel modules (take 2)

On Tuesday 30 December 2008 07:04:54 Helge Deller wrote:
> This is the second take of the patch series.
> Changes to previous version:
> - new CONFIG_HAVE_MODULE_SECTION_STUBS config option
> - put stub entries of a code section in front of the section
> 
> ____________
> The parisc port (esp. the 32bit kernel) currently lacks the ability to
> load large kernel modules like xfs or ipv6. This is a long outstanding
> bug and has already been reported a few times, e.g.:
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=350482,
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=401439,
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=508489
> 
> The symptom is like this:
> # modprobe xfs
> FATAL: Error inserting xfs
> (/lib/modules/2.6.26-1-parisc/kernel/fs/xfs/xfs.ko): Invalid module
> format
> 
> In dmesg:
> module xfs relocation of symbol xfs_btree_read_bufs is out of range
> (0x3ffefffe in 17 bits)
> 
> The reason for the failure is, that the architecture only provides the
> R_PARISC_PCREL17F (for 32bit kernels) and R_PARISC_PCREL22F (for PA2.0
> and 64bit kernels) relocations, which sometimes can't reach the target
> address of the stub entry if the kernel module is too large. Currently
> parisc (like other architectures) creates one big PLT section for all
> stubs at the beginning of the init and core sections.
> 
> The following two patches changes the parisc module loader to put stubs
> for the code sections in front of each section, so that the distance to
> the stubs more easily fits into the available 17/22 bits.

So now any one section has to pass 17 bits to break?  How close are you with
the xfs module?

But it's kind of nasty, overloading sh_entsize further.  Could we instead
do something like add a arch_module_section_size() weak fn which you can
overload?  We'd use that in get_offset() so our layout and size calculations
were correct, and use sh_size everywhere else.

Cheers,
Rusty.
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