Re: String literals in __init functions

[Joe Perches <joe@perches.com>]

On Thu, 2015-03-26 at 13:40 +0100, Mason wrote:
> On 25/03/2015 19:01, Joe Perches wrote:
> > On Wed, 2015-03-25 at 18:56 +0100, Mason wrote:
> >
> >> AFAIU, functions only used at system init are tagged __init to have
> >> the linker store them in a separate .init.text section, so memory can
> >> be reclaimed once initialization is complete. Is that correct?
> >>
> >> The corresponding tag for data is __initdata (section .init.data)
> >>
> >> I started wondering if the string literals used in an __init functions
> >> were automatically marked __initdata.
> >>
> >> Looking at the objdump output, I see that the string literals are,
> >> in fact, stored in the .rodata section. I suppose that .rodata is NOT
> >> reclaimed after init?
> >>
> >> This way seems to work:
> >>
> >> static       char XyZa[] __initdata  = KERN_ALERT "foo";
> >> static const char XyZb[] __initconst = KERN_ALERT "bar";
> >> void __init XyZc(void) { printk(XyZa); printk(XyZb); }
> >>
> >> $ arm-linux-gnueabihf-objdump -xd arch/arm/mach-tangox/time.o | grep XyZ
> >> 00000000 l     O .init.data	00000006 XyZa
> >> 00000000 l     O .init.rodata	00000006 XyZb
> >> 00000000 g     F .init.text	00000028 XyZc
> >> 00000000 <XyZc>:
> >>
> >> $ arm-linux-gnueabihf-objdump -xd vmlinux | grep XyZ
> >> c021e360 l     O .init.data	00000006 XyZa
> >> c0220090 l     O .init.data	00000006 XyZb
> >> c020d928 g     F .init.text	00000028 XyZc
> >> c020d928 <XyZc>:
> >>
> >> c020d928 <XyZc>:
> >> c020d928:       e1a0c00d        mov     ip, sp
> >> c020d92c:       e92dd800        push    {fp, ip, lr, pc}
> >> c020d930:       e24cb004        sub     fp, ip, #4
> >> c020d934:       e30e0360        movw    r0, #58208      ; 0xe360
> >> c020d938:       e34c0021        movt    r0, #49185      ; 0xc021
> >> c020d93c:       ebfe00c9        bl      c018dc68 <printk>
> >> c020d940:       e3000090        movw    r0, #144        ; 0x90
> >> c020d944:       e34c0022        movt    r0, #49186      ; 0xc022
> >> c020d948:       ebfe00c6        bl      c018dc68 <printk>
> >> c020d94c:       e89da800        ldm     sp, {fp, sp, pc}
> >>
> >> Did I miss something in init.h?
> >> Or should it be done like above to reclaim string literals?
> >
> > No, you didn't miss anything.
> >
> > One proposal:
> >
> > https://lkml.org/lkml/2014/8/21/255
> 
> Thanks for the link!
> 
> Here's the equivalent gmane link for my own reference:
> http://thread.gmane.org/gmane.linux.kernel/1771969
> 
> Basically, if I understand correctly, Ingo NAKed the patch, saying
> this should be done automatically by the toolchain. That would make
> for an interesting side-project...

True.  It's probably not feasible though.

Tracking string deduplication/reuse would be pretty difficult.