Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

[Konstantin Khlebnikov]

On 2014-11-18 20:34, Russell King - ARM Linux wrote:
> On Tue, Nov 18, 2014 at 08:21:46PM +0400, Konstantin Khlebnikov wrote:
> > Usually modules are loaded into small area prior to the kernel
> > text because they are linked with the kernel using short calls.
> > Compile-time instrumentation like GCOV or KASAN bloats code a lot,
> > and as a result huge modules no longer fit into reserved area.
> >
> > This patch adds option CONFIG_MODULES_USE_VMALLOC which lifts
> > limitation on amount of loaded modules. It links modules using
> > long-calls (option -mlong-calls) and loads them into vmalloc area.
> >
> > In few places exported symbols are called from inline assembly.
> > This patch adds macro for such call sites: __asmbl and __asmbl_clobber.
> > Call turns into single 'bl' or sequence 'movw; movt; blx' depending on
> > context and state of config option.
> >
> > Unfortunately this option isn't compatible with CONFIG_FUNCTION_TRACER.
> > Compiler emits short calls to profiling function despite of -mlong-calls.
> > This is a bug in GCC, but ftrace anyway needs an update to handle this.
> It also isn't compatible with the older architectures which don't have
> "blx".

Ok, I'll add "depends on CPU_V6 || CPU_V7" I don't think that it is 
necessary for older cpus.

> > diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> > index 9d2cdda..9e0c4f4 100644
> > --- a/arch/arm/mm/mmu.c
> > +++ b/arch/arm/mm/mmu.c
> > @@ -1161,16 +1161,15 @@ void __init sanity_check_meminfo(void)
> >   
> >   static inline void prepare_page_table(void)
> >   {
> > -	unsigned long addr;
> > +	unsigned long addr = 0;
> >   	phys_addr_t end;
> >   
> >   	/*
> >   	 * Clear out all the mappings below the kernel image.
> >   	 */
> > -	for (addr = 0; addr < MODULES_VADDR; addr += PMD_SIZE)
> > -		pmd_clear(pmd_off_k(addr));
> > -
> >   #ifdef CONFIG_XIP_KERNEL
> > +	for ( ; addr < MODULES_VADDR; addr += PMD_SIZE)
> > +		pmd_clear(pmd_off_k(addr));
> >   	/* The XIP kernel is mapped in the module area -- skip over it */
> >   	addr = ((unsigned long)_etext + PMD_SIZE - 1) & PMD_MASK;
> >   #endif
> This seems to be an unrelated change.  In any case, it looks wrong to
> me.  Please describe what you are trying to achieve here.

My patch changes  MODULES_VADDR, if option enabled it becomes alias for 
VMALLOC_START

It's used as border between two loops, before patch this code looks like:

    /*
     * Clear out all the mappings below the kernel image.
     */
    for (addr = 0; addr < MODULES_VADDR; addr += PMD_SIZE)
        pmd_clear(pmd_off_k(addr));

#ifdef CONFIG_XIP_KERNEL
    /* The XIP kernel is mapped in the module area -- skip over it */
    addr = ((unsigned long)_etext + PMD_SIZE - 1) & PMD_MASK;
#endif
    for ( ; addr < PAGE_OFFSET; addr += PMD_SIZE)
        pmd_clear(pmd_off_k(addr));

It's written with assumption that MODULES_VADDR < PAGE_OFFSET,
but this no longer true if modules are placed inside vmalloc area.


After patch:

    unsigned long addr = 0;

    /*
     * Clear out all the mappings below the kernel image.
     */
#ifdef CONFIG_XIP_KERNEL
    for ( ; addr < MODULES_VADDR; addr += PMD_SIZE)
        pmd_clear(pmd_off_k(addr));
    /* The XIP kernel is mapped in the module area -- skip over it */
    addr = ((unsigned long)_etext + PMD_SIZE - 1) & PMD_MASK;
#endif
    for ( ; addr < PAGE_OFFSET; addr += PMD_SIZE)
        pmd_clear(pmd_off_k(addr));


Without XIP there is no reason for splitting it into two loops.
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