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SubjectRe: [PATCH, V2] i386: instead of poisoning .init zone, change protection bits to force a fault
Eric Dumazet <dada1@cosmosbay.com> wrote:
>
> > --- devel/arch/i386/mm/init.c~i386-instead-of-poisoning-init-zone-change-protection-fix 2006-02-04 14:33:33.000000000 -0800
> > +++ devel-akpm/arch/i386/mm/init.c 2006-02-04 14:34:07.000000000 -0800
> > @@ -751,11 +751,15 @@ void free_init_pages(char *what, unsigne
> > ClearPageReserved(virt_to_page(addr));
> > set_page_count(virt_to_page(addr), 1);
> > #ifdef CONFIG_DEBUG_INITDATA
> > + /*
> > + * Unmap the page, and leak it. So any further accesses will
> > + * oops.
> > + */
> > change_page_attr(virt_to_page(addr), 1, __pgprot(0));
> > #else
> > memset((void *)addr, 0xcc, PAGE_SIZE);
> > -#endif
> > free_page(addr);
> > +#endif
> > totalram_pages++;
> > }
> > printk(KERN_INFO "Freeing %s: %ldk freed\n", what, (end - begin) >> 10);
> > _
> >
> > But is there much point in doing this? Does it offer much more than
> > CONFIG_DEBUG_PAGEALLOC?
> >
> >
>
> 1) CONFIG_DEBUG_PAGEALLOC is very generic (and expensive), while
> CONFIG_DEBUG_INITDATA only makes sure init data becomes unreadable.
>
> 2) If CONFIG_DEBUG_INITDATA is on, the redzone is in action in the virtual
> memory that hold the initdata, while the physical pages that contained the
> initdata where freed and might be reused for other needs.
>
> I think we have two different things here : Virtual mem redzoning (my patch),
> and physical ram poisoning (your patch).
>
> CONFIG_DEBUG_INITDATA uses only a virtual mem redzoning (no underlying memory
> cost, apart of the page tables), while your solution doesnt free the pages,
> and the poisoining wont catch further accesses, just make some results funny
> or false.
>
> The only bad effect of my patch is about the TLB cost, because of the
> hugepage(s) that should revert to 512 normal 4K pages.

Your patch made my kernel oops! The oops was prevented by either enabling
CONFIG_DEBUG_PAGEALLOC or by the above patch.
-
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