Re: [BUGFIX][PATCH -tip] x86: fix text_poke to handle highmem pages

Masami Hiramatsu
2009-04-08T14:58:59Z

Ingo Molnar wrote:
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>
>> @@ -514,27 +515,39 @@ void *__kprobes text_poke(void *addr, const void *opcode, size_t len)
>> {
>> unsigned long flags;
>> char *vaddr;
>> - struct page *pages[2];
>> + struct page *page;
>> int i;
>> + unsigned long endp = ((unsigned long)addr + len) & PAGE_MASK;
>>
>> - if (!core_kernel_text((unsigned long)addr)) {
>> - pages[0] = vmalloc_to_page(addr);
>> - pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
>> - } else {
>> - pages[0] = virt_to_page(addr);
>> - WARN_ON(!PageReserved(pages[0]));
>> - pages[1] = virt_to_page(addr + PAGE_SIZE);
>> + /*
>> + * If the written range covers 2 pages, we'll split it, because
>> + * vmalloc pages are not always continuous -- e.g. 1st page is
>> + * lowmem and 2nd page is highmem.
>> + */
>> + if (((unsigned long)addr & PAGE_MASK) != endp) {
>> + text_poke(addr, opcode, endp - (unsigned long)addr);
>> + addr = (void *)endp;
>> + opcode = (char *)opcode + (endp - (unsigned long)addr);
>> + len -= endp - (unsigned long)addr;
>> }
>> - BUG_ON(!pages[0]);
>> +
>> + if (!core_kernel_text((unsigned long)addr))
>> + page = vmalloc_to_page(addr);
>> + else
>> + page = virt_to_page(addr);
>
> hm, the bug is upstream now. And your fix turns a
> supposed-to-be-simpler kmap based patching thing back into something
> fragile looking again. We might be better off with a revert - or we
> do a real clean patch.
>
> Firstly, that core_kernel_text() distinction above looks
> artificially open-coded - dont we have a proper, generic
> "look-up-the-page" variant in the MM somewhere?

Actually, vmalloc_to_page() is generic one. It decodes
the kernel page table directly to find struct page *.
virt_to_page() is just a short-cut api.

>
>> + BUG_ON(!page);
>> local_irq_save(flags);
>> - set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
>> - if (pages[1])
>> - set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
>> - vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
>> + if (PageHighMem(page))
>> + vaddr = kmap_atomic(page, KM_TEXT_POKE);
>> + else {
>> + set_fixmap(FIX_TEXT_POKE, page_to_phys(page));
>> + vaddr = (char *)fix_to_virt(FIX_TEXT_POKE);
>> + }
>
> that too looks artificially complex. Why cannot we kmap lowmem pages
> too? If the API isnt available on !HIGHMEM kernels .. then the
> solution is to make it available, not to branch our way around it.

Hmm, why don't we enhance fixmap to handle highmem pages?
(e.g. adding set_fixmap_page())
Since kmap is only for highmem kernels, I think changing it will effects
more users...

Thank you,

>
>> memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
>> - clear_fixmap(FIX_TEXT_POKE0);
>> - if (pages[1])
>> - clear_fixmap(FIX_TEXT_POKE1);
>> + if (PageHighMem(page))
>> + kunmap_atomic(vaddr, KM_TEXT_POKE);
>> + else
>> + clear_fixmap(FIX_TEXT_POKE);
>
> ditto.
>
> Thanks,
>
> Ingo