Re: oops in copy_page_rep()

[Simon Jeons <simon.jeons@gmail.com>]

On Tue, 2013-01-08 at 18:49 +0100, Andrea Arcangeli wrote:
> Hi Kirill,
> 
> On Tue, Jan 08, 2013 at 07:30:58PM +0200, Kirill A. Shutemov wrote:
> > Merged patch is obviously broken: huge_pmd_set_accessed() can be called
> > only if the pmd is under splitting.
> 
> Of course I assume you meant "only if the pmd is not under splitting".
> 
> But no, setting a bitflag like the young bit or clearing or setting
> the numa bit won't screw with split_huge_page and it's safe even if
> the pmd is under splitting.
> 
> Those bits are only checked here at the last stage of
> split_huge_page_map after taking the PT lock:
> 
> 	spin_lock(&mm->page_table_lock);
> 	pmd = page_check_address_pmd(page, mm, address,
> 				     PAGE_CHECK_ADDRESS_PMD_SPLITTING_FLAG);
> 	if (pmd) {
> 		pgtable = pgtable_trans_huge_withdraw(mm);
> 		pmd_populate(mm, &_pmd, pgtable);
> 
> 		haddr = address;
> 		for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
> 			pte_t *pte, entry;
> 			BUG_ON(PageCompound(page+i));
> 			entry = mk_pte(page + i, vma->vm_page_prot);
> 			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> 			if (!pmd_write(*pmd))
> 				entry = pte_wrprotect(entry);
> 			else
> 				BUG_ON(page_mapcount(page) != 1);
> 			if (!pmd_young(*pmd))
> 				entry = pte_mkold(entry);
> 			if (pmd_numa(*pmd))
> 				entry = pte_mknuma(entry);
> 			pte = pte_offset_map(&_pmd, haddr);
> 			BUG_ON(!pte_none(*pte));
> 			set_pte_at(mm, haddr, pte, entry);
> 			pte_unmap(pte);
> 		}
> 
> If "young" or "numa" bitflags changed on the original *pmd for the
> previous part of split_huge_page, nothing will go wrong by the time we
> get to split_huge_page_map (the same is not true if the pfn changes!).
> 

But this time BUG_ON(mapcount != mapcount2) in function
__split_huge_page will be trigged.

> If you think this is too tricky, we could also decide to forbid
> huge_pmd_set_accessed if the pmd is in splitting state, but I don't
> think that flipping young/numa bits while in splitting state, can
> cause any problem (if done correctly with PT lock + pmd_same).
> 
> Thanks!
> 
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