Re: oops in copy_page_rep()

[Andrea Arcangeli <aarcange@redhat.com>]

On Fri, Jan 11, 2013 at 01:50:44AM -0600, Simon Jeons wrote:
> 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.

"young" or "numa" bitflags in the pmd don't alter
rmap/mapcount/pagecount/pfn or anything that could affect such BUG_ON,
so I'm not sure why you think so.

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