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    SubjectRe: oops in copy_page_rep()
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    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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