linux-kernel - Re: oops in copy_page

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Message-ID: <20130108174951.GG9163@redhat.com>
Date:	Tue, 8 Jan 2013 18:49:51 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hillf Danton <dhillf@...il.com>,
	Hugh Dickins <hughd@...gle.com>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux-MM <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: oops in copy_page_rep()

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!).

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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