From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx167.postini.com [74.125.245.167]) by kanga.kvack.org (Postfix) with SMTP id 4853B6B005A for ; Tue, 8 Jan 2013 12:49:55 -0500 (EST) Date: Tue, 8 Jan 2013 18:49:51 +0100 From: Andrea Arcangeli Subject: Re: oops in copy_page_rep() Message-ID: <20130108174951.GG9163@redhat.com> References: <20130105152208.GA3386@redhat.com> <20130108163141.GA27555@shutemov.name> <20130108173058.GA27727@shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130108173058.GA27727@shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Linus Torvalds , Hillf Danton , Hugh Dickins , Dave Jones , Linux Kernel , Andrew Morton , Mel Gorman , Linux-MM , Rik van Riel 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! -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org