From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 7 May 2008 14:25:39 -0700 From: Andrew Morton Subject: Re: [PATCH] more ZERO_PAGE handling in follow_page() Message-Id: <20080507142539.758d30f6.akpm@linux-foundation.org> In-Reply-To: <20080507163643.d4da0ed0.kamezawa.hiroyu@jp.fujitsu.com> References: <20080507163643.d4da0ed0.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: KAMEZAWA Hiroyuki Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, nickpiggin@yahoo.com.au, tonyb@cybernetics.com, mika.penttila@kolumbus.fi List-ID: On Wed, 7 May 2008 16:36:43 +0900 KAMEZAWA Hiroyuki wrote: > Rewrote the description of patch. (no changes in the logic.) > > Thank you for all help. > -Kame > == > follow_page() is called from get_user_pages(), which returns specified user page. > follow_page() can return 1) a page or 2) NULL or 3)ZERO_PAGE. > If NULL, handle_mm_fault() is called. > > Now, follow_page() to unused pte returns NULL if page table exists. As a result > get_user_pages() calls handle_mm_fault() and allocate new memory. > This behavior increases memory consumption at coredump, which does > read-once-but-never-written page fault. > By returning ZERO_PAGE() against READ/ANON request, we can avoid it. > > (Because exec's arguments copy needs to call handle_mm_fault at WRITE/ANON > request, we just handle READ/ANON case here.) > > Change log: > - Rewrote patch description and Added comments. > - fixed to check pte_present()/pte_none() in proper way. So... how serious is the problem which we're fixing here? I can see that if one is core-dumping large sparse address spaces this could improve things a lot, but please help us understand the implications so we can decide whether we need this in 2.6.26, thanks. > Index: linux-2.6.25/mm/memory.c > =================================================================== > --- linux-2.6.25.orig/mm/memory.c > +++ linux-2.6.25/mm/memory.c > @@ -926,15 +926,15 @@ struct page *follow_page(struct vm_area_ > page = NULL; > pgd = pgd_offset(mm, address); > if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd))) > - goto no_page_table; > + goto null_or_zeropage; > > pud = pud_offset(pgd, address); > if (pud_none(*pud) || unlikely(pud_bad(*pud))) > - goto no_page_table; > + goto null_or_zeropage; > > pmd = pmd_offset(pud, address); > if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd))) The mainline kernel does not have " || unlikely(pmd_bad(*pmd))" here. That got changed yesterday by commit aeed5fce37196e09b4dac3a1c00d8b7122e040ce Author: Hugh Dickins Date: Tue May 6 20:49:23 2008 +0100 x86: fix PAE pmd_bad bootup warning So please confirm that the patch which I merged is still OK (I'd be surprised if it isn't...) -- 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