From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with SMTP id B65A76B007D for ; Wed, 10 Mar 2010 17:19:08 -0500 (EST) Date: Wed, 10 Mar 2010 23:19:03 +0100 From: Andrea Arcangeli Subject: Re: mm/ksm.c seems to be doing an unneeded _notify. Message-ID: <20100310221903.GC5967@random.random> References: <20100310191842.GL5677@sgi.com> <4B97FED5.2030007@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4B97FED5.2030007@redhat.com> Sender: owner-linux-mm@kvack.org To: Izik Eidus Cc: Robin Holt , Hugh Dickins , Chris Wright , linux-mm@kvack.org List-ID: On Wed, Mar 10, 2010 at 10:19:33PM +0200, Izik Eidus wrote: > On 03/10/2010 09:18 PM, Robin Holt wrote: > > While reviewing ksm.c, I noticed that ksm.c does: > > > > if (pte_write(*ptep)) { > > pte_t entry; > > > > swapped = PageSwapCache(page); > > flush_cache_page(vma, addr, page_to_pfn(page)); > > /* > > * Ok this is tricky, when get_user_pages_fast() run it doesnt > > * take any lock, therefore the check that we are going to make > > * with the pagecount against the mapcount is racey and > > * O_DIRECT can happen right after the check. > > * So we clear the pte and flush the tlb before the check > > * this assure us that no O_DIRECT can happen after the check > > * or in the middle of the check. > > */ > > entry = ptep_clear_flush(vma, addr, ptep); > > /* > > * Check that no O_DIRECT or similar I/O is in progress on the > > * page > > */ > > if (page_mapcount(page) + 1 + swapped != page_count(page)) { > > set_pte_at_notify(mm, addr, ptep, entry); > > goto out_unlock; > > } > > entry = pte_wrprotect(entry); > > set_pte_at_notify(mm, addr, ptep, entry); > > > > > > I would think the error case (where the page has an elevated page_count) > > should not be using set_pte_at_notify. In that event, you are simply > > restoring the previous value. Have I missed something or is this an > > extraneous _notify? > > > > Yes, I think you are right set_pte_at(mm, addr, ptep, entry); would be > enough here. > > I can`t remember or think any reason why I have used the _notify... > > Lets just get ACK from Andrea and Hugh that they agree it isn't needed _notify it's needed, we're downgrading permissions here. -- 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