From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id C4B0E6B00D0 for ; Thu, 11 Mar 2010 08:18:56 -0500 (EST) Message-ID: <4B98EE31.80502@redhat.com> Date: Thu, 11 Mar 2010 15:20:49 +0200 From: Izik Eidus MIME-Version: 1.0 Subject: Re: mm/ksm.c seems to be doing an unneeded _notify. References: <20100310191842.GL5677@sgi.com> <4B97FED5.2030007@redhat.com> <20100310221903.GC5967@random.random> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Hugh Dickins Cc: Andrea Arcangeli , Robin Holt , Chris Wright , linux-mm@kvack.org List-ID: On 03/11/2010 08:23 AM, Hugh Dickins wrote: > On Wed, 10 Mar 2010, Andrea Arcangeli wrote: > >> 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. >> > Robin is not questioning that it's needed in the success case; > but in the case where we back out because the counts don't match, > and just put back the original entry, he's suggesting that then > the _notify isn't needed. > Yes exactly, and at that 'counts don`t match' path - there is no need to call to _notify. > (I'm guessing that Robin is not making a significant improvement to KSM, > but rather trying to clarify his understanding of set_pte_at_notify.) > Yea, it won`t run unless at very rare cases > Hugh > -- 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