Return-Path: Received: from mail-wm0-f51.google.com ([74.125.82.51]:35362 "EHLO mail-wm0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751565AbbKAKAZ (ORCPT ); Sun, 1 Nov 2015 05:00:25 -0500 Message-ID: <5635E2B4.5070308@electrozaur.com> Date: Sun, 01 Nov 2015 12:00:20 +0200 From: Boaz Harrosh MIME-Version: 1.0 To: Hugh Dickins CC: Andrew Morton , Benny Halevy , Trond Myklebust , Christoph Lameter , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, osd-dev@open-osd.org Subject: Re: [PATCH] osd fs: __r4w_get_page rely on PageUptodate for uptodate References: In-Reply-To: Content-Type: text/plain; charset=utf-8 Sender: linux-nfs-owner@vger.kernel.org List-ID: On 10/29/2015 08:43 PM, Hugh Dickins wrote: > Patch "mm: migrate dirty page without clear_page_dirty_for_io etc", > presently staged in mmotm and linux-next, simplifies the migration of > a PageDirty pagecache page: one stat needs moving from zone to zone > and that's about all. > > It's convenient and safest for it to shift the PageDirty bit from old > page to new, just before updating the zone stats: before copying data > and marking the new PageUptodate. This is all done while both pages > are isolated and locked, just as before; and just as before, there's > a moment when the new page is visible in the radix_tree, but not yet > PageUptodate. What's new is that it may now be briefly visible as > PageDirty before it is PageUptodate. > > When I scoured the tree to see if this could cause a problem anywhere, > the only places I found were in two similar functions __r4w_get_page(): > which look up a page with find_get_page() (not using page lock), then > claim it's uptodate if it's PageDirty or PageWriteback or PageUptodate. > > I'm not sure whether that was right before, but now it might be wrong > (on rare occasions): only claim the page is uptodate if PageUptodate. > Or perhaps the page in question could never be migratable anyway? > Hi Sir Hugh I'm sorry, I admit the code is clear as mud, but your patch below is wrong. The *uptodate return from __r4w_get_page is not really "up-to-date" at all actually it means: "do we need to read the page from storage" writable/dirty pages we do not read from storage but use the newest data in memory. r4w means read-for-write which is when we need to bring in the full stripe to re-calculate raid5/6 . (when only the partial stripe is written) The scenario below of: "briefly visible as PageDirty before it is PageUptodate" is fine in this case because in both cases we do not need to read the page. Thanks for looking Boaz > Signed-off-by: Hugh Dickins This patch is not correct! > --- > > fs/exofs/inode.c | 5 +---- > fs/nfs/objlayout/objio_osd.c | 5 +---- > 2 files changed, 2 insertions(+), 8 deletions(-) > > --- 4.3-next/fs/exofs/inode.c 2015-08-30 11:34:09.000000000 -0700 > +++ linux/fs/exofs/inode.c 2015-10-28 16:55:18.795554294 -0700 > @@ -592,10 +592,7 @@ static struct page *__r4w_get_page(void > } > unlock_page(page); > } > - if (PageDirty(page) || PageWriteback(page)) > - *uptodate = true; > - else > - *uptodate = PageUptodate(page); > + *uptodate = PageUptodate(page); > EXOFS_DBGMSG2("index=0x%lx uptodate=%d\n", index, *uptodate); > return page; > } else { > --- 4.3-next/fs/nfs/objlayout/objio_osd.c 2015-10-21 18:35:07.620645439 -0700 > +++ linux/fs/nfs/objlayout/objio_osd.c 2015-10-28 16:53:55.083686639 -0700 > @@ -476,10 +476,7 @@ static struct page *__r4w_get_page(void > } > unlock_page(page); > } > - if (PageDirty(page) || PageWriteback(page)) > - *uptodate = true; > - else > - *uptodate = PageUptodate(page); > + *uptodate = PageUptodate(page); > dprintk("%s: index=0x%lx uptodate=%d\n", __func__, index, *uptodate); > return page; > } >