From mboxrd@z Thu Jan 1 00:00:00 1970 From: KOSAKI Motohiro Subject: Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page Date: Tue, 14 Apr 2009 22:39:54 +0900 Message-ID: <2f11576a0904140639l426e137ewdc46296cdb377dd@mail.gmail.com> References: <20090414151204.C647.A69D9226@jp.fujitsu.com> <200904141925.46012.nickpiggin@yahoo.com.au> <2f11576a0904140502h295faf33qcea9a39ff7f230a5@mail.gmail.com> <200904142225.10788.nickpiggin@yahoo.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: LKML , Linus Torvalds , Andrew Morton , Andrea Arcangeli , Jeff Moyer , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Hugh Dickins To: Nick Piggin Return-path: Received: from wf-out-1314.google.com ([209.85.200.173]:32912 "EHLO wf-out-1314.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752260AbZDNNjz convert rfc822-to-8bit (ORCPT ); Tue, 14 Apr 2009 09:39:55 -0400 In-Reply-To: <200904142225.10788.nickpiggin@yahoo.com.au> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: >> >> @@ -547,7 +549,13 @@ int reuse_swap_page(struct page *page) >> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 SetPageDirty(page); >> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 } >> >> =A0 =A0 =A0 } >> >> - =A0 =A0 return count =3D=3D 1; >> >> + >> >> + =A0 =A0 /* >> >> + =A0 =A0 =A0* If we can re-use the swap page _and_ the end >> >> + =A0 =A0 =A0* result has only one user (the mapping), then >> >> + =A0 =A0 =A0* we reuse the whole page >> >> + =A0 =A0 =A0*/ >> >> + =A0 =A0 return count + page_count(page) =3D=3D 2; >> >> =A0} >> > >> > I guess this patch does work to close the read-side race, but I sl= ightly don't >> > like using page_count for things like this. page_count can be temp= orarily >> > raised for reasons other than access through their user mapping. S= wapcache, >> > page reclaim, LRU pagevecs, concurrent do_wp_page, etc. >> >> Yes, that's trade-off. >> your early decow also can misjudge and make unnecessary copy. > > Yes indeed it can. Although it would only ever do so in case of pages > that have had get_user_pages run against them previously, and not fro= m > random interactions from any other parts of the kernel. Agreed. > I would be interested, using an anon vma field as you say for keeping > a gup count... it could potentially be used to avoid the extra copy. > But hmm, I don't have much time to go down that path so long as the > basic concept of my proposal is in question. ok, I try to make it. thanks. > + =A0 =A0 if (PageSwapCache(page) && > + =A0 =A0 =A0 =A0 page_count(page) !=3D page_mapcount(page) + 2) { > + =A0 =A0 =A0 =A0 =A0 =A0 ret =3D SWAP_FAIL; > + =A0 =A0 =A0 =A0 =A0 =A0 goto out_unmap; > + =A0 =A0 } > > Now if another thread does a get_user_pages_fast after it passes this > check, it can take a gup reference to the page which is now about to > be unmapped. Then after it is unmapped, if a wp fault is caused on th= e > page, then it will not be reused and thus you lose data as explained > in your big comment. Grrr, I lose. I've misunderstood get_user_pages_fast() also grab pte_lo= ck. I must think it again. I guess you dislike get_user_page_fast() grab pte_lock too, right? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html