From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 69A5AC636D6 for ; Thu, 16 Feb 2023 21:59:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230258AbjBPV7L (ORCPT ); Thu, 16 Feb 2023 16:59:11 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48756 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229541AbjBPV7J (ORCPT ); Thu, 16 Feb 2023 16:59:09 -0500 Received: from mail-pj1-x1032.google.com (mail-pj1-x1032.google.com [IPv6:2607:f8b0:4864:20::1032]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E57AF3609C for ; Thu, 16 Feb 2023 13:59:07 -0800 (PST) Received: by mail-pj1-x1032.google.com with SMTP id d2so3278533pjd.5 for ; Thu, 16 Feb 2023 13:59:07 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=2cQYnkXv7L7qhiGEdO0H9rKipVy8lLGlyCZOqqsAcqU=; b=gmPBUl59wTFyJIl/E/EcCS3HT6nMuh+KY8yAitPi9tUu6iS10t+FlMb3mW+ybZOIio sqdUmnyeWF+vRVa/23T9dqWvrDOv8EBLsjOcjtBEDXHx/ImTx5XluaHbQZbPiqM4sQfr YLOgHIUMeHbnNFLEJNn3ZIhiGxYseq+rLjQviMKWdOdjnAJC9PSL3OMTcahyyWrAMETj 6FQmxMRPIX+wg+ohlEZTUyHpYYj6cVn0chYCycA7W/DK7ZfWAbZBkonMqPDTkrMP/czI i/ejcHcmF9FGMe2xxmGsc5nsK3CHSDkNXmfc8j8foKzzNJhlEKPljAwkMLTpHTj9pPhY k6NQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=2cQYnkXv7L7qhiGEdO0H9rKipVy8lLGlyCZOqqsAcqU=; b=iVNDgOcSjQMjIu9MtGr95ydnBKwpG5ispK6pbL9ed4iAi2bx+lueE8/hRGIyhMMmkH 3/j28Ztcwa/vLK2uhoh7puzKV0wm+muAP+agMb9/Lofm5zEWujJu/Ok355irTNk8LBvI pOrQaz6aq4JoMtUln4qjb9tQr7XX5OJ5+dyOdzekHvdg8TRQRJhz/JvJr/05mCJ9HI78 BFMkpz8stIwzKPyPbgUISa5WVnAtsWDOkChaaUz6lbu4X6pB4YoD40DOYXjNcaI5hzq3 eM9aUn0C/HxoyeG+ip0Vqu/Jw5GL+bWu0ZwpZ9W0EE7wqvY1wTfnlSTfsGZNzh9bcm0r OPbA== X-Gm-Message-State: AO0yUKVyVuLTIljs1qSDXj2uSrrCN6iFeQd/5GLe5/bHgP0A5IIA1+mM QTw31PfHG43DDjfheEDONh29pRSk7iII8EYdUn4= X-Google-Smtp-Source: AK7set8Gij72ajPVD9Xe2MJng02VISC3uIHSu7lyyH+Mf3nS1U+xR+oo072qbV8rbGgihEewLSGRE1AzpKA6eUfbyWk= X-Received: by 2002:a17:90b:1e47:b0:230:b973:a726 with SMTP id pi7-20020a17090b1e4700b00230b973a726mr943751pjb.23.1676584747311; Thu, 16 Feb 2023 13:59:07 -0800 (PST) MIME-Version: 1.0 References: <20230214075710.2401855-1-stevensd@google.com> <20230214075710.2401855-2-stevensd@google.com> In-Reply-To: From: Yang Shi Date: Thu, 16 Feb 2023 13:58:55 -0800 Message-ID: Subject: Re: [PATCH 2/2] mm/khugepaged: skip shmem with userfaultfd To: Peter Xu Cc: David Stevens , linux-mm@kvack.org, Matthew Wilcox , Andrew Morton , "Kirill A . Shutemov" , David Hildenbrand , Hugh Dickins , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 16, 2023 at 6:41 AM Peter Xu wrote: > > On Thu, Feb 16, 2023 at 10:37:47AM +0900, David Stevens wrote: > > On Thu, Feb 16, 2023 at 7:48 AM Peter Xu wrote: > > > > > > On Tue, Feb 14, 2023 at 04:57:10PM +0900, David Stevens wrote: > > > > From: David Stevens > > > > > > > > Make sure that collapse_file respects any userfaultfds registered with > > > > MODE_MISSING. If userspace has any such userfaultfds registered, then > > > > for any page which it knows to be missing, it may expect a > > > > UFFD_EVENT_PAGEFAULT. This means collapse_file needs to take care when > > > > collapsing a shmem range would result in replacing an empty page with a > > > > THP, so that it doesn't break userfaultfd. > > > > > > > > Synchronization when checking for userfaultfds in collapse_file is > > > > tricky because the mmap locks can't be used to prevent races with the > > > > registration of new userfaultfds. Instead, we provide synchronization by > > > > ensuring that userspace cannot observe the fact that pages are missing > > > > before we check for userfaultfds. Although this allows registration of a > > > > userfaultfd to race with collapse_file, it ensures that userspace cannot > > > > observe any pages transition from missing to present after such a race. > > > > This makes such a race indistinguishable to the collapse occurring > > > > immediately before the userfaultfd registration. > > > > > > > > The first step to provide this synchronization is to stop filling gaps > > > > during the loop iterating over the target range, since the page cache > > > > lock can be dropped during that loop. The second step is to fill the > > > > gaps with XA_RETRY_ENTRY after the page cache lock is acquired the final > > > > time, to avoid races with accesses to the page cache that only take the > > > > RCU read lock. > > > > > > > > This fix is targeted at khugepaged, but the change also applies to > > > > MADV_COLLAPSE. MADV_COLLAPSE on a range with a userfaultfd will now > > > > return EBUSY if there are any missing pages (instead of succeeding on > > > > shmem and returning EINVAL on anonymous memory). There is also now a > > > > window during MADV_COLLAPSE where a fault on a missing page will cause > > > > the syscall to fail with EAGAIN. > > > > > > > > The fact that intermediate page cache state can no longer be observed > > > > before the rollback of a failed collapse is also technically a > > > > userspace-visible change (via at least SEEK_DATA and SEEK_END), but it > > > > is exceedingly unlikely that anything relies on being able to observe > > > > that transient state. > > > > > > > > Signed-off-by: David Stevens > > > > --- > > > > mm/khugepaged.c | 66 +++++++++++++++++++++++++++++++++++++++++++------ > > > > 1 file changed, 58 insertions(+), 8 deletions(-) > > > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > > index b648f1053d95..8c2e2349e883 100644 > > > > --- a/mm/khugepaged.c > > > > +++ b/mm/khugepaged.c > > > > @@ -55,6 +55,7 @@ enum scan_result { > > > > SCAN_CGROUP_CHARGE_FAIL, > > > > SCAN_TRUNCATED, > > > > SCAN_PAGE_HAS_PRIVATE, > > > > + SCAN_PAGE_FILLED, > > > > > > PS: You may want to also touch SCAN_STATUS in huge_memory.h next time. > > > > > > > }; > > > > > > > > #define CREATE_TRACE_POINTS > > > > @@ -1725,8 +1726,8 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff, > > > > * - allocate and lock a new huge page; > > > > * - scan page cache replacing old pages with the new one > > > > * + swap/gup in pages if necessary; > > > > - * + fill in gaps; > > > > > > IIUC it's not a complete removal, but just moved downwards: > > > > > > > * + keep old pages around in case rollback is required; > > > > + * - finalize updates to the page cache; > > > > > > + fill in gaps with RETRY entries > > > + detect race conditions with userfaultfds > > > > > > > * - if replacing succeeds: > > > > * + copy data over; > > > > * + free old pages; > > > > @@ -1805,13 +1806,12 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, > > > > result = SCAN_TRUNCATED; > > > > goto xa_locked; > > > > } > > > > - xas_set(&xas, index); > > > > + xas_set(&xas, index + 1); > > > > } > > > > if (!shmem_charge(mapping->host, 1)) { > > > > result = SCAN_FAIL; > > > > goto xa_locked; > > > > } > > > > - xas_store(&xas, hpage); > > > > nr_none++; > > > > continue; > > > > } > > > > @@ -1970,6 +1970,56 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr, > > > > put_page(page); > > > > goto xa_unlocked; > > > > } > > > > + > > > > + if (nr_none) { > > > > + struct vm_area_struct *vma; > > > > + int nr_none_check = 0; > > > > + > > > > + xas_unlock_irq(&xas); > > > > + i_mmap_lock_read(mapping); > > > > + xas_lock_irq(&xas); > > > > + > > > > + xas_set(&xas, start); > > > > + for (index = start; index < end; index++) { > > > > + if (!xas_next(&xas)) { > > > > + xas_store(&xas, XA_RETRY_ENTRY); > > > > + nr_none_check++; > > > > + } > > > > + } > > > > + > > > > + if (nr_none != nr_none_check) { > > > > + result = SCAN_PAGE_FILLED; > > > > + goto immap_locked; > > > > + } > > > > + > > > > + /* > > > > + * If userspace observed a missing page in a VMA with an armed > > > > + * userfaultfd, then it might expect a UFFD_EVENT_PAGEFAULT for > > > > + * that page, so we need to roll back to avoid suppressing such > > > > + * an event. Any userfaultfds armed after this point will not be > > > > + * able to observe any missing pages due to the previously > > > > + * inserted retry entries. > > > > + */ > > > > + vma_interval_tree_foreach(vma, &mapping->i_mmap, start, start) { > > > > + if (userfaultfd_missing(vma)) { > > > > + result = SCAN_EXCEED_NONE_PTE; > > > > + goto immap_locked; > > > > + } > > > > + } > > > > + > > > > +immap_locked: > > > > + i_mmap_unlock_read(mapping); > > > > + if (result != SCAN_SUCCEED) { > > > > + xas_set(&xas, start); > > > > + for (index = start; index < end; index++) { > > > > + if (xas_next(&xas) == XA_RETRY_ENTRY) > > > > + xas_store(&xas, NULL); > > > > + } > > > > + > > > > + goto xa_locked; > > > > + } > > > > + } > > > > + > > > > > > Until here, all look fine to me (ignoring patch 1 for now; assuming the > > > hpage is always uptodate). > > > > > > My question is after here we'll release page cache lock again before > > > try_to_unmap_flush(), but is it safe to keep RETRY entries after releasing > > > page cache lock? It means other threads can be spinning. I assume page > > > lock is always safe and sleepable, but not sure about the page cache lock > > > here. > > > > We insert the multi-index entry for hpage before releasing the page > > cache lock, which should replace all of the XA_RETRY_ENTRYs. So the > > page cache will be fully up to date when we release the lock, at least > > in terms of which pages it contains. > > IIUC we released it before copying the pages: The huge page is locked until the copy is done. It should be fine unless the users inspect the page content without acquiring page lock. > > xa_locked: > xas_unlock_irq(&xas); <-------------------------------- here > xa_unlocked: > > /* > * If collapse is successful, flush must be done now before copying. > * If collapse is unsuccessful, does flush actually need to be done? > * Do it anyway, to clear the state. > */ > try_to_unmap_flush(); > > Before insertion of the multi-index: > > /* Join all the small entries into a single multi-index entry. */ > xas_set_order(&xas, start, HPAGE_PMD_ORDER); > xas_store(&xas, hpage); > > Thanks, > > -- > Peter Xu >