From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f71.google.com (mail-oi0-f71.google.com [209.85.218.71]) by kanga.kvack.org (Postfix) with ESMTP id C931C6B0010 for ; Fri, 13 Jul 2018 20:34:49 -0400 (EDT) Received: by mail-oi0-f71.google.com with SMTP id w204-v6so46143025oib.9 for ; Fri, 13 Jul 2018 17:34:49 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id d6-v6sor17793962oig.67.2018.07.13.17.34.48 for (Google Transport Security); Fri, 13 Jul 2018 17:34:48 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20180713063125.GA10034@hori1.linux.bs1.fc.nec.co.jp> References: <153074042316.27838.17319837331947007626.stgit@dwillia2-desk3.amr.corp.intel.com> <153074044986.27838.16910122305490506387.stgit@dwillia2-desk3.amr.corp.intel.com> <20180713063125.GA10034@hori1.linux.bs1.fc.nec.co.jp> From: Dan Williams Date: Fri, 13 Jul 2018 17:34:48 -0700 Message-ID: Subject: Re: [PATCH v5 05/11] mm, madvise_inject_error: Let memory_failure() optionally take a page reference Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Naoya Horiguchi Cc: "linux-nvdimm@lists.01.org" , Michal Hocko , "hch@lst.de" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "jack@suse.cz" , "ross.zwisler@linux.intel.com" On Thu, Jul 12, 2018 at 11:31 PM, Naoya Horiguchi wrote: > Hello Dan, > > On Wed, Jul 04, 2018 at 02:40:49PM -0700, Dan Williams wrote: >> The madvise_inject_error() routine uses get_user_pages() to lookup the >> pfn and other information for injected error, but it does not release >> that pin. The assumption is that failed pages should be taken out of >> circulation. >> >> However, for dax mappings it is not possible to take pages out of >> circulation since they are 1:1 physically mapped as filesystem blocks, >> or device-dax capacity. They also typically represent persistent memory >> which has an error clearing capability. >> >> In preparation for adding a special handler for dax mappings, shift the >> responsibility of taking the page reference to memory_failure(). I.e. >> drop the page reference and do not specify MF_COUNT_INCREASED to >> memory_failure(). >> >> Cc: Michal Hocko >> Cc: Naoya Horiguchi >> Signed-off-by: Dan Williams >> --- >> mm/madvise.c | 18 +++++++++++++++--- >> 1 file changed, 15 insertions(+), 3 deletions(-) >> >> diff --git a/mm/madvise.c b/mm/madvise.c >> index 4d3c922ea1a1..b731933dddae 100644 >> --- a/mm/madvise.c >> +++ b/mm/madvise.c >> @@ -631,11 +631,13 @@ static int madvise_inject_error(int behavior, >> >> >> for (; start < end; start += PAGE_SIZE << order) { >> + unsigned long pfn; >> int ret; >> >> ret = get_user_pages_fast(start, 1, 0, &page); >> if (ret != 1) >> return ret; >> + pfn = page_to_pfn(page); >> >> /* >> * When soft offlining hugepages, after migrating the page >> @@ -651,17 +653,27 @@ static int madvise_inject_error(int behavior, >> >> if (behavior == MADV_SOFT_OFFLINE) { >> pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n", >> - page_to_pfn(page), start); >> + pfn, start); >> >> ret = soft_offline_page(page, MF_COUNT_INCREASED); >> if (ret) >> return ret; >> continue; >> } >> + >> pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n", >> - page_to_pfn(page), start); >> + pfn, start); >> + >> + ret = memory_failure(pfn, 0); >> + >> + /* >> + * Drop the page reference taken by get_user_pages_fast(). In >> + * the absence of MF_COUNT_INCREASED the memory_failure() >> + * routine is responsible for pinning the page to prevent it >> + * from being released back to the page allocator. >> + */ >> + put_page(page); >> >> - ret = memory_failure(page_to_pfn(page), MF_COUNT_INCREASED); > > MF_COUNT_INCREASED means that the page refcount for memory error handling > is taken by the caller so you don't have to take one inside memory_failure(). > So this code don't keep with the definition, then another refcount can be > taken in memory_failure() in normal LRU page's case for example. > As a result the error message "Memory failure: %#lx: %s still referenced by > %d users\n" will be dumped in page_action(). > > So if you want to put put_page() in madvise_inject_error(), I think that > > put_page(page); > ret = memory_failure(pfn, 0); > > can be acceptable because the purpose of get_user_pages_fast() here is > just getting pfn, and the refcount itself is not so important. > IOW, memory_failure() is called only with pfn which never changes depending > on the page's status. Ok, I'll resend with the put_page() moved before memory_failure() to make it more clear that memory_failure() is responsible for taking its own reference and that there is no dependency to hold the reference in madvise_inject_error(). > In production system memory_failure() is called via machine check code > without taking any pagecount, so I don't think the this injection interface > is properly mocking the real thing. So I'm feeling that this flag will be > wiped out at some point. Ok, makes sense.