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[192.55.52.120]) by mx.google.com with ESMTPS id f64si20738477pfc.168.2019.04.09.13.23.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 09 Apr 2019 13:23:22 -0700 (PDT) Received-SPF: pass (google.com: domain of ira.weiny@intel.com designates 192.55.52.120 as permitted sender) client-ip=192.55.52.120; Authentication-Results: mx.google.com; spf=pass (google.com: domain of ira.weiny@intel.com designates 192.55.52.120 as permitted sender) smtp.mailfrom=ira.weiny@intel.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=intel.com X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga104.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 09 Apr 2019 13:23:21 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,330,1549958400"; d="scan'208";a="141425019" Received: from iweiny-desk2.sc.intel.com ([10.3.52.157]) by fmsmga007.fm.intel.com with ESMTP; 09 Apr 2019 13:23:21 -0700 Date: Tue, 9 Apr 2019 13:23:16 -0700 From: Ira Weiny To: Huang Shijie Cc: Matthew Wilcox , akpm@linux-foundation.org, william.kucharski@oracle.com, palmer@sifive.com, axboe@kernel.dk, keescook@chromium.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] mm/gup.c: fix the wrong comments Message-ID: <20190409202316.GA22989@iweiny-DESK2.sc.intel.com> References: <20190408023746.16916-1-sjhuang@iluvatar.ai> <20190408141313.GU22763@bombadil.infradead.org> <20190409010832.GA28081@hsj-Precision-5520> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190409010832.GA28081@hsj-Precision-5520> User-Agent: Mutt/1.11.1 (2018-12-01) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Apr 09, 2019 at 09:08:33AM +0800, Huang Shijie wrote: > On Mon, Apr 08, 2019 at 07:13:13AM -0700, Matthew Wilcox wrote: > > On Mon, Apr 08, 2019 at 10:37:45AM +0800, Huang Shijie wrote: > > > When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own > > > get_user_pages_fast(). > > > > > > In the following scenario, we will may meet the bug in the DMA case: > > > ..................... > > > get_user_pages_fast(start,,, pages); > > > ...... > > > sg_alloc_table_from_pages(, pages, ...); > > > ..................... > > > > > > The root cause is that sg_alloc_table_from_pages() requires the > > > page order to keep the same as it used in the user space, but > > > get_user_pages_fast() will mess it up. > > > > I don't understand how get_user_pages_fast() can return the pages in a > > different order in the array from the order they appear in userspace. > > Can you explain? > Please see the code in gup.c: > > int get_user_pages_fast(unsigned long start, int nr_pages, > unsigned int gup_flags, struct page **pages) > { > ....... > if (gup_fast_permitted(start, nr_pages)) { > local_irq_disable(); > gup_pgd_range(addr, end, gup_flags, pages, &nr); // The @pages array maybe filled at the first time. > local_irq_enable(); > ret = nr; > } > ....... > if (nr < nr_pages) { > /* Try to get the remaining pages with get_user_pages */ > start += nr << PAGE_SHIFT; > pages += nr; // The @pages is moved forward. > > if (gup_flags & FOLL_LONGTERM) { > down_read(¤t->mm->mmap_sem); > ret = __gup_longterm_locked(current, current->mm, // The @pages maybe filled at the second time > Neither this nor the get_user_pages_unlocked is filling the pages a second time. It is adding to the page array having moved start and the page array forward. Are you doing a FOLL_LONGTERM GUP? Or are you in the else clause below when you get this bug? Ira > start, nr_pages - nr, > pages, NULL, gup_flags); > up_read(¤t->mm->mmap_sem); > } else { > /* > * retain FAULT_FOLL_ALLOW_RETRY optimization if > * possible > */ > ret = get_user_pages_unlocked(start, nr_pages - nr, // The @pages maybe filled at the second time. > pages, gup_flags); > } > } > > > ..................... > > BTW, I do not know why we mess up the page order. It maybe used in some special case. > > Thanks > Huang Shijie >