From: Eric Sandeen Subject: Re: [Ocfs2-devel] [PATCH 1/3] ext3/ext4: Factor out disk addressability check Date: Sun, 15 Aug 2010 22:36:36 -0500 Message-ID: <4C68B244.6080406@redhat.com> References: <874ofr2myq.fsf@patl.com> <20100812174215.GC6561@mail.oracle.com> <1F3EDC08-AC93-4D4D-8F83-A13C418DFC88@dilger.ca> <20100812201534.GA22777@mail.oracle.com> <209AEA97-E284-4ADB-8774-50C2630606B9@dilger.ca> <20100812222949.GC22777@mail.oracle.com> <20100813163006.GB4329@quack.suse.cz> <4C6821A8.5080805@redhat.com> <20100816025436.GA9061@mail.oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit To: Eric Sandeen , Jan Kara , Andreas Dilger , "Ted Ts'o" , ocfs2-devel@oss.oracle.com, linux-fsdevel@vger.kernel.org, Return-path: Received: from mx1.redhat.com ([209.132.183.28]:49713 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751551Ab0HPDgm (ORCPT ); Sun, 15 Aug 2010 23:36:42 -0400 In-Reply-To: <20100816025436.GA9061@mail.oracle.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: Joel Becker wrote: > On Sun, Aug 15, 2010 at 12:19:36PM -0500, Eric Sandeen wrote: >>>> + (last_fs_block > >>>> + (pgoff_t)(~0ULL) >> (PAGE_CACHE_SHIFT - blocksize_bits))) { >>> ^^^ I don't get the pgoff_t check. Shouldn't it rather be >>> (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits)? >> Argh that was my fault... Thankfully not too many 1k-blocksize-formatted >> 16T devices out there, I guess. >> >> I went through the math again and also came up with: >> >> total fs pages is blocks / (blocks per page) >> total pages is blocks / (1 << PAGE_CACHE_SHIFT / 1 << blocksize_bits) >> total pages is blocks / (1 << (PAGE_CACHE_SHIFT - blocksize_bits)) >> total pages is blocks * (1 >> (PAGE_CACHE_SHIFT - blocksize_bits)) >> total pages is blocks >> (PAGE_CACHE_SHIFT - blocksize_bits) >> >> too big if total pages is > (pgoff_t)(~0ULL) >> too big if blocks >> (PAGE_CACHE_SHIFT - blocksize_bits) > (pgoff_t)(~0ULL) > > Why not stop here, which is what I put in my other email? > "blocks >> SHIFT-bits" is "how many pages do I need?". yeah, ok. Was going for pointless symmetry w/ the other test... >> too big if blocks > (pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits) >> and to not overflow: >> too big if blocks > (u64)(pgoff_t)(~0ULL) << (PAGE_CACHE_SHIFT - blocksize_bits) > > This still overflows. pgoff_t is a u64 on 64bit machines, > right? So shift that left by anything and you wrap. Er, yeah. I had 32 bits in my head since that's the case we're checking for... whoops. So I guess your ... || ((last_fs_block >> (PAGE_CACHE_SHIFT - blocksize_bits)) > (pgoff_t)(!0ULL))) { is right :) (my feeble brain has a hard time reading that, though, TBH) -Eric > Joel >