Return-Path: Received: from mail-eopbgr750091.outbound.protection.outlook.com ([40.107.75.91]:17717 "EHLO NAM02-BL2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726011AbeL2CPB (ORCPT ); Fri, 28 Dec 2018 21:15:01 -0500 Date: Fri, 28 Dec 2018 21:11:57 -0500 From: "Theodore Y. Ts'o" To: Peter Maydell CC: Andreas Dilger , Florian Weimer , linux-fsdevel , Linux API , Ext4 Developers List , , , Arnd Bergmann , , , lkml - Kernel Mailing List , QEMU Developers , , Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation Message-ID: <20181229021157.GG5864@mit.edu> References: <87bm56vqg4.fsf@mid.deneb.enyo.de> <9C6A7D45-CF53-4C61-B5DD-12CA0D419972@dilger.ca> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Fri, Dec 28, 2018 at 11:18:18AM +0000, Peter Maydell wrote: > In general inodes and offsets start from 0 and work up -- > so almost all of the time they don't actually overflow. > The problem with ext4 directory hash "offsets" is that they > overflow all the time and immediately, so instead of "works > unless you have a weird edge case" like all the other filesystems,h > it's "never works". Actually, XFS uses the inode number to encode the location of the inode (it doesn't have a fixed inode table, so it's effectively the block number shifted left by 3 or 4 bits, with the low bits indicating the slot in the 4k block). It has a hack to provide backwards compatibility for 32-bit API's, but there is a similar, "oh, we're on a non-paleolithic CPU, let's use the full 64-bits" sort of logic that ext4 has. > The problem is that there is no 32-bit API in some cases > (unless I have misunderstood the kernel code) -- not all > host architectures implement compat syscalls or allow them > to be called from 64-bit processes or implement all the older > syscall variants that had smaller offets. If there was a guaranteed > "this syscall always exists and always gives me 32-bit offsets" > we could use it. Are there going to be cases where a process or a thread will sometimes want the 64-bit interface, and sometimes want the 32-bit interface? Or is it always going to be one or the other? I wonder if we could simply add a new flag to the process personality(2) flags. > Yes, that has been suggested, but it seemed a bit dubious > to bake in knowledge of ext4's internal implementation details. > Can we rely on this as an ABI promise that will always work > for all versions of all file systems going forwards? Yeah, that seems dubious because I'm pretty sure there are other file systems that may have their own 32/64-bit quirks. - Ted