From mboxrd@z Thu Jan 1 00:00:00 1970 From: Theodore Ts'o Subject: Re: ext4 build errors Date: Mon, 2 Oct 2017 10:55:17 -0400 Message-ID: <20171002145517.652p7p7q4vv5rqcc@thunk.org> References: <1506954181.985.9.camel@infinera.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "linux-ext4@vger.kernel.org" To: Joakim Tjernlund Return-path: Received: from imap.thunk.org ([74.207.234.97]:53186 "EHLO imap.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751188AbdJBOzT (ORCPT ); Mon, 2 Oct 2017 10:55:19 -0400 Content-Disposition: inline In-Reply-To: <1506954181.985.9.camel@infinera.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Oct 02, 2017 at 02:23:02PM +0000, Joakim Tjernlund wrote: > Hi ext4 devs > > Adding the patch last in this mail cause lots of build errors in ext4, here is a few: Why did you need this patch to fix problems in VirtualBox? Cleaning this up is going to be a little tricky, because one of the implications the void * declaration in the __set_bit_le() declaration is that there isn't any particular alignment requirement with the __le functions. But the long * declaration implies that the bitmaps have to be aligned to sizeof(long). For the ext4 bitmap, we use it on bh->b_data, for which we can safely assume is long-aligned. But the mballoc buddy bitmaps use mb_set_bit() in ways that are _not_ guaranteed to be long aligned. So fixing this is going to be a bit painful, and will likely result in a performance regression for ext4. We can make our own version that open codes it as C functions --- but then we lose all of the architecture optimized bitop functions. I believe the reason why the standard bitop functions are made long * aligned is that on some BE architectures --- I suspect it was PowerPC but I'm not 100% sure about that --- the native bitop functions required a long * alignment. Fortunately all of the little endian architectures didn't have these alignment restrictions, so we could keep the __set_bit_le functions to not have any long alignment restrictions. The fact that bitop and the bitop_le functions are not the same is... inelegant, but if it represents a practical optimization that is possible on LE systems but not on BE systems (where bitop_le gets open coded in C, in an inefficient way, but oh, well, BE systems aren't for the cool kids anyway :-), I have to ask whether it's really worth it to do the cleanup. Cheers, - Ted