From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f71.google.com (mail-wm0-f71.google.com [74.125.82.71]) by kanga.kvack.org (Postfix) with ESMTP id 2B5DF6B0253 for ; Fri, 25 Nov 2016 11:18:40 -0500 (EST) Received: by mail-wm0-f71.google.com with SMTP id g23so23522777wme.4 for ; Fri, 25 Nov 2016 08:18:40 -0800 (PST) Received: from mout.kundenserver.de (mout.kundenserver.de. [217.72.192.74]) by mx.google.com with ESMTPS id k8si43261280wjv.25.2016.11.25.08.18.38 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 25 Nov 2016 08:18:39 -0800 (PST) From: Arnd Bergmann Subject: Re: [PATCH] z3fold: use %z modifier for format string Date: Fri, 25 Nov 2016 17:11:49 +0100 Message-ID: <4155555.0I0VNuaa72@wuerfel> In-Reply-To: References: <20161124163158.3939337-1-arnd@arndb.de> <3177176.drX8hSSUx4@wuerfel> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: owner-linux-mm@kvack.org List-ID: To: Vitaly Wool Cc: Joe Perches , Andrew Morton , Dan Streetman , zhong jiang , Linux-MM , LKML On Friday, November 25, 2016 4:51:03 PM CET Vitaly Wool wrote: > On Fri, Nov 25, 2016 at 9:41 AM, Arnd Bergmann wrote: > > On Friday, November 25, 2016 8:38:25 AM CET Vitaly Wool wrote: > >> >> diff --git a/mm/z3fold.c b/mm/z3fold.c > >> >> index e282ba073e77..66ac7a7dc934 100644 > >> >> --- a/mm/z3fold.c > >> >> +++ b/mm/z3fold.c > >> >> @@ -884,7 +884,7 @@ static int __init init_z3fold(void) > >> >> { > >> >> /* Fail the initialization if z3fold header won't fit in one chunk */ > >> >> if (sizeof(struct z3fold_header) > ZHDR_SIZE_ALIGNED) { > >> >> - pr_err("z3fold: z3fold_header size (%d) is bigger than " > >> >> + pr_err("z3fold: z3fold_header size (%zd) is bigger than " > >> >> "the chunk size (%d), can't proceed\n", > >> >> sizeof(struct z3fold_header) , ZHDR_SIZE_ALIGNED); > >> >> return -E2BIG; > >> > > >> > The embedded "z3fold: " prefix here should be removed > >> > as there's a pr_fmt that also adds it. > >> > > >> > The test looks like it should be a BUILD_BUG_ON rather > >> > than any runtime test too. > >> > >> It used to be BUILD_BUG_ON but we deliberately changed that because > >> sizeof(spinlock_t) gets bloated in debug builds, so it just won't > >> build with default CHUNK_SIZE. > > > > Could this be improved by making the CHUNK_SIZE bigger depending on > > the debug options? > > I don't see how silently enforcing a suboptimal configuration is > better than failing the initialization (so that you can adjust > CHUNK_SIZE yourself). I can add something descriptive to > Documentation/vm/z3fold.txt for that matter. Failing at runtime when you know it's broken at compile-time seems wrong, too. If you can't use z3fold with spinlock debugging, you may as well hide the option in Kconfig based on the other ones. Printing a runtime warning for the suboptimal configuration but making it work anyway is probably better than just failing. > > Alternatively, how about using a bit_spin_lock instead of raw_spin_lock? > > That would guarantee a fixed size for the lock and make z3fold_header > > always 24 bytes (on 32-bit architectures) or 40 bytes > > (on 64-bit architectures). You could even play some tricks with the > > first_num field to make it fit in the same word as the lock and make the > > structure fit into 32 bytes if you care about that. > > That is interesting. Actually I can have that bit in page->private and > then I don't need to handle headless pages in a special way, that > sounds appealing. However, there is a warning about bit_spin_lock > performance penalty. Do you know how big it is? No idea, sorry. On x86, test_and_set_bit() seems to be only one instruction to test/set the bit, followed by a conditional branch, compared to a cmpxchg() for the raw_spin_lock(), so the fast path seems pretty much the same. Arnd -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org