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Mon, 12 Sep 2022 18:54:39 -0700 (PDT) Date: Tue, 13 Sep 2022 10:54:34 +0900 From: Sergey Senozhatsky To: Brian Geffon Cc: Sergey Senozhatsky , Minchan Kim , Andrew Morton , Nitin Gupta , LKML , linux-mm Subject: Re: [PATCHv2] zram: do not waste zram_table_entry flags bits Message-ID: References: <20220912152744.527438-1-senozhatsky@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1663034081; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=pE3z9bD3wrpH25CppWpqm9Q7/WCYzJnth3D82Qxm1v4=; b=n0T0Cw8JLCBa43QPO7vpab7T8R3xUmROzH15yxbRkws7a21G253PHGyr5eB/Vf6fmwqaNc rNyxY7z01+c7HMCRwJsZT/LkLcbDQLyB0kmOL0Tk9u9zumuWfSYGAB7XYvAL7qvbkKCpS4 tMGdAo7H1QwDJ7yPgcT7bcsd8ZL1OlU= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=chromium.org header.s=google header.b="TwgGk/wD"; spf=pass (imf26.hostedemail.com: domain of senozhatsky@chromium.org designates 209.85.215.178 as permitted sender) smtp.mailfrom=senozhatsky@chromium.org; dmarc=pass (policy=none) header.from=chromium.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1663034081; a=rsa-sha256; cv=none; b=P+/9X6IDDlawTK5sMs2HZCILQc9GJQl/RzgDvrm18tNSgnENuEuiS2/rmj+qTLB/48jVnp unvM5iRiHSU6WxK/I1hWIUrpRDQ8m/jyrznjrs/aTvr8sp5FUiW8mv4zF7w1fWX4AbKJYx UPSSnLQ9j2WGevfMvfN5V9mygNov0YM= X-Stat-Signature: 5mdmsunxrumpkqhxrpm6di9qz75g7joh X-Rspamd-Queue-Id: 5736F1400BB Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=chromium.org header.s=google header.b="TwgGk/wD"; spf=pass (imf26.hostedemail.com: domain of senozhatsky@chromium.org designates 209.85.215.178 as permitted sender) smtp.mailfrom=senozhatsky@chromium.org; dmarc=pass (policy=none) header.from=chromium.org X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1663034081-68928 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 (22/09/12 11:39), Brian Geffon wrote: > > zram_table_entry::flags stores object size in the lower bits and > > zram pageflags in the upper bits. However, for some reason, we > > use 24 lower bits, while maximum zram object size is PAGE_SIZE, > > which requires PAGE_SHIFT bits (up to 16 on arm64). This wastes > > 24 - PAGE_SHIFT bits that we can use for additional zram pageflags > > instead. > > > > Also add a BUILD_BUG_ON() to alert us should we run out of bits > > in zram_table_entry::flags. > > > > Signed-off-by: Sergey Senozhatsky > > --- > > drivers/block/zram/zram_drv.c | 2 ++ > > drivers/block/zram/zram_drv.h | 15 +++++++-------- > > 2 files changed, 9 insertions(+), 8 deletions(-) > > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c > > index f3948abce2f7..07913bcdb5c2 100644 > > --- a/drivers/block/zram/zram_drv.c > > +++ b/drivers/block/zram/zram_drv.c > > @@ -2449,6 +2449,8 @@ static int __init zram_init(void) > > { > > int ret; > > > > + BUILD_BUG_ON(__NR_ZRAM_PAGEFLAGS > BITS_PER_LONG); > > Should this be >= BITS_PER_LONG? __NR_ZRAM_PAGEFLAGS == BITS_PER_LONG == 64 (e.g. on 64 bit host) means that the last valid zram pageflag (and __NR_ZRAM_PAGEFLAGS is not a valid pageflag) is __NR_ZRAM_PAGEFLAGS - 1, which is 63 and which is a valid BIT() offset for u64. So __NR_ZRAM_PAGEFLAGS == BITS_PER_LONG should be a valid case.