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[115.124.30.44]) by mx.google.com with ESMTPS id l7si2400497plt.244.2019.05.23.21.06.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 23 May 2019 21:06:50 -0700 (PDT) Received-SPF: pass (google.com: domain of aaron.lu@linux.alibaba.com designates 115.124.30.44 as permitted sender) client-ip=115.124.30.44; Authentication-Results: mx.google.com; spf=pass (google.com: domain of aaron.lu@linux.alibaba.com designates 115.124.30.44 as permitted sender) smtp.mailfrom=aaron.lu@linux.alibaba.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=alibaba.com X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07487;MF=aaron.lu@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0TSXA6aZ_1558670807; Received: from 30.17.232.208(mailfrom:aaron.lu@linux.alibaba.com fp:SMTPD_---0TSXA6aZ_1558670807) by smtp.aliyun-inc.com(127.0.0.1); Fri, 24 May 2019 12:06:47 +0800 Subject: Re: [PATCH] mm, swap: use rbtree for swap_extent To: Andrew Morton Cc: linux-mm@kvack.org, Huang Ying , Hugh Dickins References: <20190523142404.GA181@aaronlu> <20190523120035.efb7c3bf4c91e3aef255621c@linux-foundation.org> From: Aaron Lu Message-ID: <357d963e-2657-4926-bab0-a87096a82230@linux.alibaba.com> Date: Fri, 24 May 2019 12:06:47 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190523120035.efb7c3bf4c91e3aef255621c@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 2019/5/24 3:00, Andrew Morton wrote: ... > On Thu, 23 May 2019 22:24:15 +0800 Aaron Lu wrote: >> ... >> >> +static struct swap_extent * >> +offset_to_swap_extent(struct swap_info_struct *sis, unsigned long offset) >> +{ >> + struct swap_extent *se; >> + struct rb_node *rb; >> + >> + rb = sis->swap_extent_root.rb_node; >> + while (rb) { >> + se = rb_entry(rb, struct swap_extent, rb_node); >> + if (offset < se->start_page) >> + rb = rb->rb_left; >> + else if (offset >= se->start_page + se->nr_pages) >> + rb = rb->rb_right; >> + else >> + return se; >> + } >> + /* It *must* be present */ >> + BUG_ON(1); > > I'm surprised this doesn't generate a warning about the function Ah right, I'm also surprised after you mentioned. This BUG_ON(1) here is meant to serve the same purpose as the original code in map_swap_entry(): static sector_t map_swap_entry(swp_entry_t entry, struct block_device **bdev) { ... offset = swp_offset(entry); start_se = sis->curr_swap_extent; se = start_se; for ( ; ; ) { if (se->start_page <= offset && offset < (se->start_page + se->nr_pages)) { return se->start_block + (offset - se->start_page); } se = list_next_entry(se, list); sis->curr_swap_extent = se; BUG_ON(se == start_se); /* It *must* be present */ } } I just copied the pattern and changed the condition to 1 without much thought. > failing to return a value. I guess the compiler figured out that > BUG_ON(non-zero-constant) is equivalent to BUG(), which is noreturn. > > Let's do this? Yes, it doesn't make much sense to use BUG_ON when the condition is 1...Thanks for the cleanup. > > --- a/mm/swapfile.c~mm-swap-use-rbtree-for-swap_extent-fix > +++ a/mm/swapfile.c > @@ -218,7 +218,7 @@ offset_to_swap_extent(struct swap_info_s > return se; > } > /* It *must* be present */ > - BUG_ON(1); > + BUG(); > } >