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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id h138sm1230701oic.8.2019.03.15.10.21.46 (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Fri, 15 Mar 2019 10:21:47 -0700 (PDT) Date: Fri, 15 Mar 2019 10:21:18 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Oscar Salvador cc: Michal Hocko , akpm@linux-foundation.org, anshuman.khandual@arm.com, william.kucharski@oracle.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jan Kara , Hugh Dickins Subject: Re: [PATCH] mm: Fix __dump_page when mapping->host is not set In-Reply-To: <20190315143304.pkuvj4qwtlzgm7iq@d104.suse.de> Message-ID: References: <20190315121826.23609-1-osalvador@suse.de> <20190315124733.GE15672@dhcp22.suse.cz> <20190315143304.pkuvj4qwtlzgm7iq@d104.suse.de> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 15 Mar 2019, Oscar Salvador wrote: > On Fri, Mar 15, 2019 at 01:47:33PM +0100, Michal Hocko wrote: > > diff --git a/mm/debug.c b/mm/debug.c > > index 1611cf00a137..499c26d5ebe5 100644 > > --- a/mm/debug.c > > +++ b/mm/debug.c > > @@ -78,6 +78,9 @@ void __dump_page(struct page *page, const char *reason) > > else if (PageKsm(page)) > > pr_warn("ksm "); > > else if (mapping) { > > + if (PageSwapCache(page)) > > + mapping = page_swap_info(page)->swap_file->f_mapping; > > + > > pr_warn("%ps ", mapping->a_ops); > > if (mapping->host->i_dentry.first) { > > struct dentry *dentry; > > This looks like a much nicer fix, indeed. > I gave it a spin and it works. > > Since the mapping is set during the swapon, I would assume that this should > always work for swap. > Although I am not sure if once you start playing with e.g zswap the picture can > change. > > Let us wait for Hugh and Jan. > > Thanks Michal Sorry, I don't agree that Michal's more sophisticated patch is nicer: the appropriate patch was your original, just checking for NULL. Though, would I be too snarky to suggest that your patch description would be better at 2 lines than 90? Swap mapping->host is NULL, so of course __dump_page() needs to be careful about that. I was a little disturbed to see __dump_page() now getting into dentries, but admit that it can sometimes be very helpful to see the name of the file involved; so if that is not in danger of breaking anything, okay. It is very often useful to see if a page is PageSwapCache (typically because that should account for 1 of its refcount); I cannot think of a time when it's been useful to know the name of the underlying swap device (if that's indeed what f_mapping leads to: it's new to me). And if you need swp_type and swp_offset, they're in the raw output. The cleverer __dump_page() tries to get, the more likely that it will itself crash just when you need it most. Please just keep it simple. Thanks, Hugh