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Biederman" , chenzhou Subject: Re: [PATCH v14 01/11] x86: kdump: replace the hard-coded alignment with macro CRASH_ALIGN Message-ID: <20210302074327.GC13714@MiWiFi-R3L-srv> References: <20210130071025.65258-1-chenzhou10@huawei.com> <20210130071025.65258-2-chenzhou10@huawei.com> <20210224141939.GA28965@arm.com> <20210225072426.GH3553@MiWiFi-R3L-srv> <121fa1e6-f1a3-d47f-bb1d-baaacf96fddc@huawei.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210302_024350_513207_28A888F6 X-CRM114-Status: GOOD ( 30.03 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: wangkefeng.wang@huawei.com, linux-doc@vger.kernel.org, Catalin Marinas , huawei.libin@huawei.com, guohanjun@huawei.com, will@kernel.org, corbet@lwn.net, mingo@redhat.com, dyoung@redhat.com, John.P.donnelly@oracle.com, arnd@arndb.de, xiexiuqi@huawei.com, horms@verge.net.au, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, robh+dt@kernel.org, james.morse@arm.com, rppt@kernel.org, prabhakar.pkin@gmail.com, nsaenzjulienne@suse.de Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 02/26/21 at 09:38am, Eric W. Biederman wrote: > chenzhou writes: > > > On 2021/2/25 15:25, Baoquan He wrote: > >> On 02/24/21 at 02:19pm, Catalin Marinas wrote: > >>> On Sat, Jan 30, 2021 at 03:10:15PM +0800, Chen Zhou wrote: > >>>> Move CRASH_ALIGN to header asm/kexec.h for later use. Besides, the > >>>> alignment of crash kernel regions in x86 is 16M(CRASH_ALIGN), but > >>>> function reserve_crashkernel() also used 1M alignment. So just > >>>> replace hard-coded alignment 1M with macro CRASH_ALIGN. > >>> [...] > >>>> @@ -510,7 +507,7 @@ static void __init reserve_crashkernel(void) > >>>> } else { > >>>> unsigned long long start; > >>>> > >>>> - start = memblock_phys_alloc_range(crash_size, SZ_1M, crash_base, > >>>> + start = memblock_phys_alloc_range(crash_size, CRASH_ALIGN, crash_base, > >>>> crash_base + crash_size); > >>>> if (start != crash_base) { > >>>> pr_info("crashkernel reservation failed - memory is in use.\n"); > >>> There is a small functional change here for x86. Prior to this patch, > >>> crash_base passed by the user on the command line is allowed to be 1MB > >>> aligned. With this patch, such reservation will fail. > >>> > >>> Is the current behaviour a bug in the current x86 code or it does allow > >>> 1MB-aligned reservations? > >> Hmm, you are right. Here we should keep 1MB alignment as is because > >> users specify the address and size, their intention should be respected. > >> The 1MB alignment for fixed memory region reservation was introduced in > >> below commit, but it doesn't tell what is Eric's request at that time, I > >> guess it meant respecting users' specifying. > > > > I think we could make the alignment unified. Why is the alignment system reserved and > > user specified different? Besides, there is no document about the 1MB alignment. > > How about adding the alignment size(16MB) in doc if user specified > > start address as arm64 does. > > Looking at what the code is doing. Attempting to reserve a crash region > at the location the user specified. Adding unnecessary alignment > constraints is totally broken. > > I am not even certain enforcing a 1MB alignment makes sense. I suspect > it was added so that we don't accidentally reserve low memory on x86. > Frankly I am not even certain that makes sense. > > Now in practice there might be an argument for 2MB alignment that goes > with huge page sizes on x86. But until someone finds that there are > actual problems with 1MB alignment I would not touch it. > > The proper response to something that isn't documented and confusing is > not to arbitrarily change it and risk breaking users. Especially in > this case where it is clear that adding additional alignment is total > nonsense. The proper response to something that isn't clear and > documented is to dig in and document it, or to leave it alone and let it Sounds reasonable. Then adding document or code comment around looks like a good way to go further so that people can easily get why its alignment is different than other reservation. > be the next persons problem. > > In this case there is no reason for changing this bit of code. > All CRASH_ALIGN is about is a default alignment when none is specified. > It is not a functional requirement but just something so that things > come out nicely. > > > Eric > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel