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Fri, 2 Feb 2024 09:40:18 -0500 (EST) Message-ID: Date: Fri, 2 Feb 2024 09:40:20 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v3 2/4] dax: Check for data cache aliasing at runtime Content-Language: en-US From: Mathieu Desnoyers To: Dan Williams , Arnd Bergmann , Dave Chinner Cc: linux-kernel@vger.kernel.org, Andrew Morton , Linus Torvalds , linux-mm@kvack.org, linux-arch@vger.kernel.org, Vishal Verma , Dave Jiang , Matthew Wilcox , Russell King , nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, dm-devel@lists.linux.dev References: <20240131162533.247710-1-mathieu.desnoyers@efficios.com> <20240131162533.247710-3-mathieu.desnoyers@efficios.com> <65bab567665f3_37ad2943c@dwillia2-xfh.jf.intel.com.notmuch> <0a38176b-c453-4be0-be83-f3e1bb897973@efficios.com> <65bac71a9659b_37ad29428@dwillia2-xfh.jf.intel.com.notmuch> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 3B4541A0021 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: bgm64g36gtdzdx896e5wikaw8jx9nfa1 X-HE-Tag: 1706884820-24346 X-HE-Meta: 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 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: List-Subscribe: List-Unsubscribe: On 2024-02-01 10:44, Mathieu Desnoyers wrote: [...] >> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c >> index 4e8fdcb3f1c8..b69c9e442cf4 100644 >> --- a/drivers/nvdimm/pmem.c >> +++ b/drivers/nvdimm/pmem.c >> @@ -560,17 +560,19 @@ static int pmem_attach_disk(struct device *dev, >>       dax_dev = alloc_dax(pmem, &pmem_dax_ops); >>       if (IS_ERR(dax_dev)) { >>           rc = PTR_ERR(dax_dev); >> -        goto out; >> +        if (rc != -EOPNOTSUPP) >> +            goto out; > > If I compare the before / after this change, if previously > pmem_attach_disk() was called in a configuration with FS_DAX=n, it would > result in a NULL pointer dereference. I was wrong. drivers/nvdimm/Kconfig has: config BLK_DEV_PMEM select DAX and drivers/nvdimm/Makefile has: obj-$(CONFIG_BLK_DEV_PMEM) += nd_pmem.o nd_pmem-y := pmem.o which means that anything in pmem.c can assume that alloc_dax() is implemented. [...] >> diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c >> index 4b7ecd4fd431..f911e58a24dd 100644 >> --- a/drivers/s390/block/dcssblk.c >> +++ b/drivers/s390/block/dcssblk.c >> @@ -681,12 +681,14 @@ dcssblk_add_store(struct device *dev, struct >> device_attribute *attr, const char >>       if (IS_ERR(dev_info->dax_dev)) { >>           rc = PTR_ERR(dev_info->dax_dev); >>           dev_info->dax_dev = NULL; >> -        goto put_dev; >> +        if (rc != -EOPNOTSUPP) >> +            goto put_dev; > > config DCSSBLK selects FS_DAX_LIMITED and DAX. > > I'm not sure what selecting DAX is trying to achieve here, because the > Kconfig option is "FS_DAX". > > So depending on the real motivation behind this select, we may want to > consider failure rather than success in the -EOPNOTSUPP case. > I missed that alloc_dax() is implemented as not supported based on CONFIG_DAX (not CONFIG_FS_DAX). Therefore DCSSBLK Kconfig does the right thing and always selects DAX, and thus an implemented version of alloc_dax(). This takes care of two of my open questions at least. :) Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com