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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1rE9XT-000SMS-Kc; Fri, 15 Dec 2023 14:51:43 +0000 Date: Fri, 15 Dec 2023 14:51:43 +0000 From: Matthew Wilcox To: Baolin Wang Cc: akpm@linux-foundation.org, david@redhat.com, ying.huang@intel.com, ziy@nvidia.com, xuyu@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: migrate: fix getting incorrect page mapping during page migration Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 293AF140018 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 1173bsd6ad6k3ke798i9k39ftdxxkkwf X-HE-Tag: 1702651921-725969 X-HE-Meta: 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 OV3Lv4ku NIZ265IWUf1EZQA0rghdNYwlMSpJFC3gSMVoAOHHgmu03gK+3hhYFBuYVPmNxUBDe4hjmkUc42QTZYZt4uQNgD2k8X4hu72DuPqFm+8ZzXaxbKkyrlTbtSaWUCoT2bG+skVnADmE18aADtyk= 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 Fri, Dec 15, 2023 at 08:07:52PM +0800, Baolin Wang wrote: > When running stress-ng testing, we found below kernel crash after a few hours: > > Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000 > pc : dentry_name+0xd8/0x224 > lr : pointer+0x22c/0x370 > sp : ffff800025f134c0 > ...... > Call trace: > dentry_name+0xd8/0x224 > pointer+0x22c/0x370 > vsnprintf+0x1ec/0x730 > vscnprintf+0x2c/0x60 > vprintk_store+0x70/0x234 > vprintk_emit+0xe0/0x24c > vprintk_default+0x3c/0x44 > vprintk_func+0x84/0x2d0 > printk+0x64/0x88 > __dump_page+0x52c/0x530 > dump_page+0x14/0x20 [...] > There are seveval ways to fix this issue: > (1) Setting the PAGE_MAPPING_ANON flag for target page's ->mapping when saving > 'anon_vma', but this can confuse PageAnon() for PFN walkers, since the target > page has not built mappings yet. > (2) Getting the page lock to call page_mapping() in __dump_page() to avoid crashing > the system, however, there are still some PFN walkers that call page_mapping() > without holding the page lock, such as compaction. > (3) Using target page->private field to save the 'anon_vma' pointer and 2 bits > page state, just as page->mapping records an anonymous page, which can remove > the page_mapping() impact for PFN walkers and also seems a simple way. > > So I choose option 3 to fix this issue, and this can also fix other potential > issues for PFN walkers, such as compaction. I'm not saying no to this fix, but dump_mapping() is supposed to be resilient against this. Is the issue that 'dentry' is NULL, or is it some field within dentry that is NULL? eg, would this fix your case? +++ b/fs/inode.c @@ -588,7 +588,7 @@ void dump_mapping(const struct address_space *mapping) } dentry_ptr = container_of(dentry_first, struct dentry, d_u.d_alias); - if (get_kernel_nofault(dentry, dentry_ptr)) { + if (get_kernel_nofault(dentry, dentry_ptr) || !dentry) { pr_warn("aops:%ps ino:%lx invalid dentry:%px\n", a_ops, ino, dentry_ptr); return; Just to be clear, I think we should fix both the dumping and the migration code.