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Received: from viro by zeniv.linux.org.uk with local (Exim 4.99 #2 (Red Hat Linux)) id 1vPo4k-0000000F1yA-2jlj; Sun, 30 Nov 2025 20:31:18 +0000 Date: Sun, 30 Nov 2025 20:31:18 +0000 From: Al Viro To: david laight Cc: Linus Torvalds , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-alpha@vger.kernel.org Subject: Re: [RFC][alpha] saner vmalloc handling (was Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context) Message-ID: <20251130203118.GW3538@ZenIV> References: <20251126090505.3057219-1-wozizhi@huaweicloud.com> <20251126185545.GC3538@ZenIV> <20251129033728.GH3538@ZenIV> <20251130030146.GN3538@ZenIV> <20251130113213.40c8e7a0@pumpkin> <20251130164348.GV3538@ZenIV> <20251130190331.40385ddc@pumpkin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251130190331.40385ddc@pumpkin> X-Rspamd-Queue-Id: DFA5D4000F X-Rspamd-Server: rspam02 X-Stat-Signature: 7mukcjz9orzzyagai73gx1ewrrw5gfyj X-Rspam-User: X-HE-Tag: 1764534669-708591 X-HE-Meta: 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 P4bgbZba VygnKu6sPHRUFlSSeB/P8kMKOPd895uX3SlyIuXZOODtbubmqzZcpWUsXED2awplj0xnrE26TgecRWGJX8Ha7W5+5J1D3LnEaB4J1It+buLm6gmfkD/y3ZGWIyurG6BZD9NcWDwtAhraglgMvRYQJzZTiGU6a/HMIQRMBWq2Hwl7EhwlqdmJDGJddOOSX6Q0lFv0kCIBDnVikuQqyalRQ8ntMFTmuXxKQ/vwTmK9TR3BGQyNaNq4Yh5nb+lVFFGh2N9oEFCXy99dDAmw/OV2nvvkiogDS7EUBtT4LCIXk6IZ1AtPwMeeMFBVMnHmmSy1DemNnBIbyBxKODMcOW0YKuP8MNU6N/wLZKW224lkgN4vwMGyFsE/aU20tMBevEhKlS0KvGdR5kUuV1bvc4pg7EvmV0q6V9l09+0sLO5gKwV8PtF4ZHg/66cnVUplrkUpi/n2FYCH92nUJmlSH0ad5cXjhI1FA790qiQH99Uno6yYQcZM= 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 Sun, Nov 30, 2025 at 07:03:31PM +0000, david laight wrote: > (Doing the updates in the page fault handler definitely sounds like a recipe > for disaster.) See the comments in arch/x86/mm/fault.c regarding the need to do it in #PF on i386. Basically, you have CPU1: does vmalloc(), needs to grab a new slot in top-level table. Updates the page table tree for init_mm (rooted at swapper_pg_dir). Starts propagating that change to other threads. CPU2: does vmalloc(), which grabs another address in the range covered by the same slot. It works with the same page table tree, so it sees that slot already occupied, inserts a new page reference into the page table hanging off it. No top-level changes to propagate, so it returns the address it has grabbed to the caller. CPU2: caller of vmalloc() dereferences the returned object. If propagation from CPU1 has not reached the top-level page table CPU2 is using, the top-level slot is empty and MMU of CPU2 raises #PF. BTW, it might be possible for two parallel allocations to grab two areas, each requiring grabbing its own top-level slot ;-/ It certainly can happen on x86 - two vmalloc(SZ_4M) in parallel will do just that, but with NUMA you can get it 64bit boxen as well. So simple generation count on root page table won't solve it either...