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b=DW+Lzp9HdPWLoHdkJz7iOn+k+Z1Cetdo1F/t4kgwns9ANb8hZ2oHNjsL6dcpj0nb2 TKKuLdr/tyJ4ukjNShLQQ2ZaWdvXybQKk5k0WolzqgvDjiVb+SWQIt4aSKSeo3/S/1 crz3L+LskPut0T8BbdoJMAQ689QSTlbYc2AIf+rIR/dEcFcFEHAhKrWKzIKi6JJpMQ OjJNXHtw6YTAZyMJX5OnWmekoc4Oy63ZSvAljoqu9p3m0ZXFtRVmRBPob4IJobVyNv abktZzol5yoSlbzDhCxx4gC/a9SgkkWq9caWAbRKmbe3DlQK3PbRPqFky6OBSYk1Wo +ZdxE+6/cLK7g== Date: Wed, 14 May 2025 16:13:46 +0100 From: Will Deacon To: Ryan Roberts Cc: Catalin Marinas , Pasha Tatashin , Andrew Morton , Uladzislau Rezki , Christoph Hellwig , David Hildenbrand , "Matthew Wilcox (Oracle)" , Mark Rutland , Anshuman Khandual , Alexandre Ghiti , Kevin Brodsky , linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+5c0d9392e042f41d45c5@syzkaller.appspotmail.com Subject: Re: [PATCH] arm64/mm: Disable barrier batching in interrupt contexts Message-ID: <20250514151345.GA10762@willie-the-truck> References: <20250512102242.4156463-1-ryan.roberts@arm.com> <20250513204603.GA9866@willie-the-truck> <8d4e66ef-a292-45ff-9c4a-0248aff44fd3@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8d4e66ef-a292-45ff-9c4a-0248aff44fd3@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 36CE0C0009 X-Stat-Signature: zqy6ayri535e3gstjcxjw6byfseogwxe X-HE-Tag: 1747235634-463208 X-HE-Meta: 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 mGnG0OBe MV/MbA13FYv03wsOOtTlXtaMYq9huAGSWKXUaKxTRu3xh1o3zT+b6pivp4rryEuUawq+FNEJwlAs6VopOnYEDUQPKEKR6CzICeGazi0gybKWEb4aTfvDZon8VHpCDTV1R+zgKn8jzGFFIS9OidqanZzlnWx1wdo8/wWQ3RnYTOqR2YypxZlWi9UMXUdsUUOPyE1xDcHHJE7bvVwWA62X38+wro6E7qgNpw3FeG31/aheVjgteul0gsfwQ2D48qrkkECsQfooEhof5yHAgIS4O/MBwmFRhq0in8vJVXtER/q15kyu0e1LbZ4yfWuSnwgkw82/4 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 Wed, May 14, 2025 at 10:29:17AM +0100, Ryan Roberts wrote: > On 13/05/2025 21:46, Will Deacon wrote: > > On Mon, May 12, 2025 at 11:22:40AM +0100, Ryan Roberts wrote: > >> Commit 5fdd05efa1cd ("arm64/mm: Batch barriers when updating kernel > >> mappings") enabled arm64 kernels to track "lazy mmu mode" using TIF > >> flags in order to defer barriers until exiting the mode. At the same > >> time, it added warnings to check that pte manipulations were never > >> performed in interrupt context, because the tracking implementation > >> could not deal with nesting. > >> > >> But it turns out that some debug features (e.g. KFENCE, DEBUG_PAGEALLOC) > >> do manipulate ptes in softirq context, which triggered the warnings. > > > > Hmm. Do we also have to worry about the case where a softirq is triggered > > off the back of a hardirq *and* that hardirq is taken while we're in the > > middle of e.g. queue_pte_barriers()? In that case, I think we can end > > up in strange situations, such as having LAZY_MMU_PENDING set when > > LAZY_MMU is clear, although it looks like things still work even in that > > case. > > I don't see any problem here. This change ensures that we always behave the > "old" way in interrupt context. So the interrupt context will never even look at > those TIF flags, so it doesn't matter that the task context is midway through > changing the flags when the interrupt comes in. > > (although somehow I feel like I should be bracing for a zinger :) Ha, for some reason, I was looking at the code _without_ your fix applied. Although it's quite hard to think about, I couldn't spot any other issues with nesting beyond the one you call out at the end of the commit message. Your patch makes all of this a lot simpler, though, so I'll pick it up (along with the other one). Thanks, Will