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[100.36.248.188]) by smtp.gmail.com with ESMTPSA id af79cd13be357-907b9772b87sm1195078585a.5.2026.05.11.19.50.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2026 19:50:51 -0700 (PDT) Date: Mon, 11 May 2026 22:50:48 -0400 From: Gregory Price To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, "David Hildenbrand (Arm)" , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , Muchun Song , Oscar Salvador , Andrew Morton , Lorenzo Stoakes , "Liam R. 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Howlett" , Harry Yoo , Hao Li Subject: Re: [PATCH resend v6 03/30] mm: thread user_addr through page allocator for cache-friendly zeroing Message-ID: References: <9b53972f4854c1064b92cefc464f51949afeb83f.1778489843.git.mst@redhat.com> <20260511114853-mutt-send-email-mst@kernel.org> <20260511175032-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260511175032-mutt-send-email-mst@kernel.org> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 78E4FC000B X-Rspam-User: X-Stat-Signature: na4476qmuhzhri69qxezbtqgnkiyfjyn X-HE-Tag: 1778554252-619656 X-HE-Meta: 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 Epj2mZ02 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 11, 2026 at 05:59:23PM -0400, Michael S. Tsirkin wrote: > On Mon, May 11, 2026 at 12:52:10PM -0400, Gregory Price wrote: > > On Mon, May 11, 2026 at 11:55:40AM -0400, Michael S. Tsirkin wrote: > > > On Mon, May 11, 2026 at 11:37:37AM -0400, Gregory Price wrote: > > > > On Mon, May 11, 2026 at 05:01:55AM -0400, Michael S. Tsirkin wrote: > > > > > > > > > +/* > > > > > + * Sentinel for user_addr: indicates a non-user allocation. > > > > > + * Cannot use 0 because address 0 is a valid userspace mapping. > > > > > + */ > > > > > +#define USER_ADDR_NONE ((unsigned long)-1) > > > > > > > > Ehm, hm. Does -1 hold as a non-user address across all architectures? > > > > > > > > What about in linear addressing / no VM mode? > > > > > > this is used on a fault. I don't think there are any faults then? > > > But maybe FAULT_ADDR_NONE would be clearer. > > > > > > > Meh, naming here is less relevant than the sentinel correctness. > > > > My only concern is really whether -1 could end up being a valid address > > in some horrid future timeline and this all going belly up. > > > > Is why I asked about whether this is correct on all architectures. > > I think the answer is yes: on all architectures Linux supports, the last > page of the address space is never a valid user mapping. The kernel > enforces this -- mmap will not create a mapping whose end wraps past -1. > > if (addr > TASK_SIZE - len) > return -ENOMEM; > > > So the maximum vm_end = addr + len <= TASK_SIZE. > On every architecture TASK_SIZE <= (unsigned long)-1, so vm_end can > never wrap around to 0 and address -1 can never be within any > VMA. > That is worth documenting at least in the changelog if not the comment with the define there's a clue if anything ever changes. ~Gregory