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Thu, 06 Feb 2025 11:44:17 -0800 (PST) X-Relaying-Domain: purestorage.com Received: from dev-ushankar.dev.purestorage.com (dev-ushankar.dev.purestorage.com [10.7.70.36]) by c7-smtp-2023.dev.purestorage.com (Postfix) with ESMTP id EF15534068B; Thu, 6 Feb 2025 12:44:15 -0700 (MST) Received: by dev-ushankar.dev.purestorage.com (Postfix, from userid 1557716368) id E3D88E55FD7; Thu, 6 Feb 2025 12:44:15 -0700 (MST) Date: Thu, 6 Feb 2025 12:44:15 -0700 From: Uday Shankar To: Oscar Salvador Cc: Muchun Song , Andrew Morton , Joern Engel , linux-mm@kvack.org Subject: Re: [bug report?] unintuitive behavior when mapping over hugepage-backed PROT_NONE regions Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Stat-Signature: ybgfb991dpgiidqjzcgahojt6daderd5 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 03900100010 X-HE-Tag: 1738871058-918529 X-HE-Meta: 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 9lUcWUl+ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.433491, 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 Thu, Feb 06, 2025 at 10:01:05AM +0100, Oscar Salvador wrote: > On Wed, Feb 05, 2025 at 11:18:34PM -0700, Uday Shankar wrote: > > I was debugging an issue with a malloc implementation when I noticed > > some unintuitive behavior that happens when someone attempts to > > overwrite part of a hugepage-backed PROT_NONE mapping with another > > mapping. I've isolated the issue and reproduced it with the following > > program: > ... > > > First, we map a 2G PROT_NONE region using hugepages. This succeeds. Then > > we try to map a 4096-length PROT_READ | PROT_WRITE region at the > > beginning of the PROT_NONE region, still using hugepages. This fails, as > > expected, because 4096 is much smaller than the hugepage size configured > > on the system (this is x86 with a default hugepage size of 2M). The > > Not really, see how ksys_mmap_pgoff() aligns len to huge_page_size if we > set MAP_HUGETLB. > It fails with ENOMEM because likely you did not preallocate any hugetlb > pages, so by the time we do hugetlbfs_file_mmap()->hugetlb_reserve_pages(), > it sees that we do not have enough hugetlb pages in the pool to be reserved, > so it bails out. Yeah, you're right, this is a system without any hugepages preallocated, and that makes the error code make a lot more sense. Sorry for any confusion. This is why I should report issues when I discover them and not 2 months later :)