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Fri, 27 Mar 2026 09:23:13 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id pbhvNIFMxmlkGQAAD6G6ig (envelope-from ); Fri, 27 Mar 2026 09:23:13 +0000 Date: Fri, 27 Mar 2026 09:23:12 +0000 From: Pedro Falcato To: Andrew Morton Cc: "Lorenzo Stoakes (Oracle)" , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jianzhou Zhao , Oscar Salvador Subject: Re: [PATCH 0/3] mm: improve map count checks Message-ID: <2bkj3z4mgwmobscv4huqu26cwykn57juljqf46ljwagghzh2kx@lycizofaukl7> References: <20260326224239.9052f8be3ddb9cac600f59f5@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260326224239.9052f8be3ddb9cac600f59f5@linux-foundation.org> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 3723514000E X-Stat-Signature: wt1etq17bp6zk9hfbwph379b6pq851zh X-Rspam-User: X-HE-Tag: 1774603395-837830 X-HE-Meta: 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 fbERKbYy CA7MxpffHAtDuLsrsYVelQe/b02ecpooozk86+JvcOfQt79JcGdKhNd9PNJR3mm6dsk6boOqoXR3mPDDo2dXzvRZ8VDMdD0rGnAqb4LMmw2ruotXUvRcj34znnAbO9TzKyURqNZ85+6ryKYGvdOjBgZUKLlQcjq5Cidx3/2up+TyEgC+ytH2yrxM0xkvvn4m3SV0ACxWzmbHNUCb0zZeia5tbRK/DwtvXi/HKoY7kvMS4t4bBd9rKaEatogYvywzwq9qDcCkzOEcR5FggBGbxO/ArR+GNWheyi4ZCF7P96nkWCCE955Z3xBt7wwnYp1c/0lJw+JydJ7KjFb4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Mar 26, 2026 at 10:42:39PM -0700, Andrew Morton wrote: > On Wed, 11 Mar 2026 17:24:35 +0000 "Lorenzo Stoakes (Oracle)" wrote: > > > Firstly, in mremap(), it appears that our map count checks have been overly > > conservative - there is simply no reason to require that we have headroom > > of 4 mappings prior to moving the VMA, we only need headroom of 2 VMAs > > since commit 659ace584e7a ("mmap: don't return ENOMEM when mapcount is > > temporarily exceeded in munmap()"). > > > > Likely the original headroom of 4 mappings was a mistake, and 3 was > > actually intended. > > > > Next, we access sysctl_max_map_count in a number of places without being > > all that careful about how we do so. > > > > We introduces a simple helper that READ_ONCE()'s the field > > (get_sysctl_max_map_count()) to ensure that the field is accessed > > correctly. The WRITE_ONCE() side is already handled by the sysctl procfs > > code in proc_int_conv(). > > > > We also move this field to internal.h as there's no reason for anybody else > > to access it outside of mm. Unfortunately we have to maintain the extern > > variable, as mmap.c implements the procfs code. > > > > Finally, we are accessing current->mm->map_count without holding the mmap > > write lock, which is also not correct, so this series ensures the lock is > > head before we access it. > > > > We also abstract the check to a helper function, and add ASCII diagrams to > > explain why we're doing what we're doing. > > This little series has no reviews, if anyone is wanting to boost our > R-b stats. Thanks for the heads up! This was too stale for too long :) -- Pedro