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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , 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: <20260326224239.9052f8be3ddb9cac600f59f5@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 7BD3C180004 X-Stat-Signature: qo71ga6yc3me1up8hdwkmo1ubnc7n5c7 X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1774590161-533870 X-HE-Meta: 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 +OfSsNLh HFe17ji1eFxKMSo4Es4ROiX09ReMLIf4N0xPx8zymBI3ghnp+QJbg6S7BIH2vBMSsP58rVAGsXp/fQQuN9XRTycCfqYK5BS7s9O9SrH8iCCIlXaXyQPgI8/7WQD18UQ056TJwSWG/k7ZdYHRvX5BucLEUdszX9QzE3u9D/Fp4GZNr7Mge73W01pfmjJcx3e40AuDh3zFLqg39PFiqusGBuV58JOfMkvmdLnXv/sTaUJi/2sG9j3/csWbFHzWMqfswgF+/ppIXO2klz4wSTypCJ2DAFA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Or not. I plan to upstream it unless someone stops me.