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Howlett" , Helge Deller , Yang Shi , Helge Deller , linux-kernel@vger.kernel.org, Andrew Morton , linux-mm@kvack.org, linux-parisc@vger.kernel.org Subject: Re: [PATCH] [RFC] mm: mmap: Allow mmap(MAP_STACK) to map growable stack Message-ID: References: <95c4efe9-e92a-46fe-bf41-9141e125332d@gmx.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: cymwet9i943kay9dudzd4jxpyspcnyos X-Rspamd-Queue-Id: 281CD180002 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1726164750-344432 X-HE-Meta: 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 2twHgLfR QDk9yBWuz1IoX+3CpfJ09VMpCMZ+YkS2N6XYtD2TQHgICuUlktgBOX/nUAzVFhP66dFWvIXEo7b+Vl2Hb9PhqC6QjZRdj7zR/v/R8FAJ0K4NC2q3BaUE7YIoRdEkrVxUpWiEZICU4bnRTKXMBaUIjb8rglbnAXFM2Y3UfVeNQx/SAIy4m5ATHTPu0OV5EDjmHeDcj0j18EmXKnNQqmuh6sytHc7E6ySj7jPtvdA0l4bhptub3KJYpYHePU3x7cAABlwV7ZrdlzlFDwrlhctH/ukrk1g== 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, Sep 11, 2024 at 09:32:29PM -0400, Liam R. Howlett wrote: > * Helge Deller [240911 20:51]: > > On 9/12/24 01:05, Liam R. Howlett wrote: > > > If someone is using MAP_STACK to avoid having a huge page, they will > > > also get a mapping that grows - which is different than what happens > > > today. > > > > > > I'm not saying that's right, but someone could be abusing the existing > > > flag and this will change the behaviour. > > > > Wouldn't a plain mmap() followed by madvise(MADV_NOHUGEPAGE) do exactly that? > > Why abusing MAP_STACK for that? > > I can think of two answers: > 1. An error that has worked without issues so far > 2. One less system call > > I'm not saying this really is a blocker, but the change is not without > risk as it does change behaviour the user could see. > > Interestingly enough, the man page is incorrect as it is written because > the flag is not strictly a no-op; it ensures no huge pages. So the > feature of applying VM_NOHUGEPAGE with the use of MAP_STACK is not > documented today. It's a recent change and I don't think it's something we necessarily want to document. It was c4608d1bf7c6 which was December 2023.