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[2003:d8:2f12:1b00:5d6b:db26:e2b7:12]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6e80fed75sm3939229f8f.66.2025.06.25.00.49.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 00:49:49 -0700 (PDT) Message-ID: <6bda0de6-1ade-40c9-aa52-16bc02d98bee@redhat.com> Date: Wed, 25 Jun 2025 09:49:48 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled To: Lorenzo Stoakes Cc: Hugh Dickins , Baolin Wang , akpm@linux-foundation.org, ziy@nvidia.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, zokeefe@google.com, shy828301@gmail.com, usamaarif642@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <75c02dbf-4189-958d-515e-fa80bb2187fc@google.com> <9cb94544-f65a-4394-b1e2-bfb226ead31c@redhat.com> <8691d74b-67ee-4e26-81ac-f6bf1725361e@redhat.com> <3b6db0c3-aef3-4a21-a154-6aafd639dbc7@lucifer.local> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 25.06.25 09:42, Lorenzo Stoakes wrote: > On Wed, Jun 25, 2025 at 09:36:53AM +0200, David Hildenbrand wrote: >> On 25.06.25 09:30, Lorenzo Stoakes wrote: >>> I _guarantee_ you that's what nearly everybody except a handful of people will >>> expect. >> >> I know, See my other mail, the problem is rather if there is no somebody >> relying on never+MADV_COLLAPSE from doing the MADV_COLLAPSE-documented >> thing. >> >> It's a mess. > > Well now we have an almost philosophical debate - we have different sets of > users, 99% of whom believe the uAPI is X, and 1% of whom believe it is Y. > > Now what is the uAPI? What is 'breaking userspace'? :) Yeah, that's why I mentioned that I think we broke "something" when we changed the semantics. But that breakage probably only affects real corner cases (debugging, customer workarounds). I think the whole use case of using MADV_COLLAPSE to completely control THP allocation in a system is otherwise pretty hard to achieve, if there is no other way to tame THP allocation through page faults+khugepaged. Mess. -- Cheers, David / dhildenb