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[2003:d8:2f12:1b00:5d6b:db26:e2b7:12]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6e80f2a4csm3959490f8f.65.2025.06.25.01.16.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Jun 2025 01:16:47 -0700 (PDT) Message-ID: <28051538-d3ea-4064-aef3-89f6dd98b119@redhat.com> Date: Wed, 25 Jun 2025 10:16:46 +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 From: David Hildenbrand 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> <6bda0de6-1ade-40c9-aa52-16bc02d98bee@redhat.com> 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 09:49, David Hildenbrand wrote: > 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. Just want to add: for an app itself, it's doable in "madvise" mode perfectly fine. If your app does a MADV_HUGEPAGE, it can get a THP during page-fault + khugepaged. If your app does not do a MADV_HUGEPAGE, it can get a THP through MADV_COLLAPSE. So the "madvise" mode actually works. The problem appears as soon as we want to control other processes that might be setting MADV_HUGEPAGE, and we actually want to control the behavior using process_madvise(MADV_COLLAPSE), to say "well, the MADV_HUGEPAGE" should be ignored. Then, you configure "never" system-wide and use process_madvise(MADV_COLLAPSE) to drive it all manually. Curious to learn if there is such a user out there. -- Cheers, David / dhildenb