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(p200300d82f251e00ce4c0be397b36587.dip0.t-ipconnect.de. [2003:d8:2f25:1e00:ce4c:be3:97b3:6587]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b9ab7c7dbsm118390555e9.11.2025.09.04.12.02.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 04 Sep 2025 12:02:31 -0700 (PDT) Message-ID: <8e1881b8-3867-4cea-b03e-50c05ed8148d@redhat.com> Date: Thu, 4 Sep 2025 21:02:30 +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] mm: centralize and fix max map count limit checking To: "Liam R. Howlett" , Kalesh Singh , Lorenzo Stoakes , akpm@linux-foundation.org, minchan@kernel.org, kernel-team@android.com, android-mm@google.com, Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250903232437.1454293-1-kaleshsingh@google.com> <827c844f-7106-4f62-a108-1f83544aa56e@lucifer.local> <43ryds7hzhs5bpaxznco7fppmakdb4f46agwtsc5erudqfoz2x@7y4jgbtft7jj> <413ee338-1795-433c-b3d4-72c870488d95@lucifer.local> <84aad392-3bff-4f98-b612-5e9a046edb36@redhat.com> <4chxl7uxr4exy2z2dcshxla3c5nzzo2tbnelsbbky7pdzrih6a@hzfnpbenfmub> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <4chxl7uxr4exy2z2dcshxla3c5nzzo2tbnelsbbky7pdzrih6a@hzfnpbenfmub> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 04.09.25 20:49, Liam R. Howlett wrote: > * Kalesh Singh [250904 13:51]: >> On Thu, Sep 4, 2025 at 10:42 AM David Hildenbrand wrote: >>> >>> On 04.09.25 19:33, Lorenzo Stoakes wrote: >>>> On Thu, Sep 04, 2025 at 01:22:51PM -0400, Liam R. Howlett wrote: >>>>>>> diff --git a/mm/mremap.c b/mm/mremap.c >>>>>>> index e618a706aff5..793fad58302c 100644 >>>>>>> --- a/mm/mremap.c >>>>>>> +++ b/mm/mremap.c >>>>>>> @@ -1040,7 +1040,7 @@ static unsigned long prep_move_vma(struct vma_remap_struct *vrm) >>>>>>> * We'd prefer to avoid failure later on in do_munmap: >>>>>>> * which may split one vma into three before unmapping. >>>>>>> */ >>>>>>> - if (current->mm->map_count >= sysctl_max_map_count - 3) >>>>>>> + if (exceeds_max_map_count(current->mm, 4)) >>>>>>> return -ENOMEM; >>>>>> >>>>>> In my version this would be: >>>>>> >>>>>> if (map_count_capacity(current->mm) < 4) >>>>>> return -ENOMEM; >>>>>> >>>>> >>>>> Someone could write map_count_capacity(current->mm) <= 4 and reintroduce >>>>> what this is trying to solve. And with the way it is written in this >>>>> patch, someone could pass in the wrong number. >>>> >>>> Right, but I think 'capacity' is pretty clear here, if the caller does something >>>> silly then that's on them... >>>> >>>>> >>>>> I'm not sure this is worth doing. There are places we allow the count >>>>> to go higher. >>>> >>>> ...But yeah, it's kinda borderline as to how useful this is. >>>> >>>> I _do_ however like the 'put map count in one place statically' rather than >>>> having a global, so a minimal version of this could be to just have a helper >>>> function that gets the sysctl_max_map_count, e.g.: >>>> >>>> if (current->mm->mmap_count >= max_map_count() - 3) >>> >>> I enjoy seeing sysctl_max_map_count hidden. But map_count_capacity() is >>> even more readable, so I like it. >>> >>> I don't complete like the "capacity" term, but I cannot think of >>> something better right now. Maybe something around "free" or >>> "remaining", not sure. >>> >>> I also don't completely like "map_count" (I know, I know, we call it >>> like that in structures), because it reminds me of the mapcount ... >>> talking somehow about "vmas" would be quite clear. >> >> Thanks David, my original implementation started with vma_limit() :). >> Maybe something like vma_count_remaining() ? > > Yes, reducing this confusion would very much be helpful. In fact, if > you put it in its own function we could change the actual name with > lower impact. map_count vs mapcount is annoying. > > vma_headroom() ? > additional_vma_space() ? VMA space might be interpreted as VA space. I think basing it on "vma_count" would be good. vma_count_capacity() vma_count_headroom() vma_count_remaining() vma_count_avail() vma_count_left() > > Maybe David would like: > remedy_vma_space() which would be !poison_vma_space().. that's pretty > clear.. :) :D Careful, sensitive topic; I know that Lorenzo is still crying about this late at night when nobody watches, screaming "DAVID, WHYYYYYYY" ;) -- Cheers David / dhildenb