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b=HGxHi5X1wkkztSUobbCILvTlRsfOY7LnJ3KTOjPX0l7odb6o+J/uI1/R4WGZxpeRU XPCYkK7HhyFgwz2d2/SvcVzlAvFJcpQ3+2z2lcc86ZH0TsW45xY00eBI8jNnRmLeWU kA35nBILIMVHtdySHkZ8M+wNEKBFLCEWZS2JEgzNIM86ZS0lkdTEzIlEwKjEtBNr8x YqZHodcEhWYOhxQA0+cTt52EuqjmEA8ls55FqclzZriy0oHpCymA8EPlYTETCzQwy3 U3HLWZTfIe9w80700dWV3ZI25HSrA4Zl0+9bkoouf7MiwmbmKnbtkSbQsUkeKB1DCC /2blUit7vaygQ== Message-ID: Date: Wed, 9 Sep 2026 18:59:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: fix overbroad MMU notifiers for unshared PMDs To: Li Zhe , Andrew Morton Cc: muchun.song@linux.dev, osalvador@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, aiqi.i7@bytedance.com References: <20260831091023.66581-1-lizhe.67@bytedance.com> <20260831173220.72ab28190a44a305dacf4d04@linux-foundation.org> <47a6817e-e98e-4a00-8e2c-6a66d15ddb37@kernel.org> <874e7793-877a-40a5-97ce-11dd5b9ee9ba@bytedance.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+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: <874e7793-877a-40a5-97ce-11dd5b9ee9ba@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 831C340002 X-Stat-Signature: sqm1abcdxbhrssar9yh3bji9xudfdfqi X-Rspam-User: X-HE-Tag: 1788973197-158405 X-HE-Meta: U2FsdGVkX19AsQY5ZN828K/Gn6insC/0XE7lbL4NZwpHoMi9UcNwokQ2fT0e0F9RXW7OENqPXwrGUthJPb1z1MjqCF7FfYXR/JM7CDyC8mM2GBclIg2PvWQUEdEZYfuOIjCelmuEi4sBOpNIRhVUVEctu1LzbE5KXpduxL1zYnvb0Ej+lYWmKQKFSb1incPwcQ6rnqBggcOlpN8d7oRt28sYP4+yzLGxOnt8XUJSoyUZzKeNCqKfHC4+eSfUmO4NgX/REgbS2gHg6BjFbgv0VHUo04c17y2twTbLWsflfILbVI1Mvah47UKQ/uwUcTKQMZg6CIvcpsVwKpqFOjBapmR1XBEcmReFFDa/wUPw7JKHURFngEBaTrszHN9EA3Xe0RSSLsQnEtWlj/Zx3GAqvdIY41U0BdAONXmql7C419fc8Wr5eK05eeiqERKoDWmAqbKtyLQu9Kfn150RFYVJwf/T1fEZKP1nZKxCzIELDG2MoVxSo66CeqO2WZxfqWUauOH0AKe/wwgOoNYTNNab+mbhobj58k2tCWDplFuEKKnskpHSh9GeZVXJRjM53TcixCjpoNtvD5dVm73Gddp5IvCQxJxgFd6ipqe8+P0rgTaOAOt8asTj9BR7r/nuS22pYDEtuO7oLdN/qqQYfFFDsOXP/VQx5eZd8xqSW2WdsR3OH4eh04J79qN3ZIyo526R8R4L1o5RcNfHHaJhbeRCLoUcEQiBNYNZNg7ocBQFQPqgamh3/8rXGsVUyughLVPVz1WY5LX2pJhrQLMM9SiE5S5XzXDDz4i3lHh0Z0nMLg53UiE0MEgRUdwD6YdYfI0s+XJTLGRZxHsjVUCTMH6llUT+8oOQia/7aLbVWz0cVV6N8O21Keh0DHy6TguFuHjg0bzyaemUyZQaSP1J0sAZGQTBVYKEUwTQMB+xKBuuJkHheUSiRoA3duTbkYLgu2IVYa5ROemZJI2K0HW4tuX d4f5pU8Y F+B6GWFdIlYTLrePVDRtdTdcCZWxqQhmsbFceTM9REbgoVCLvbxvjmOYkqTilz3AlIxH6tWXlzc9X730PlAkWSvOXAiyabuVWPPREbeQWPHSsBRaeh9qrCA7M2cuXP+U+ND38T76YItlfNLAZkkoBQRhvNo6t2nfewTCDuPnvx20CymIoqyohBl1JvYbTQMLTEBQ5ECHxJ2AGmk/nFQl5V+C6YjJohNhMMd5ybeCCIbqN7JwlUkr6iDQQztcw/BtBykg4vdl8NJ8+g9OK0Q7PlHJ+Ui6w378Y4tWn Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/8/26 09:09, Li Zhe wrote: > On 9/7/26 11:29 PM, David Hildenbrand (Arm) wrote: >> On 9/1/26 02:32, Andrew Morton wrote: >>> >>> Thanks. >>> >>> So a modest performance improvement? >> Is that worth the complexity, though? > > > That is a fair concern. In you setup, are the page tables ever being shared? I suspect you just run a VM with no other processes actually sharing the memory? One idea would be to just remember whether any sharing ever happened for a hugetlb file. > > I tried to simplify the approach. Instead of computing the exact PUD > sub-ranges that contain shared PMD tables, this version keeps the > existing adjust_range_if_pmd_sharing_possible() logic unchanged and only > adds an actual shared-PMD check as a gate before it. > > So the behavior becomes: > >   - if no shared PMD table is found, keep the notifier range unchanged; >   - if any shared PMD table is found, fall back to the existing >     conservative PUD-sized expansion. > > This should still avoid the unnecessary 1G KVM invalidation for the > common unshared-PMD case, while keeping the range expansion policy > unchanged when a shared PMD is present. > > The resulting diff would look like this: > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 816dde7..7a61fc3 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -5355,10 +5357,12 @@ void __hugetlb_zap_begin(struct vm_area_struct *vma, >      if (!vma->vm_file)    /* hugetlbfs_file_mmap error */ >          return; > > -    adjust_range_if_pmd_sharing_possible(vma, start, end); >      hugetlb_vma_lock_write(vma); > -    if (vma->vm_file) > +    if (vma->vm_file) { >          i_mmap_lock_write(vma->vm_file->f_mapping); > +        if (range_has_shared_pmd(vma, *start, *end)) > +            adjust_range_if_pmd_sharing_possible(vma, start, end); > +    } >  } > >  void __hugetlb_zap_end(struct vm_area_struct *vma, > @@ -5397,7 +5401,9 @@ void unmap_hugepage_range(struct vm_area_struct > *vma, unsigned long start, > >      mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, >                  start, end); > -    adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end); > +    if (range_has_shared_pmd(vma, range.start, range.end)) > +        adjust_range_if_pmd_sharing_possible(vma, &range.start, > +                             &range.end); >      mmu_notifier_invalidate_range_start(&range); >      tlb_gather_mmu(&tlb, vma->vm_mm); > > @@ -6958,6 +6964,52 @@ void adjust_range_if_pmd_sharing_possible(struct > vm_area_struct *vma, >          *end = ALIGN(*end, PUD_SIZE); >  } > > +static bool range_has_shared_pmd(struct vm_area_struct *vma, > +                 unsigned long start, unsigned long end) > +{ > +    unsigned long v_start = ALIGN(vma->vm_start, PUD_SIZE); > +    unsigned long v_end = ALIGN_DOWN(vma->vm_end, PUD_SIZE); > +    struct hstate *h = hstate_vma(vma); > +    struct mm_struct *mm = vma->vm_mm; > +    unsigned long address; > + > +    if (huge_page_size(h) != PMD_SIZE) > +        return false; > + > +    /* > +     * First apply the same cheap test as > +     * adjust_range_if_pmd_sharing_possible(). Only the PUD-aligned > +     * intersection can contain shared PMD tables. > +     */ > +    if (!(vma->vm_flags & VM_MAYSHARE) || !(v_end > v_start) || > +        end <= v_start || start >= v_end) > +        return false; > + > +    start = max(ALIGN_DOWN(start, PUD_SIZE), v_start); > +    end = min(ALIGN(end, PUD_SIZE), v_end); > + > +    hugetlb_vma_assert_locked(vma); > +    i_mmap_assert_write_locked(vma->vm_file->f_mapping); > + > +    for (address = start; address < end; address += PUD_SIZE) { > +        pte_t *ptep; > +        bool shared; > + > +        ptep = hugetlb_walk(vma, address, PMD_SIZE); > +        if (!ptep) > +            continue; > + > +        spin_lock(huge_pte_lockptr(h, mm, ptep)); > +        shared = ptdesc_pmd_is_shared(virt_to_ptdesc(ptep)); > +        spin_unlock(huge_pte_lockptr(h, mm, ptep)); I really don't like this piece of code to optimize something that is already questionable in practice: overcommiting hugetlb folios for VMs. Can you share some more details which mechanism ends up zapping hugetlb folios for the VM? If it's virtio-balloon's free-page-reporting, you should likely disable that for the VM. -- Cheers, David