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b=nJrnAQ1yhxtMBG44ZNjv1H1OdQEHQj6Tb3C+lAHtIhaV2h8jNYInA4ake/qXooMSz LU1J/Mo30EGj2iYo+8htzhcOQPH00kbXyEFAZlj8XXBxVn5DoOxhqsMtfWg0enSNYS PCnpzoPNoxuc4eQ7NipBRw7MaX4nrMCNrkBDTvA71D2/UmrV0ABJDUm7YXAKDUiu7l cQeE02ugsBcBGUW4hQeB/i1v+MMa8ps/OQPoNDOh/NI/FAOyUfqbnPHHTZFWLIaCpE y0Hypz5mbIHBhVRc2bbYCeOyuWNPNIuIj9edgDR7ojCIfSpMw0oe1s7M8ScIQmmMJf nFeoLXU8PmDcw== Message-ID: <42087cd7-473e-40a7-ae63-3c841c615c8f@kernel.org> Date: Mon, 12 Oct 2026 12:09:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/madvise: keep the hardware dirty bit in MADV_COLD's THP path To: samarth.colleges@gmail.com, akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org Cc: vbabka@kernel.org, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261011132227.413217-1-samarth.colleges@gmail.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: <20261011132227.413217-1-samarth.colleges@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: y5fmttqfrsq4dm1c9uoo1qbjfudf47k8 X-Rspam-User: X-Rspamd-Queue-Id: 270E6C0007 X-Rspamd-Server: rspam08 X-HE-Tag: 1791799754-856246 X-HE-Meta: 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 jb4qVrjs MsQsc4utrvuwLCrZZLSJFx1llTInpsRvNH6krpofsRwm8D1iaNI+Wj3+aOA7oBzVNDw5b8PuYXMx0TdqMDHHhzXmFbe1fbgmaD0obEyfajH9W60D+WqiFdIBaRth7ptO+lQHp0sCjxn4JV5q+SXJK+hy9bAtJv6N/1T8CW3oX7ORcXiwxYPtmIrSqEtJEWeE/CHA4Y8r7rd6mg9U+it5QSNr0DFRzGCfSl9KRplVpQGBZxgDTxMJTAKqRuklt3xUCt1iC9r2M/PHCQ/Pq0IEnl/aN+8nS2VBy1CtsMSLMbOc0HZRqhPQ0nOEaZROWoEPFx9UyZiRy7AHT/2Mh5wrRpXVRZB9T28sMmW+HI1QCB01LKeTE4Hq8wz183RHn4NzYnptFIBuCFFUw8ZzemEri2ZGhpAR2dkAqZnXFiSiZ/D5R+vA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/11/26 15:22, samarth.colleges@gmail.com wrote: > From: Samarth Narang > > For a young, PMD-mapped THP, madvise_cold_or_pageout_pte_range() handles > MADV_COLD by reading orig_pmd = *pmd, calling pmdp_invalidate(), > discarding its return value, and writing back pmd_mkold(orig_pmd): > > if (!pageout && pmd_young(orig_pmd)) { > pmdp_invalidate(vma, addr, pmd); > orig_pmd = pmd_mkold(orig_pmd); > set_pmd_at(mm, addr, pmd, orig_pmd); > tlb_remove_pmd_tlb_entry(tlb, pmd, addr); > } > > pmdp_invalidate() atomically clears the present bit and returns the PMD > as it was in the page table at that instant. A store from another CPU > that lands after orig_pmd is read but before the invalidate sets the > hardware dirty bit in the live entry. Because the written-back value is > derived from the stale orig_pmd, that dirty bit is erased. > > If the THP had been marked lazyfree (MADV_FREE), reclaim later finds a > clean PMD and a clean folio and discards it rather than swapping it out > (__discard_anon_folio_pmd_locked()). The store is then silently lost: a > subsequent read returns stale/zero data, although the MADV_FREE contract > is that a write after the advice call cancels the free. > > pmdp_invalidate() returns the pre-invalidate entry precisely so callers > can carry the accessed/dirty bits hardware may have set concurrently > (d52605d7cb30). change_huge_pmd() and set_pmd_migration_entry() > (8a8683ad9ba4) already do this; the MADV_COLD path does not. Rebuild > orig_pmd from the returned entry. > > Reproduced unprivileged on x86-64: a 2 MiB anon THP is marked MADV_FREE, > then MADV_COLD races a store from another CPU, then MADV_PAGEOUT; the > stored byte reads back zero. On current mainline built and booted under > KVM, the race lost 131 of 8000 stores before this patch and 0 of 8000 > after; a store-before-COLD control and a MADV_NORMAL control lose none. > > Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD") > Cc: stable@vger.kernel.org > Signed-off-by: Samarth Narang > --- > mm/madvise.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/mm/madvise.c b/mm/madvise.c > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -426,8 +426,14 @@ > } > > if (!pageout && pmd_young(orig_pmd)) { > - pmdp_invalidate(vma, addr, pmd); > - orig_pmd = pmd_mkold(orig_pmd); > + /* > + * Rebuild from the entry pmdp_invalidate() exchanged > + * out, not from orig_pmd: hardware may have set the > + * dirty bit since orig_pmd was read, and a MADV_FREE'd > + * (lazyfree) THP whose dirty bit is lost is discarded > + * by reclaim, silently dropping the store. > + */ > + orig_pmd = pmd_mkold(pmdp_invalidate(vma, addr, pmd)); Okay, you are saying we can lose the dirty bit that could get set concurrently. And if we did an MADV_FREE before, we might not detect that the content was changed. I suspect this found through the help of an LLM and not an actual use case in practice? Using MADV_COLD following MADV_FREE sounds odd at thirst. In particular, when having MADV_COLD race with actual access. On the PTE path we use clear_young_dirty_ptes(CYDP_CLEAR_YOUNG) which achieves the same through ptep_test_and_clear_young(). ... where we also just do a pte = ptep_get(ptep); set_pte_at(vma->vm_mm, address, ptep, pte_mkold(pte)); in the generic variant and rely on the per-arch variant to do the right thing. Which makes me wonder whether we should actually have per-architecture PMD helpers for that purpose? -- Cheers, David