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b=MbQYG4T+HrUcgGCJB8Mr03WSESTKYUFJ+qZJSTPAvzfVoBtgEeSK+dmKLTgClqmu3 Br9W2bQAi2zTh5qhSrSzPZ/4/LthoT+d2d0HCfgeGBe8fCLqVuhUsF+T40JocKxYBA qq4U7Ch/Dng+qFOv7tZUfMZalopkfx2AzXal8G1hyqz7m5WHU4Zf6voNZWLLmFSiTr e11LREhFwp7RHVy01ngL6IiRJ55rj89LCM6NmoVpAg2g12j/v5JnGFMjHN64n0frFA CzXv0dX+0NwVq9yUbT5cdkIew2c1xUy8lJF9pKHn6ThLoEJf1PJ/A7iItNkTaSGiXe O/cD+MI1FqhYA== Message-ID: <60634ab5-3fcc-42fb-8722-4ba6771acfda@kernel.org> Date: Wed, 23 Sep 2026 15:49:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/migrate_device: consolidate compound folio handling To: Hui Su , Andrew Morton Cc: Balbir Singh , Matthew Brost , Zi Yan , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260923112041.2103427-1-sh_def@163.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: <20260923112041.2103427-1-sh_def@163.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: B379EC000C X-Stat-Signature: 6w43nq9agxpq9s4dz6gpq3hxn7gcwnh6 X-Rspam-User: X-HE-Tag: 1790171376-349613 X-HE-Meta: U2FsdGVkX185lUVPzOeo8fSt2RyFktFizSMh8XYo2iV1yWuJTkBY4G0rx2TJpNu+ngPysf9VajY/Pzon3Gz+zU1dIUPEykA8g9qF3PUDqBNTWQwGCSZHKIOaos20b7SaKMFdNlHqfaErzbMeS3PFJfSKoGd+CozGym5h9YAcEIO1sdZB6sH3mE8nGj2Oll9Ad3W2QBzUY52gCI+xo3UPJJdlKrOkP/8eX6lDz0JzZVgzithV5RUoyB7Kr7QosobJ+Pd69qGq4bebRFJkab6FulhrfKE0At36coOWZnznwtotvWTBAw8W2lZlbYdZGKt+S4XKPJL9Xn3PIkaCv/rbwzXwD2fr6gE0HKuIX8kQuyFOW8CA0mvWc70rfpMdrejhue52mPQlHj3Tzez5cHD1GvHSOcBtR3hh15xWrtTpoGY+bvPo5erG6uYsXnerNdJX238Y5sdZ1jqZy+OebG8vgpDdiWs/DULP/hYXkjA5VYcR5y3VpZO1XcVFxBTIOG4i11XQpXEHGoEifWBvir6VzYVl/Jn66i21hiWIoK1D+xCBrkN4gQzpFPtR2WdZgYzP/jgrGhuK1OdE3h5a5czr7AReDfQZBcObCelgTiJvEcbL5RYOCACGOtjZ9JSavDpRMannhwgFVIwXQSEPkFG/zuTddIO1xclObg+6wWCG3EaKqW4rvY4+Pg64h098np/txT+GbYuUORhK5k00voHOH8aqVJ1qXRk1lteyTxMB24OEBlE2kxTFaFI4h2F9ZzJCVAC8zDM/S0WLbNE3CUib0X2o+Nr9Npf4bo1tJprLYIF8gdhmbaWajs8Z804C064MECSjfLcyt5a0+i83MFzqO2K3nm18Mw/S4zTXPeswqVohxi6po5XNk4eyfukjWtahlBqAVj9USLXb+6mOSP1/7vO/ckrfIT6Kqc5suZAbhLUfkf6DLa5q8Fg7PEcavZjA08XU8Fwduo5bPvhAHnj 0P4hwMn3 NTxriskQ5LZKCBFpLBMLLoy8N9edERUW6U2ms6qg+6NIi1SbAQuG8x9X3x0PlNdhrT8osGoBUK9cG8yZxCq+qqOZexqUvZvFXpEymrS7TRN0qa8S9apopFqDPrhgUG1ecdLBwY4ya41X0JkojmHP7XkT4YrF7uBZKEXA9fUdXr7He9ss/eWnvzkHVpsgfsewRAign0QF2b3Cw1DiOZ91sCbofSq4U5bSmu9J5gcxLWEC8o/Bczutx6/wzSE7dMiDMMDeKvG5APwB9rnLkbXj9WaHhjazQu4XVqpcFCarCKEWPRNSYlY1SmJjbJHt92GMVXYq1Mg2ek+OOwR22mC8MxONwQbPGKpbdWSbqnFeKGbffhhfEzfWzJdJyrvGcnLkqwrNnkOPkhOMH+ZuOisNpK9QfhqcGdQXpyJeGB+1/NnvGw2JMYcOKOfwPz3GzueAWcKkQwmuj3gcE89c= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/23/26 13:20, Hui Su wrote: > Commit dc41e961a269 ("mm/migrate_device: avoid out-of-bounds writes for > compound folios") added handling for compound folios that do not fit in > the remaining PFN array. > > migrate_device_range() and migrate_device_pfns() duplicate the logic for > locking device PFNs, encoding compound folios, and handling this boundary > condition. > > A compound folio cannot be represented partially for migration. Use > VM_WARN_ON_ONCE() when one does not fit in the remaining PFN array, while > retaining the existing defensive handling: release any lock and reference > acquired for the current folio, clear the remaining entries, and stop > collecting. > > Move the shared collection and encoding logic into a helper so both > interfaces handle compound folios consistently. Also use memset() for the > compound-folio tail entries instead of open-coding the clearing loop. > > If locking a folio fails, leave its source entry zero while still consuming > and clearing the slots belonging to the whole folio. This prevents a tail > page from being treated as a new source PFN by the caller's next iteration. > > Document that an encountered compound folio must fit entirely in the > remaining range or PFN array. > > Link: https://lore.kernel.org/r/c99ca53a-73ef-4a0c-8738-eba1cc89bea2@kernel.org > Suggested-by: David Hildenbrand > Acked-by: Balbir Singh > Signed-off-by: Hui Su > --- > > Changes in v3: > - Keep the source entry zero when migrate_device_pfn_lock() fails instead > of setting MIGRATE_PFN_COMPOUND, while still consuming and clearing all > slots belonging to the compound folio. > - Use VM_WARN_ON_ONCE() for the truncated compound-folio invariant while > keeping the defensive clear-and-stop path independent of CONFIG_DEBUG_VM, > as suggested by Balbir Singh. > > Changes in v2: > - Fix the helper parameter indentation and keep the declaration to two > lines, as suggested by David Hildenbrand. > > mm/migrate_device.c | 91 +++++++++++++++++++++++++-------------------- > 1 file changed, 50 insertions(+), 41 deletions(-) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 009bfa8b212d..68b787bdc8be 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -1392,6 +1392,40 @@ static unsigned long migrate_device_pfn_lock(unsigned long pfn) > return migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE; > } > > +/* > + * Collect a device folio into the page-granular PFN array. > + * > + * Return the number of entries consumed, or 0 if the folio does not fit in > + * the remaining array. > + */ > +static unsigned int migrate_device_collect_folio(unsigned long *src_pfn, > + unsigned long pfn, unsigned long remaining) > +{ > + struct folio *folio = page_folio(pfn_to_page(pfn)); > + unsigned int nr; > + > + *src_pfn = migrate_device_pfn_lock(pfn); Let's rant about migrate_device_pfn_lock(). It looks up a folio (which we do as well !?) to then get+lock that folio, but doesn't tell us about the folio, but we're supposed to magically unlock and put that folio on the backout path?. And if it fails (e.g., folio_get_nontail_page() failed) we simply go ahead and work on that folio (folio_nr_pages() etc)? What?! Now that we have a single caller of migrate_device_pfn_lock(), can we please get rid of this abomination and figure out what to do when migrate_device_pfn_lock() would have returned 0? > + nr = folio_nr_pages(folio); > + > + VM_WARN_ON_ONCE(nr > remaining); > + if (nr > remaining) { It's odd to add recovery paths for something that is mostly an internal assertion. The pattern is odd. I'd either if (WARN_ON_ONCE(nr > remaining)) { ... } Or only do the VM_WARN_ON_ONCE() and drop recovery, expecting this to be found early during testing. > + if (*src_pfn & MIGRATE_PFN_MIGRATE) { > + folio_unlock(folio); > + folio_put(folio); > + } > + memset(src_pfn, 0, remaining * sizeof(*src_pfn)); > + return 0; > + } > + > + if (nr > 1) { > + if (*src_pfn) > + *src_pfn |= MIGRATE_PFN_COMPOUND; > + memset(src_pfn + 1, 0, (nr - 1) * sizeof(*src_pfn)); > + } > + > + return nr; > +} -- Cheers, David