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b=I+90rJ7JiNyGDHmpESb2muBx4f0icgsKo4YOj3+vqmnnqX0wx2tvnwsbBzyQnF/Jv AdGYGGhLUIEtxOdwe0c4cWeU7+jH4CmT5w2B6xl4YRL5ZCxKcNODp+eE+Yl4tKYZlT TfOajiI8OUldT9FU/5QDfzpt53TBc9dhiRQTMvtbOiCU97GsgMuWGCG9V71tnMz3j1 KRqc8eY5D87flUUHMAYzFQ1C4pSY4O3oKCNIO25MjkwGIW62AyRV4HiHgcFopI6+EV sCI4M9DpSvpJHvgT4ANS7XzlTXj441gK+GPg5phi7mxi7rLYBDBaL3iMcTWgrYEZAY 0rwobFkScUFgg== Message-ID: <4a07d02a-6f0d-4748-976b-503dd3db1e5d@kernel.org> Date: Wed, 8 Apr 2026 09:11:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: ksm: add mremap selftests for ksm_rmap_walk To: Hugh Dickins Cc: xu.xin16@zte.com.cn, akpm@linux-foundation.org, michel@lespinasse.org, ljs@kernel.org, chengming.zhou@linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20260407140805858ViqJKFhfmYSfq0FynsaEY@zte.com.cn> <2513cca9-0f5f-3b5b-e758-b9cc304c8699@google.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: <2513cca9-0f5f-3b5b-e758-b9cc304c8699@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8E191A0008 X-Stat-Signature: kr6ueiz6tcspdt7ymw1j6tqw1qt3ugue X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1775632266-694065 X-HE-Meta: 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 reyGQgDU ewi1aQajQsheWAAopXMMWsBW5Hq/Vw7vOECRUOTa50TaMu1l2hcSJHpPkNp896CaEfq+2DzHIXczAR0fQGTB0xvjKQx2BjlE9FBPHODf/D8UValVg/qYES6aRN6UJYz59RbtUx1qCx7zZpYICPRrpRzmp/HM+Ex3oMYd8VgOeGh+UFh+SUzMvWn4efjpBgO/AQSTKN701kw/ERzw7vU9R0gZCT83fzal9QyX/pkU8g+aBNNXrdw1wuGazR8mYizcMbSdLfuCP8n4G1xygMlqq0r63MaJg01a9IEqXtzDV+tveZleLmm384gN9pGd2Te7HVNgB3ps1VwY//Q0h39rtPLESD3BGh3yOjH6g4ALv3C06fIhSvWsrpZQlXeVfKpSUGNmv Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/8/26 02:08, Hugh Dickins wrote: > On Tue, 7 Apr 2026, David Hildenbrand (Arm) wrote: >> On 4/7/26 08:08, xu.xin16@zte.com.cn wrote: >>> From: xu xin >>> >>> The existing tools/testing/selftests/mm/rmap.c has already one testcase >>> for ksm_rmap_walk in TEST_F(migrate, ksm), which takes use of migration >>> of page from one NUMA node to another NUMA node. However, it just lacks >>> the senario of mremapped VMAs. >>> >>> Before migrating, we add the calling of mremap() to address mapped with KSM >>> pages, which is specailly to test a optimization which is introduced by this >>> patch ("ksm: Optimize rmap_walk_ksm by passing a suitable address range") >>> https://lore.kernel.org/all/20260212193045556CbzCX8p9gDu73tQ2nvHEI@zte.com.cn/ >>> >>> Result: >>> TAP version 13 >>> 1..5 >>> ok 1 migrate.anon >>> ok 2 migrate.shm >>> ok 3 migrate.file # SKIP Failed in worker >>> ok 4 migrate.ksm >>> ok 5 migrate.ksm_and_mremap >>> >>> Signed-off-by: xu xin >>> --- >>> tools/testing/selftests/mm/rmap.c | 69 ++++++++++++++++++++++++++++ >>> tools/testing/selftests/mm/vm_util.c | 38 +++++++++++++++ >>> tools/testing/selftests/mm/vm_util.h | 2 + >>> 3 files changed, 109 insertions(+) >>> >>> diff --git a/tools/testing/selftests/mm/rmap.c b/tools/testing/selftests/mm/rmap.c >>> index 53f2058b0ef2..65470def2bf1 100644 >>> --- a/tools/testing/selftests/mm/rmap.c >>> +++ b/tools/testing/selftests/mm/rmap.c >>> @@ -430,4 +430,73 @@ TEST_F(migrate, ksm) >>> propagate_children(_metadata, data); >>> } >>> >>> +/* To test if ksm page can be migrated when it's mremapped */ >>> +int merge_mremap_and_migrate(struct global_data *data) >>> +{ >>> + int ret = 0; >>> + /* Allocate range and set the same data */ >>> + data->mapsize = 3*getpagesize(); >>> + data->region = mmap(NULL, data->mapsize, PROT_READ|PROT_WRITE, >>> + MAP_PRIVATE|MAP_ANON, -1, 0); >>> + if (data->region == MAP_FAILED) >>> + ksft_exit_fail_perror("mmap failed"); >>> + >>> + memset(data->region, 0x77, data->mapsize); > > (Not crucial at all, but to avoid confusion between our results, I'll > point out that my testcase only memset 2*getpagesize() there, leaving > the last page unpopulated - just one less complication.) > >> >> What happens if you mremap() after faulting, but before merging? >> >> rmap_item->address always holds the user space address of the entry in >> the parent process. It must match the one in the child process, because >> mremap() will unmerge/unshare in the child. >> >> And it must match the one in the parent, as mremap() would similarly >> unmerge/unshare. >> >> Maybe doing the mremap() before merging (but after faulting) would >> trigger what Hugh described. >> >> break_cow() and friends don't care about the rmap, as they simply jump >> directly to the user space address in the process. >> >> In rmap_walk_ksm(), I think the concern Hugh raised is that we are using >> >> const pgoff_t pgoff = rmap_item->address >> PAGE_SHIFT; >> >> but we'd actually need a pgoff into the anon_vma. Without mremap, it >> does not matter, they are the same (tests keep passing). But with mremap >> it's not longer the same. >> >> I think one could store it in the ksm_rmap_item, but that would increase >> it's size. It's essentially the folio->index of the original page we are >> replacing. >> >> Or we could just remember "pgoff is not that simple because mremap was >> involved, so walk the whole damn thing". >> >> We could also just try walking all involved processes, looking only at >> that user space address (but that gets more tricky with rmap locking etc >> ...). >> >> Anyhow, I think that's the concern Hugh raised, IIUC. > > Yes, you and Lorenzo are seeing the same seed for doubt as I saw: > as you say, "pgoff is not that simple because mremap involved"; > but it is confusing, so hard for us to be sure about it. > >> >>> + >>> + if (ksm_start() < 0) >>> + return FAIL_ON_CHECK; >>> + >>> + /* 1 2 expected */ >>> + ksft_print_msg("Shared: %ld (1 expected) Sharing: %ld (2 expected)\n", >>> + ksm_get_pages_shared(), ksm_get_pages_sharing()); >>> + >>> + /* >>> + * Mremap the second pagesize address range into the third pagesize >>> + * address. >>> + */ >>> + data->region = mremap(data->region + getpagesize(), getpagesize(), getpagesize(), >>> + MREMAP_MAYMOVE|MREMAP_FIXED, data->region + 2*getpagesize()); >> >> There would not be a KSM page after this mremap(), no? > > Oh, good thinking: yes, a side-effect of mremap's non-persistent > MADV_UNMERGEABLE would be that there's no KSM page in the mremapped > "subregion" at this instant, so the try_to_move_page() which follows > is likely to have no trouble succeeding; so, as it stands, this test > is not testing what's required. > > Sorry, I won't be able to give this more attention for a week: I > wanted to advertise my doubt before 7.1 merge window, while awkwardly > knowing I'd have to back out of ensuing discussion and testing for a > few days. It seems agreed that we won't endanger 7.1 until this is > resolved: I just hope I'm not guilty of raising a false alarm. Not at all. During my review I primarily focused on the "do we handle fork'ed child processes in all cases correctly". Given that our tests keep passing, I was assuming that non-fork should be fine. But seems like we might be missing some test coverage even there, with respect to mremap(). -- Cheers, David