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b=mEzPIQnv7lpcbrKWbQSKEdwI+BmCAqv9+EhC7qg6Um/kMDsgtjDEe93CG4shWLgkg gqTkxBO8owX1eCgVCyx4jJCC43iB7ou0V0GuWAGcW4rXjypPMQUMV1pHreHaoZSDKe 4doGDFgYMIwDauXbDCHFPPI6z2ZD4esYqqhXADOo1YNu725Iay0VVjaoKiCt1hKmJc 5z1re9/loSTGJ3veQ+m5147YICU/ztkucG7JjDj1XR6jieHJI39l6UJDOsCaKjCuI1 GdaQ43Yxx/XLOEARcmwCeIU/9xKt1ar5josIKAZi6/s6pDw42HW20TQ4j3OQbO9tXM vv3OIO8m9oBhQ== Message-ID: Date: Thu, 12 Mar 2026 09:55:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: prevent droppable mappings from being locked To: anthony.yznaga@oracle.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Cc: akpm@linux-foundation.org, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jannh@google.com, pfalcato@suse.de, Jason@zx2c4.com, shuah@kernel.org References: <20260310155821.17869-1-anthony.yznaga@oracle.com> <9d90dd0b-1d60-4e45-922a-84637cab16a3@oracle.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: D543A180002 X-Rspamd-Server: rspam07 X-Stat-Signature: omkxb9sffgeoody6fox5fqhen16kkxd7 X-Rspam-User: X-HE-Tag: 1773305734-98621 X-HE-Meta: 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 mnVQ2w6A 9qQoJcu8qb+i8eFMgf1oOdz2X/uQHTio+CgMaQYD7VBZ+vqIWDC9Tr9FbiM8UM+jPqVqWsxwrBa5Yia7Q84SI4tC/P9OjKGY2TjDSCIJiHgtgWaWXVL29jZqZHI2553zGELI/4mWWSs2MCIe/bq0SEHKxO5eex2FZICtxGUaR/s2bwDGwvmWLJ60lkMS9/Jyv5tATbrTxmvJ8VFjf2BHTtNC7qHOoolB9Xb67zg900gn1A5pZHGViBoZ7IkrN+Y4mZOsHaeEzqCK1IR30aZicIyjwgQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/12/26 03:01, anthony.yznaga@oracle.com wrote: > > On 3/11/26 2:27 AM, David Hildenbrand (Arm) wrote: >> On 3/10/26 16:58, Anthony Yznaga wrote: >>> Droppable mappings must not be lockable. There is a check for VMAs with >>> VM_DROPPABLE set in mlock_fixup() along with checks for other types of >>> unlockable VMAs which ensures this when calling mlock()/mlock2(). >>> >>> For mlockall(MCL_FUTURE), the check for unlockable VMAs is different. >>> In apply_mlockall_flags(), if the flags parameter has MCL_FUTURE set, >>> the >>> current task's mm's default VMA flag field mm->def_flags has VM_LOCKED >>> applied to it. VM_LOCKONFAULT is also applied if MCL_ONFAULT is also >>> set. >>> When these flags are set as default in this manner they are cleared in >>> __mmap_complete() for new mappings that do not support mlock. A check >>> for >>> VM_DROPPABLE in __mmap_complete() is missing resulting in droppable >>> mappings created with VM_LOCKED set. To fix this and reduce that >>> chance of >>> similar bugs in the future, introduce and use vma_supports_mlock(). >>> >>> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always >>> lazily freeable mappings") >> Should we Cc: stable? I think we should, to fix mlockall(MCL_FUTURE) >> behavior. > > I found this issue through code inspection while doing mshare dev work. > I don't have a strong idea how likely it is to happen in practice. If it > did it might not be easily diagnosed so I'm fine adding the tag. IIUC, mlockall(MCL_FUTURE) will result in future MAP_DROPPABLE mappings to get mlocked. I assume that implies that all pages will get faulted in and might not be reclaimable. With a quick test program that mmaps 4M, we indeed fault in all these pages (I assume you test does something similar). 7f5fc4600000-7f5fc4a00000 rw-p 00000000 00:00 0 Size: 4096 kB KernelPageSize: 4 kB MMUPageSize: 4 kB Rss: 4096 kB Pss: 4096 kB Pss_Dirty: 4096 kB Shared_Clean: 0 kB Shared_Dirty: 0 kB Private_Clean: 0 kB Private_Dirty: 4096 kB Referenced: 4096 kB Anonymous: 4096 kB KSM: 0 kB LazyFree: 0 kB AnonHugePages: 4096 kB ShmemPmdMapped: 0 kB FilePmdMapped: 0 kB Shared_Hugetlb: 0 kB Private_Hugetlb: 0 kB Swap: 0 kB SwapPss: 0 kB Locked: 4096 kB It's a good question whether memory reclaim would still be able to free these pages. I'd assume the folios would get mlocked and essentially turned unevictable -- breaking the whole concept of droppable mappings. So I think we should CC stable. -- Cheers, David