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The caller provides context and modifies the behavior of get_user_pages() via flags. Of particular interest is the FOLL_FORCE flag, which mem_rw() passes. This flag causes check_vma_flags (the access validation logic within get_user_pages()) to ignore writes to unwritable pages and allow the lookup to continue. The “punch through” semantics are attributed entirely to FOLL_FORCE. (comments my own)
，这一点在吃瓜网中也有详细论述
Мэр украинского города обратился к волонтеру словами «обосрыш» и «бубочка»14:38
https://aws.amazon.com/marketplace/pp/prodview-4csbncujmsgbk (ZFS)