Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 11 Sep 2020 09:45:33 -0700 | Subject | Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding |
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On Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev <agordeev@linux.ibm.com> wrote: > > What if the entry is still pud_present, but got remapped after > READ_ONCE(*pudp)? IOW, it is still valid, but points elsewhere?
That can't happen.
The GUP walk doesn't hold any locks, but it *is* done with interrupts disabled, and anybody who is modifying the page tables needs to do the TLB flush, and/or RCU-free them.
The interrupt disable means that on architectures where the TLB flush involves an IPI, it will be delayed until afterwards, but it also acts as a big RCU read lock hammer.
So the page tables can get modified under us, but the old pages won't be released and re-used.
Linus
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