Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 17 Sep 2020 11:26:01 -0700 | Subject | Re: [PATCH 1/4] mm: Trial do_wp_page() simplification |
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On Thu, Sep 17, 2020 at 11:14 AM Peter Xu <peterx@redhat.com> wrote: > > In my humble opinion, the real solution is still to use MADV_DONTFORK properly > so we should never share the DMA pages with others when we know the fact.
Is this all just because somebody does a fork() after doing page pinning?
If so, I feel this should be trivially fixed in copy_one_pte(). That's where we currently do
/* * If it's a COW mapping, write protect it both * in the parent and the child */ if (is_cow_mapping(vm_flags) && pte_write(pte)) { ptep_set_wrprotect(src_mm, addr, src_pte); pte = pte_wrprotect(pte); }
and I feel that that is where we could just change the code to do a COW event for pinned pages (and *not* mark the parent write protected, since the parent page now isn't a COW page).
Because if that's the case that Jason is hitting, then I feel that really is the correct fix: make sure that the pinning action is meaningful.
As mentioned, I really think the whole (and only) point of page pinning is that it should keep the page locked in the page tables. And by "locked" I mean exactly that: not just present, but writable.
And then the "we never COW a pinned page" comes not from the COW code doing magic, but by it simply never becoming non-writable - because the page table entry is locked!
Linus
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