On Mon, Sep 21, 2020 at 05:20:31PM -0400, Peter Xu wrote: > Pinned pages shouldn't be write-protected when fork() happens, because follow > up copy-on-write on these pages could cause the pinned pages to be replaced by > random newly allocated pages. > > For huge PMDs, we split the huge pmd if pinning is detected. So that future > handling will be done by the PTE level (with our latest changes, each of the > small pages will be copied). We can achieve this by let copy_huge_pmd() > return > -EAGAIN for pinned pages, so that we'll fallthrough in copy_pmd_range() and > finally land the next copy_pte_range() call. > > Huge PUDs will be even more special - so far it does not support anonymous > pages. But it can actually be done the same as the huge PMDs even if the > split > huge PUDs means to erase the PUD entries. It'll guarantee the follow up fault > ins will remap the same pages in either parent/child later. > > This might not be the most efficient way, but it should be easy and clean > enough. It should be fine, since we're tackling with a very rare case just to > make sure userspaces that pinned some thps will still work even without > MADV_DONTFORK and after they fork()ed. > > Signed-off-by: Peter Xu <[email protected]> > mm/huge_memory.c | 26 ++++++++++++++++++++++++++ > 1 file changed, 26 insertions(+) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 7ff29cc3d55c..c40aac0ad87e 100644 > +++ b/mm/huge_memory.c > @@ -1074,6 +1074,23 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct > mm_struct *src_mm, > > src_page = pmd_page(pmd); > VM_BUG_ON_PAGE(!PageHead(src_page), src_page); > + > + /* > + * If this page is a potentially pinned page, split and retry the fault > + * with smaller page size. Normally this should not happen because the > + * userspace should use MADV_DONTFORK upon pinned regions. This is a > + * best effort that the pinned pages won't be replaced by another > + * random page during the coming copy-on-write. > + */ > + if (unlikely(READ_ONCE(src_mm->has_pinned) && > + page_maybe_dma_pinned(src_page))) { > + pte_free(dst_mm, pgtable); > + spin_unlock(src_ptl); > + spin_unlock(dst_ptl); > + __split_huge_pmd(vma, src_pmd, addr, false, NULL); > + return -EAGAIN; > + }
Not sure why, but the PMD stuff here is not calling is_cow_mapping() before doing the write protect. Seems like it might be an existing bug? In any event, the has_pinned logic shouldn't be used without also checking is_cow_mapping(), so it should be added to that test. Same remarks for PUD Jason
