Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

[Jason Gunthorpe]

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 <peterx@xxxxxxxxxx>
>  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