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

[Peter Xu]

On Mon, Sep 21, 2020 at 11:41:16PM -0700, John Hubbard wrote:
> On 9/21/20 2:20 PM, Peter Xu wrote:
> ...
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 7ff29cc3d55c..c40aac0ad87e 100644
> > --- a/mm/huge_memory.c
> > +++ 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;
> > +	}
> 
> 
> Why wait until we are so deep into this routine to detect this and unwind?
> It seems like if you could do a check near the beginning of this routine, and
> handle it there, with less unwinding? In fact, after taking only the src_ptl,
> the check could be made, right?

Because that's where we've fetched the page from the pmd so I can directly
reference src_page.  Also I think at least I need to check against swp entries?
So it seems still easier to keep it here, considering it's an unlikely path.

Thanks,

-- 
Peter Xu