Re: [PATCH 3/4] thp: fix MADV_DONTNEED vs. MADV_FREE race

[Aneesh Kumar K.V]

"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:

> Basically the same race as with numa balancing in change_huge_pmd(), but
> a bit simpler to mitigate: we don't need to preserve dirty/young flags
> here due to MADV_FREE functionality.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bb2b3646bd78..324217c31ec9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1566,8 +1566,6 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		deactivate_page(page);
>  
>  	if (pmd_young(orig_pmd) || pmd_dirty(orig_pmd)) {
> -		orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
> -			tlb->fullmm);
>  		orig_pmd = pmd_mkold(orig_pmd);
>  		orig_pmd = pmd_mkclean(orig_pmd);
>  

Instead can we do a new interface that does something like

pmdp_huge_update(tlb->mm, addr, pmd, new_pmd);

We do have a variant already in ptep_set_access_flags. What we need is
something that can be used to update THP pmd, without converting it to
pmd_none and one which doens't loose reference and change bit ?

-aneesh