Re: [PATCH 1/1] userfaultfd: fix move_pages_pte() splitting folio under RCU read lock

[Peter Xu]

On Fri, Dec 29, 2023 at 06:56:07PM -0800, Suren Baghdasaryan wrote:
> @@ -1078,9 +1078,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
>  
>  		/* at this point we have src_folio locked */
>  		if (folio_test_large(src_folio)) {
> +			/* split_folio() can block */
> +			pte_unmap(&orig_src_pte);
> +			pte_unmap(&orig_dst_pte);
> +			src_pte = dst_pte = NULL;
>  			err = split_folio(src_folio);
>  			if (err)
>  				goto out;
> +			goto retry;
>  		}

Do we also need to clear src_folio and src_folio_pte?  If the folio is a
thp, I think it means it's pte mapped here. Then after the split we may
want to fetch the small folio after the split, not the head one?

-- 
Peter Xu