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

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Date: Tue, 2 Jan 2024 16:59:56 +0800
From: Peter Xu <peterx@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, viro@...iv.linux.org.uk, brauner@...nel.org,
	shuah@...nel.org, aarcange@...hat.com, lokeshgidra@...gle.com,
	david@...hat.com, ryan.roberts@....com, hughd@...gle.com,
	mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
	willy@...radead.org, Liam.Howlett@...cle.com, jannh@...gle.com,
	zhangpeng362@...wei.com, bgeffon@...gle.com, kaleshsingh@...gle.com,
	ngeoffray@...gle.com, jdduke@...gle.com, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/1] userfaultfd: fix move_pages_pte() splitting folio
 under RCU read lock

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