linux-kernel - Re: [PATCH v2 1/1] userfaultfd: fix move_pages_pte() splitting folio under RCU read lock

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Message-ID: <ZZS-crdr54WqL7Ns@x1n>
Date: Wed, 3 Jan 2024 09:54:58 +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 v2 1/1] userfaultfd: fix move_pages_pte() splitting folio
 under RCU read lock

On Tue, Jan 02, 2024 at 03:32:56PM -0800, Suren Baghdasaryan wrote:
> While testing the split PMD path with lockdep enabled I've got an
> "Invalid wait context" error caused by split_huge_page_to_list() trying
> to lock anon_vma->rwsem while inside RCU read section. The issues is due
> to move_pages_pte() calling split_folio() under RCU read lock. Fix this
> by unmapping the PTEs and exiting RCU read section before splitting the
> folio and then retrying. The same retry pattern is used when locking the
> folio or anon_vma in this function. After splitting the large folio we
> unlock and release it because after the split the old folio might not be
> the one that contains the src_addr.
> 
> Fixes: 94b01c885131 ("userfaultfd: UFFDIO_MOVE uABI")
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

Reviewed-by: Peter Xu <peterx@...hat.com>

Thanks,

-- 
Peter Xu