From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 33E473BFAD4 for ; Wed, 30 Sep 2026 10:03:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790762622; cv=none; b=fzTk9Op2H41RR76lGjlG/IXoc6aEOW5yMEzUxkrwUZajrAdLZ5gVTlVLh7jFbBBUFerglVY9wfS7er0ttmQYqxSpBTBgggS+MYJrNlfO15O8PA06YGhLoOFDwnHkY3QKj/+PqjEk3ZybhKoxFAovt1qIqibmPW/2g+LWD+S7WQk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790762622; c=relaxed/simple; bh=ySdFEvvLw8Dgy7OVR9qyWQIVIEqKTvONxjz63zztjJQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ecCjoYytaE+8XSgmdnfJCRLjfCPHekZQbiJrXwEmDTqN9bzhMTn3w0Zs/hLZzAyShL+dQXf7mi4jTtE5VaKkyX5cJna9HmXV1nZ0LzmGHjdMgad4/E0VaTXEeXnJ9G8gee1wHpq98NktdZGzt8UlCX16YOdajWKK5LJuEtp+SfI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=c39Xhitk; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="c39Xhitk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 23E321F00893; Wed, 30 Sep 2026 10:03:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790762621; bh=Ml0DuYlmFa9hQaDKnLTS0bQy6dNO9ILQKsOk1hfaR/Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=c39XhitkGKdQ/yXBH5DDEEOWKDiqpWzdQvRaLfGE0PbANlGxw3t6EWIwyiiGPY6/a 5iONMk1hgoZFGooMXpm2iSxvYrW0gbGPkpbdkYU5fqNalHrPKmOoWDIZtqT2KEUvm6 98c61JZfXQ9uNRxkzSIwvdIuqK/HkIE77N6mNBd7f92BX1xqAneUSTjBz+FDf0wj6Q RhCqBYOIinjgtNh8N7msgGmQse1xAwbAG+hKZng0Iz6lUaSRLIIwhQ7Krg4q1roAH2 OOuL/d6430Qpme+7Mrcvis+5vRvFxhm4MwV+lY/wRDF9JcEB7ZEcEoExcOe4aMl3sz kZqp+4Vb1nOiQ== Message-ID: <8f509b48-db3e-44ff-8423-1e43a35daa01@kernel.org> Date: Wed, 30 Sep 2026 12:03:36 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: avoid recursive i_mmap_rwsem in PMD sharing To: Li Zhe , akpm@linux-foundation.org, muchun.song@linux.dev, osalvador@suse.de Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Oscar Salvador , "Lorenzo Stoakes (Arm)" References: <20260930064308.58159-1-lizhe.67@bytedance.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/30/26 08:43, Li Zhe wrote: > huge_pmd_share() always takes i_mmap_rwsem for read before looking for a > shareable PMD page table. > > That is unsafe for callers that already hold the same mapping lock for > write. In particular, move_hugetlb_page_tables() takes i_mmap_rwsem for > write to prevent truncation races, then calls huge_pte_alloc() for the > destination address. If the destination PUD is empty and PMD sharing is > possible, huge_pte_alloc() can call huge_pmd_share(), which then tries to > take the same rwsem for read and can deadlock on itself. > > PMD sharing is only an optimization. If the read side of i_mmap_rwsem > cannot be acquired immediately, fall back to allocating a private PMD > table instead of blocking in the sharing path. > > Signed-off-by: Li Zhe > --- > mm/hugetlb.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index cea25773a6c95..75631101a6807 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6995,7 +6995,15 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma, > pte_t *spte = NULL; > pte_t *pte; > > - i_mmap_lock_read(mapping); > + /* > + * Some callers already hold i_mmap_rwsem for write, for example > + * move_hugetlb_page_tables(). PMD sharing is only an optimization, so > + * fall back to a private PMD table instead of blocking on the same > + * rwsem in read mode. > + */ "just an optimization" is not entirely true. There are workloads that depend on huge pmd sharing to work. So I don't think this is acceptable. There was a private discussion on this issue recently (for some weird reason the deadlock was reported as a security issue), 2 months ago I think. Oscar, I recall that you wanted to send a proper patch? -- Cheers, David