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 BF8EA472F7D for ; Mon, 21 Sep 2026 09:32:25 +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=1789983147; cv=none; b=SQLNmDqylJ4IOSmCPqCfrcwTIvmpAVfZBcEW/A/fZVOqyb2JfkV0HWKClqYuim6zhExqQmcirwSj8EYevyKJUpPZ+h6yitgyYmihYQhEk6WN57CN3YCk2+YetJEOR2KU6nLyVgi3hLi/uobPB/P38JPKiQQzCQ+kw8Q9fh7uB04= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789983147; c=relaxed/simple; bh=A+FpwmEdcgLkNBmXKcsH5Ari5n1zAVt4M/rru0icHo8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EyKxvtU5nMyZFAt6cs3ApUOmAdA/0x3PVJ3w9SOrsEhO2hRfMmpQwNMq7/xojany9Uxm6CMrSOsRrH/6mXn71om9uUkWJsMwemM0WlORkA0MfmOyM7QlsD6zlH2S/sVerlZeKcwEkZNBZ0oz5s+H7fNbr1Eba+QI5a7/YKxOuIs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EiR2ktE5; 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="EiR2ktE5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E04791F000FF; Mon, 21 Sep 2026 09:32:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789983145; bh=YYR5qVSNGypuoPk4MWOVXb7SNwRU6tu0qAlYq1SIKq4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=EiR2ktE5bWvCWZK2xZovGpxWGGzeb+rPS5X4wbkKw1+lhJsvN+ZhauW30Gmk+VICF B6WsjoFZiPQlJm6zYFnispjSdhqDgBrA83GqOYc9yPLoKD22vhubSfV+6zO/wd8vWI K6ymAsrEO0K3o7WtIvhTWCCfxjaC3L3v1df2ifYXlUI8kAtMZgqT65Tb39EwSrmiEJ XhoNLIW67BTaDDljGDfoArCWVzZkUtAUqWXrXnvVXuHSyIburK7teC1/ioEipBlbR7 1kEp5/WaQQwSk9l1q29y72XL8jMs3WHfAYYJQWYj5+zg0viR/PHATQwjo7HtFjGTN5 8ddCwV2uNcG7g== Message-ID: <80925c1f-73c2-45df-ac26-fdcc96ffe764@kernel.org> Date: Mon, 21 Sep 2026 11:32:19 +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 v3] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked() To: Lance Yang Cc: akpm@linux-foundation.org, jgg@ziepe.ca, jhubbard@nvidia.com, gregkh@linuxfoundation.org, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260919-nommu_gup_pin-v3-1-3660a8851243@kernel.org> <20260921024914.12819-1-lance.yang@linux.dev> 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 9/21/26 04:49, Lance Yang wrote: > > On Sat, Sep 19, 2026 at 12:04:36PM +0200, David Hildenbrand (arm) wrote: >> From: Greg Kroah-Hartman >> >> The !CONFIG_MMU implementation of __get_user_pages_locked() takes a bare >> get_page() reference for each page regardless of foll_flags: >> if (pages[i]) >> get_page(pages[i]); >> >> This is reached from pin_user_pages*() with FOLL_PIN set. >> unpin_user_page() is shared between MMU and NOMMU configurations and >> unconditionally calls gup_put_folio(..., FOLL_PIN), which subtracts >> GUP_PIN_COUNTING_BIAS (1024) from the folio refcount. >> >> This means that pin adds 1, and then unpin will subtract 1024. >> >> If a user maps a page (refcount 1), registers it 1023 times as an >> io_uring fixed buffer (1023 pin_user_pages calls -> refcount 1024), then >> unregisters: the first unpin_user_page subtracts 1024, refcount hits 0, >> the page is freed and returned to the buddy allocator. The remaining >> 1022 unpins write into whatever was reallocated, and the user's VMA >> still maps the freed page (NOMMU has no MMU to invalidate it). >> Reallocating the page for an io_uring pbuf_ring then lets userspace >> corrupt the new owner's data through the stale mapping. >> >> Use try_grab_folio() which adds GUP_PIN_COUNTING_BIAS for FOLL_PIN and 1 >> for FOLL_GET, mirroring the CONFIG_MMU path so pin and unpin are >> symmetric. Keep supporting the traditional behavior where users specify >> a pages array but don't set FOLL_GET. >> >> While at it, don't return NULL pointers in the page array, >> as this is really not expected for GUP users; instead, just fail and return >> -EFAULT. >> >> [ david: support traditional behavior with no FOLL_GET, extend >> description ] >> >> Cc: Andrew Morton >> Cc: David Hildenbrand >> Cc: Jason Gunthorpe >> Cc: John Hubbard >> Cc: Peter Xu >> Reported-by: Anthropic >> Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages") >> Assisted-by: gkh_clanker_t1000 >> Signed-off-by: Greg Kroah-Hartman >> Signed-off-by: David Hildenbrand (Arm) >> --- >> I added a best-guess fixes tag. Given that this has been around for a >> while ... and it's nommu, I think this is fine just going into the next >> merge window. In that case, I will pick this up myself. If we want this >> as a hotfix, Andrew please pick it up. >> --- >> mm/gup.c | 17 ++++++++++++++--- >> 1 file changed, 14 insertions(+), 3 deletions(-) >> >> diff --git a/mm/gup.c b/mm/gup.c >> index eb898ea1ee22e..1681bea8eed7e 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -1659,6 +1659,10 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, >> if (!nr_pages) >> return 0; >> >> + /* See the MMU variant: support the traditional behavior. */ >> + if (pages && !(flags & FOLL_PIN)) >> + flags |= FOLL_GET; >> + > > Wait ... isn't this the wrong __get_user_pages_locked()? The MMU version > already does this a few lines below ... > Yes, Sashiko complained as well. Will fix it up and send a v2. Lesson learned: don't work on nommu patches on Saturday ;) > Shouldn't the NOMMU version get the check instead, using foll_flags? Yes, I'll fix that up and send a v2. -- Cheers, David