netdev - Re: [PATCH v8 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <e4c92510-9756-d9a1-0055-4cd64a0c76d9@redhat.com>
Date: Thu, 4 May 2023 17:04:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
 Matthew Wilcox <willy@...radead.org>,
 Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
 Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>,
 Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
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 Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v8 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

[...]

> +static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
> +{
> +	struct address_space *mapping;
> +	unsigned long mapping_flags;
> +
> +	/*
> +	 * If we aren't pinning then no problematic write can occur. A long term
> +	 * pin is the most egregious case so this is the one we disallow.
> +	 */
> +	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
> +	    (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
> +		return true;
> +
> +	/* The folio is pinned, so we can safely access folio fields. */
> +
> +	/* Neither of these should be possible, but check to be sure. */

You can easily have anon pages that are at the swapcache at this point 
(especially, because this function is called before our unsharing 
checks), the comment is misleading.

And there is nothing wrong about pinning an anon page that's still in 
the swapcache. The following folio_test_anon() check will allow them.

The check made sense in page_mapping(), but here it's not required.

I do agree regarding folio_test_slab(), though. Should we WARN in case 
we would have one?

if (WARN_ON_ONCE(folio_test_slab(folio)))
	return false;

> +	if (unlikely(folio_test_slab(folio) || folio_test_swapcache(folio)))
> +		return false;
> +
> +	/* hugetlb mappings do not require dirty-tracking. */
> +	if (folio_test_hugetlb(folio))
> +		return true;
> +
> +	/*
> +	 * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
> +	 * cannot proceed, which means no actions performed under RCU can
> +	 * proceed either.
> +	 *
> +	 * inodes and thus their mappings are freed under RCU, which means the
> +	 * mapping cannot be freed beneath us and thus we can safely dereference
> +	 * it.
> +	 */
> +	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * However, there may be operations which _alter_ the mapping, so ensure
> +	 * we read it once and only once.
> +	 */
> +	mapping = READ_ONCE(folio->mapping);
> +
> +	/*
> +	 * The mapping may have been truncated, in any case we cannot determine
> +	 * if this mapping is safe - fall back to slow path to determine how to
> +	 * proceed.
> +	 */
> +	if (!mapping)
> +		return false;
> +
> +	/* Anonymous folios are fine, other non-file backed cases are not. */
> +	mapping_flags = (unsigned long)mapping & PAGE_MAPPING_FLAGS;
> +	if (mapping_flags)
> +		return mapping_flags == PAGE_MAPPING_ANON;

KSM pages are also (shared) anonymous folios, and that check would fail 
-- which is ok (the following unsharing checks rejects long-term pinning 
them), but a bit inconstent with your comment and folio_test_anon().

It would be more consistent (with your comment and also the 
folio_test_anon implementation) to have here:

	return mapping_flags & PAGE_MAPPING_ANON;

> +
> +	/*
> +	 * At this point, we know the mapping is non-null and points to an
> +	 * address_space object. The only remaining whitelisted file system is
> +	 * shmem.
> +	 */
> +	return shmem_mapping(mapping);
> +}
> +

In general, LGTM

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb