Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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On 02.05.23 14:52, Lorenzo Stoakes wrote:
On Tue, May 02, 2023 at 02:47:22PM +0200, David Hildenbrand wrote:
On 02.05.23 14:40, Peter Zijlstra wrote:
On Tue, May 02, 2023 at 02:08:10PM +0200, Peter Zijlstra wrote:



	if (folio_test_anon(folio))
		return true;

This relies on the mapping so belongs below the lockdep assert imo.

Oh, right you are.


	/*
	 * Having IRQs disabled (as per GUP-fast) also inhibits RCU
	 * grace periods from making progress, IOW. they imply
	 * rcu_read_lock().
	 */
	lockdep_assert_irqs_disabled();

	/*
	 * Inodes and thus address_space are RCU freed and thus safe to
	 * access at this point.
	 */
	mapping = folio_mapping(folio);
	if (mapping && shmem_mapping(mapping))
		return true;

	return false;

+}

So arguably you should do *one* READ_ONCE() load of mapping and
consistently use that, this means open-coding both folio_test_anon() and
folio_mapping().

Open-coding folio_test_anon() should not be required. We only care about
PAGE_MAPPING_FLAGS stored alongside folio->mapping, that will stick around
until the anon page was freed.


Ack, good point!

@Lorenzo, you might also want to special-case hugetlb directly using
folio_test_hugetlb().


I already am :) I guess you mean when I respin along these lines? Will port
that across to.

Ha, I only stared at that reply, good! :)

--
Thanks,

David / dhildenb




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