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

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Date:   Tue, 2 May 2023 13:13:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
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        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        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>,
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        Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 12:11:49AM +0100, Lorenzo Stoakes wrote:
> @@ -95,6 +96,77 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
>  	return folio;
>  }
>  
> +#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
> +static bool stabilise_mapping_rcu(struct folio *folio)
> +{
> +	struct address_space *mapping = READ_ONCE(folio->mapping);
> +
> +	rcu_read_lock();
> +
> +	return mapping == READ_ONCE(folio->mapping);

This doesn't make sense; why bother reading the same thing twice?

Who cares if the thing changes from before; what you care about is that
the value you see has stable storage, this doesn't help with that.

> +}
> +
> +static void unlock_rcu(void)
> +{
> +	rcu_read_unlock();
> +}
> +#else
> +static bool stabilise_mapping_rcu(struct folio *)
> +{
> +	return true;
> +}
> +
> +static void unlock_rcu(void)
> +{
> +}
> +#endif

Anyway, this all can go away. RCU can't progress while you have
interrupts disabled anyway.

> +/*
> + * Used in the GUP-fast path to determine whether a FOLL_PIN | FOLL_LONGTERM |
> + * FOLL_WRITE pin is permitted for a specific folio.
> + *
> + * This assumes the folio is stable and pinned.
> + *
> + * Writing to pinned file-backed dirty tracked folios is inherently problematic
> + * (see comment describing the writeable_file_mapping_allowed() function). We
> + * therefore try to avoid the most egregious case of a long-term mapping doing
> + * so.
> + *
> + * This function cannot be as thorough as that one as the VMA is not available
> + * in the fast path, so instead we whitelist known good cases.
> + *
> + * The folio is stable, but the mapping might not be. When truncating for
> + * instance, a zap is performed which triggers TLB shootdown. IRQs are disabled
> + * so we are safe from an IPI, but some architectures use an RCU lock for this
> + * operation, so we acquire an RCU lock to ensure the mapping is stable.
> + */
> +static bool folio_longterm_write_pin_allowed(struct folio *folio)
> +{
> +	bool ret;
> +
> +	/* hugetlb mappings do not require dirty tracking. */
> +	if (folio_test_hugetlb(folio))
> +		return true;
> +

This:

> +	if (stabilise_mapping_rcu(folio)) {
> +		struct address_space *mapping = folio_mapping(folio);

And this is 3rd read of folio->mapping, just for giggles?

> +
> +		/*
> +		 * Neither anonymous nor shmem-backed folios require
> +		 * dirty tracking.
> +		 */
> +		ret = folio_test_anon(folio) ||
> +			(mapping && shmem_mapping(mapping));
> +	} else {
> +		/* If the mapping is unstable, fallback to the slow path. */
> +		ret = false;
> +	}
> +
> +	unlock_rcu();
> +
> +	return ret;

then becomes:


	if (folio_test_anon(folio))
		return true;

	/*
	 * 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;

> +}