linux-kernel - Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

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Message-ID: <505b7df8-bb60-7564-af28-b99875eea12a@redhat.com>
Date:   Tue, 2 May 2023 21:17:53 +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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        Jonathan Lemon <jonathan.lemon@...il.com>,
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        "Paul E . McKenney" <paulmck@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
 file-backed mappings

> +static bool folio_longterm_write_pin_allowed(struct folio *folio)
> +{
> +	struct address_space *mapping;
> +
> +	/*
> +	 * GUP-fast disables IRQs - this prevents IPIs from causing page tables
> +	 * to disappear from under us, as well as preventing RCU grace periods
> +	 * from making progress (i.e. implying rcu_read_lock()).
> +	 *
> +	 * This means we can rely on the folio remaining stable for all
> +	 * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE
> +	 * and those that do not.
> +	 *
> +	 * We get the added benefit that given inodes, and thus address_space,
> +	 * objects are RCU freed, we can rely on the mapping remaining stable
> +	 * here with no risk of a truncation or similar race.
> +	 */
> +	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * If no mapping can be found, this implies an anonymous or otherwise
> +	 * non-file backed folio so in this instance we permit the pin.
> +	 *
> +	 * shmem and hugetlb mappings do not require dirty-tracking so we
> +	 * explicitly whitelist these.
> +	 *
> +	 * Other non dirty-tracked folios will be picked up on the slow path.
> +	 */
> +	mapping = folio_mapping(folio);
> +	return !mapping || shmem_mapping(mapping) || folio_test_hugetlb(folio);
> +}

BTW, try_grab_folio() is also called from follow_hugetlb_page(), which 
is ordinary GUP and has interrupts enabled if I am not wrong.

-- 
Thanks,

David / dhildenb