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: <bd75a53d-d461-488c-af8c-b7e483c22e41@lucifer.local>
Date:   Tue, 2 May 2023 20:45:22 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Hildenbrand <david@...hat.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>,
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Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing
 to file-backed mappings

On Tue, May 02, 2023 at 09:17:53PM +0200, David Hildenbrand wrote:
> > +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.

It does hold the PTL though, so can't fiddle with the entry.

But that does suggest folio_test_hugetlb() should be put _before_ the irq
disabled assertion then :)

>
> --
> Thanks,
>
> David / dhildenb
>