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Date:   Tue, 2 May 2023 10:00:16 +0200
From:   Jan Kara <jack@suse.cz>
To:     "Kirill A . Shutemov" <kirill@shutemov.name>
Cc:     David Hildenbrand <david@redhat.com>, Peter Xu <peterx@redhat.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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On Sat 29-04-23 02:43:32, Kirill A . Shutemov wrote:
> I think I found relevant snippet of code that solves similar issue.
> get_futex_key() uses RCU to stabilize page->mapping after GUP_fast:
> 
> 
> 		/*
> 		 * The associated futex object in this case is the inode and
> 		 * the page->mapping must be traversed. Ordinarily this should
> 		 * be stabilised under page lock but it's not strictly
> 		 * necessary in this case as we just want to pin the inode, not
> 		 * update the radix tree or anything like that.
> 		 *
> 		 * The RCU read lock is taken as the inode is finally freed
> 		 * under RCU. If the mapping still matches expectations then the
> 		 * mapping->host can be safely accessed as being a valid inode.
> 		 */
> 		rcu_read_lock();
> 
> 		if (READ_ONCE(page->mapping) != mapping) {
> 			rcu_read_unlock();
> 			put_page(page);
> 
> 			goto again;
> 		}
> 
> 		inode = READ_ONCE(mapping->host);
> 		if (!inode) {
> 			rcu_read_unlock();
> 			put_page(page);
> 
> 			goto again;
> 		}
> 
> I think something similar can be used inside GUP_fast too.

Yeah, inodes (and thus struct address_space) is RCU protected these days so
grabbing RCU lock in gup_fast() will get you enough protection for checking
aops if you are careful (like the futex code is).

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR