Messages in this thread | | | Date | Tue, 2 May 2023 10:00:16 +0200 | From | Jan Kara <> | 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
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