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mm: allow drivers to prevent new writable mappings

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This patch (of 6):

The i_mmap_writable field counts existing writable mappings of an
address_space.  To allow drivers to prevent new writable mappings, make
this counter signed and prevent new writable mappings if it is negative.
This is modelled after i_writecount and DENYWRITE.

This will be required by the shmem-sealing infrastructure to prevent any
new writable mappings after the WRITE seal has been set.  In case there
exists a writable mapping, this operation will fail with EBUSY.

Note that we rely on the fact that iff you already own a writable mapping,
you can increase the counter without using the helpers.  This is the same
that we do for i_writecount.

Signed-off-by: David Herrmann <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Ryan Lortie <[email protected]>
Cc: Lennart Poettering <[email protected]>
Cc: Daniel Mack <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
David Herrmann 2014-08-08 14:25:25 -07:00 • committed by Linus Torvalds
commit 4bb5f5d939
5 changed files with 54 additions and 9 deletions
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1
fs/inode.c
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@ -165,6 +165,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
mapping->a_ops = &empty_aops;
mapping->host = inode;
mapping->flags = 0;
atomic_set(&mapping->i_mmap_writable, 0);
mapping_set_gfp_mask(mapping, GFP_HIGHUSER_MOVABLE);
mapping->private_data = NULL;
mapping->backing_dev_info = &default_backing_dev_info;