On Wed, Apr 05 2017, Matthew Wilcox wrote:
> On Thu, Apr 06, 2017 at 10:02:48AM +1000, NeilBrown wrote:
>> If you are concerned about space in 'struct address_space', just prune
>> some wastage.
>
> I'm trying to (via wlists). still buggy though.
Cool.
(I wonder what a wlist is.... weighted list?)
>
>> The "host" field brings no value. It is only ever assigned in
>> inode_init_always():
>>
>> struct address_space *const mapping = &inode->i_data;
>> ......
>> mapping->host = inode;
>>
>> So you could change all references to use
>> container_of(mapping, struct inode, i_data)
>
> Alas, no:
>
> drivers/dax/dax.c: inode->i_mapping->host = dax_dev->inode;
inode->i_mapping = dax_dev->inode->i_mapping;
inode->i_mapping->host = dax_dev->inode;
so that second line is equivalent to
dax_dev->inode->i_mapping->host = dax_dev->inode;
so inode->mapping->host leads back to inode. So this doesn't break the
invariant.
> fs/gfs2/glock.c: mapping->host = s->s_bdev->bd_inode;
> fs/gfs2/ops_fstype.c: mapping->host = sb->s_bdev->bd_inode;
Hmm.. that's weird. I cannot quite follow what is happening there.
It creates an address-space for metadata which doesn't have a real
inode, and borrows bits of the bdev inode ... possibly just to be able
to find the blocksize deep in buffer.c or similar.
I suspect that using an 'inode' instead of a 'mapping' would make the
code clearer.
> fs/nilfs2/page.c: mapping->host = inode;
A nilfs inode is allocated with 2 address spaces,
one for the data and one for btree indexing metadata.
And then there are a couple of extra address spaces for the
global metadata-file (mtd).
I wonder what the ->host pointer is actually used for.
buffer.c uses it:
- to mark the inode 'dirty' when the page is marked dirty
- to find the blocksize of the inode, for creating buffer_heads
- find the size of the mapping (i_size)
I could probably argue that the 'dirty' flag (at least for the data) and
the size really belong in the address_space, not in the inode.
The blocksize, I'm less sure of.
I suspect gfs2 and nilfs2 could be changed to allocate a separate inode
(instead of address_space), or to not make use of the ->host pointer.
It would be more work than I at first thought though.
Thanks,
NeilBrown
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