Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t

[Ingo Molnar]

Al,

I was wondering about the struct page rules of 
iov_iter_get_pages_alloc(), used in various places. There's no 
documentation whatsoever in lib/iov_iter.c, nor in 
include/linux/uio.h, and the changelog that introduced it only says:

 commit 91f79c43d1b54d7154b118860d81b39bad07dfff
 Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
 Date:   Fri Mar 21 04:58:33 2014 -0400

    new helper: iov_iter_get_pages_alloc()
    
    same as iov_iter_get_pages(), except that pages array is allocated
    (kmalloc if possible, vmalloc if that fails) and left for caller to
    free.  Lustre and NFS ->direct_IO() switched to it.
    
    Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

So if code does iov_iter_get_pages_alloc() on a user address that has 
a real struct page behind it - and some other code does a regular 
get_user_pages() on it, we'll have two sets of struct page 
descriptors, the 'real' one, and a fake allocated one, right?

How does that work? Nobody else can ever discover these fake page 
structs, so they don't really serve any 'real' synchronization purpose 
other than the limited role of IO completion.

Thanks,

	Ingo
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