Re: [patch] fix O_DIRECT read of last block in a sparse file

[Jeff Moyer <jmoyer@redhat.com>]

==> Regarding Re: [patch] fix O_DIRECT read of last block in a sparse file; Andrew Morton <akpm@osdl.org> adds:

akpm> Jeff Moyer <jmoyer@redhat.com> wrote:
>> Currently, if you open a file O_DIRECT, truncate it to a size that is
>> not a multiple of the disk block size, and then try to read the last
>> block in the file, the read will return 0.  The problem is in
>> do_direct_IO, here:
>> 
>> /* Handle holes */ if (!buffer_mapped(map_bh)) { char *kaddr;
>> 
>> ...
>> 
>> if (dio->block_in_file >= i_size_read(dio->inode)>>blkbits) { /* We hit
>> eof */ page_cache_release(page); goto out; }
>> 
>> We shift off any remaining bytes in the final block of the I/O,
>> resulting in a 0-sized read.  I've attached a patch that fixes this.
>> I'm not happy about how ugly the math is getting, so suggestions are
>> more than welcome.
>> 
>> I've tested this with a simple program that performs the steps outlined
>> for reproducing the problem above.  Without the patch, we get a 0-sized
>> result from read.  With the patch, we get the correct return value from
>> the short read.

akpm> OK.  We do have some helper functions to make the math a little
akpm> clearer. How does this look?

That's much cleaner, thanks!

-Jeff
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