[Ocfs2-devel] [PATCH] Treat writes as new when holes span across page boundaries

[Goldwyn Rodrigues <rgoldwyn@gmail.com>]

Hi Joel,

On Wed, Feb 23, 2011 at 1:28 PM, Joel Becker <jlbec@evilplan.org> wrote:
> On Wed, Feb 23, 2011 at 11:13:39AM -0800, Joel Becker wrote:
>> The calling code should have made sure we are correctly passing
>> should_zero to all non-target pages. ?Having new overridden for them is
>> wrong. ?So maybe we move your check inside the target_page if. ?It only
>> really matters for the target page.
>
> ? ? ? ?Actually, I have a question: what is the clustersize of your
> test? ?Because if the filesystem is 4k/4k, the write at 4096 of 32 bytes
> should allocate a new cluster. ?'new' should already be set, and your
> check irrelevant.

It is 16K and does not happen for clustersize==pagesize/blocksize.
Don't have a machine with a bigger pagesize handy.

>
>> ? ? ? But wait! ?Check out block_write_full_page_endio() in
>> fs/buffer.c! ?Especially this part:
>>
>> ? ? ? zero_user_segment(page, offset, PAGE_CACHE_SIZE);
>>
>> It is zeroing the tail of a page when it straddles i_size to make sure
>> it goes out correctly. ?But, of course, blocks past i_size never go to
>> disk. ?And we don't care; readpage should handle them correctly, right?
>> So if writepage is going to zero for us, and readpage is going to read
>> correctly, how are you getting bad data?
>> ? ? ? I was about to think we were done, but now I'm not so sure. ?I
>> have a question: with my change to ocfs2_should_read_block(), but
>> without your patch, what do you see for corruption? ?Your pattern
>> written in the first pass, or complete random garbage?
>
> ? ? ? ?One thing I would like to see are the results with fsync()
> between writeSparse() and checkSparselyWrittenFile(). ?Forcing the file
> to disk before checking the values will tell us something. ?This is with
> my patch but without yours.
> ? ? ? ?Another good test would be to umount/mount between writeSparse()
> and checkSparselyWrittenFile(). ?This tells us what actually hit disk.
>

No, fsync does not work. I still get junk in the holes. Same is the
case if I umount and re-mount the disk.


-- 
Goldwyn