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SubjectRe: Block I/O Mystery
Block Device writes:
> Hi,
> I am trying to write to inode blocks without using the VFS layer
> (filp_open, f_op->write)
> etc.
> I could use __bread to read the inode blocks correctly. But when I'm
> trying to write a block after changing it I face a strange problem.
> My changes do not show up when I cat the file but if I use dd command on
> the block device it shows my writes.
>
> Steps I performed :
>

Blocks read by bread() and friends are cached in block device (not you
:-) struct address_space. File data are cached in the per-inode struct
address_space.

> a) Created a file of 8K ( 2blocks ) and filled it with 'a'.

Now 2 pages are cached in memory in address_space associated with inode.

> b) Read the blocks using __bread and print(k)ed the contents.
> This worked correctly.

This reads blocks directly from the disk (igrnoing cached copy), and
caches them in block device address_space.

> c) Called __getblk for a file block, locked the buffer, made
> changes to it (memcpy),
> marked buffer dirty, unlocked it and called sync_dirty_buffer.

This makes changes to the block cached in block device address_space.

> d) If I try to see contents of the file ( cat or vi ) they are
> shown unchanged, but if I

vi uses read(1), and, hence, will fetch data from inode address_space,
where original pages still live.

> use dd on the raw device or use step b) to read a block the
> changes made @ c)
> are seen.

And this will read from block device cache.

>
> Also, if I unmount the filesystem and mount it again the changes are
> visible through cat, vi etc.
>
> Can someone explain what exactly is going wrong ?

You are bypassing normal file sysetm caching and results can be
unpredictable.

>
> I'm using the 2.6.13 kernel.
>
> Thanks & Regards
> -BD

Nikita.
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