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SubjectRe: [RFC] changes to buffer.c (was Test12 ll_rw_block error)

On Thu, 21 Dec 2000, Chris Mason wrote:

> Ok guys, I think I've taken Linus' suggestion to have buffer.c use its
> own writepage a bit too far. This patch marks pages dirty when the
> buffer head is marked dirty, and changes flush_dirty_buffers and
> sync_buffers to use writepage instead of ll_rw_block. The idea is
> to allow filesystems to use the buffer lists and provide their own
> i/o mechanism.
>
> The result is a serious semantics change for writepage, which now is
> expected to do partial page writes when the page isn't up to date and
> there are dirty buffers inside. For all the obvious reasons, this isn't
> fit for 2.4.0, and if you all feel it is a 2.5. thing I'll send along
> the shorter patch Linus originally suggested. But, I think it would
> be pretty useful for the new filesystems (once I also fix
> fsync_inode_buffers and sync_page_buffers).

It is very powerful.

With this on place, the filesystem is able to do write clustering at its
writepage() function by checking if the on-disk physically nearby pages
are dirty.

> Other changes: submit_bh now cleans the buffers. I don't see how
> they were getting cleaned before, it must have been try_to_free_buffers
> sending the page through sync_page_buffers, meaning they were probably
> getting written twice. Unless someone throws a clue my way, I'll send
> this out as a separate diff.

Ouch.

> page_launder doesn't fiddle with the buffermem_pages counter, it is done
> in try_to_free_buffers instead.
>
> Obvious bug, block_write_full_page zeros out the bits past the end of
> file every time. This should not be needed for normal file writes.
>
> Most testing was on ext2, who actually calls mark_buffer_dirty, and
> supports blocksizes < intel page size. More tests are running
> overnight.

It seems your code has a problem with bh flush time.

In flush_dirty_buffers(), a buffer may (if being called from kupdate) only
be written in case its old enough. (bh->b_flushtime)

If the flush happens for an anonymous buffer, you'll end up writing all
buffers which are sitting on the same page (with block_write_anon_page),
but these other buffers are not necessarily old enough to be flushed.

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