Re: [PATCH 1/3] Preserve the dirty bit in init_page_buffers

[ebiederm@xmission.com (Eric W. Biederman)]

Nick Piggin <nickpiggin@yahoo.com.au> writes:

> Nick Piggin wrote:
>> Eric W. Biederman wrote:
>
>>> When we initialize the ramdisk by writing to /dev/ram0 usually in
>>> init/do_mounts_rd.c we don't allocate buffer heads but we do set
>>> the dirty bit, and the page is in the page cache.  So when we
>>> later call getblk it reuses the same page and then calls
>>> init_page_buffers.
>>
>>
>> Hmm, so this would be a problem for block_dev.c as well, then?
>> Because it would be possible to have a dirty block dev page
>> have its buffers reclaimed and then reinitialised via
>> init_page_buffers, AFAIKS.
>
> Oh, no, try_to_free_buffers won't drop dirty buffers. However we
> could still set_page_dirty of a block device page without buffers
> via an mmap.

After the page is made dirty via mmap we have:
sys_write -> ... -> block_prepare_write -> ... -> create_empty_buffers.

I suspect that is a pretty rare case but it does indeed seem to exist
as a problem.

Eric