Eric W. Biederman wrote:
Nick Piggin <[email protected]> writes:
Eric W. Biederman wrote:
The problem: When we are trying to free buffers try_to_free_buffers
will look at ramdisk pages with clean buffer heads and remove the
dirty bit from the page. Resulting in ramdisk pages with data that
get removed from the page cache. Ouch!
Buffer heads appear on ramdisk pages when a filesystem calls getblk,
which through a series of function calls eventually calls
init_page_buffers.
So to fix the mismatch between buffer head state and page state this
patch modifies init_page_buffers to transfer the dirty bit from the
page to the buffer heads like we currently do for the uptodate bit.
Ouch indeed!
But can we ever have a dirty page at init_page_buffers-time?
Definitely, and it was a royal pain to trace the bug that this
caused. An initial ramdisk having pieces disappear after mkfs
is called can look like the entire machine is dying.
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, the comment above grow_dev_buffers indicates this should
not happen. But contrary to the comment, it doesn't go BUG
unless you're attaching dirty buffers to a page with dirty
buffers.
I suspect this happens more frequently with rd.c, because unlike
block_dev.c, it does not create dirty buffers in prepare_write.
However it could still happen in block_dev.c via mmaped memory,
as I said earlier.
I'm not saying this patch 1/3 is wrong, but we would at least
need to revise some comments. The grow_dev_buffers comment looks
like one of Andrew's. Maybe he can shed some more light on this?
--
SUSE Labs, Novell Inc.
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