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SubjectRe: [RFC][PATCH] ext3: don't read inode block if the buffer has a write error
On Mon, 23 Jun 2008 21:46:27 +1000 Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> On Monday 23 June 2008 21:25, Hidehiro Kawai wrote:
> > A transient I/O error can corrupt inode data. Here is the scenario:
> >
> > (1) update inode_A at the block_B
> > (2) pdflush writes out new inode_A to the filesystem, but it results
> > in write I/O error, at this point, BH_Uptodate flag of the buffer
> > for block_B is cleared and BH_Write_EIO is set
> > (3) create new inode_C which located at block_B, and
> > __ext3_get_inode_loc() tries to read on-disk block_B because the
> > buffer is not uptodate
> > (4) if it can read on-disk block_B successfully, inode_A is
> > overwritten by old data
> >
> > This patch makes __ext3_get_inode_loc() not read the inode block if
> > the buffer has BH_Write_EIO flag. In this case, the buffer should
> > have the latest information, so setting the uptodate flag to the
> > buffer (this avoids WARN_ON_ONCE() in mark_buffer_dirty().)
> >
> > According to this change, we would need to test BH_Write_EIO flag for
> > the error checking. Currently nobody checks write I/O errors on
> > metadata buffers, but it will be done in other patches I'm working on.
>
> IMO there is a problem with all the buffer head and pagecache error
> handling in that uptodate gets cleared on write errors. This is not
> only wrong (because the in-memory copy continues to contain the most
> uptodate copy), but it will trigger assertions all over the mm and
> buffer code AFAIKS.
>
> I don't know why it was done like this, or if anybody actually tested
> any of it, but AFAIKS the best way to fix this is to simply not
> clear any uptodate bits upon write errors.

There's a plausible-sounding reason for this behaviour which I forgot
about three years ago. Maybe Linus remembers?

Meanwhile I guess I'll queue the patch, sad though it is.


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