Re: Question regarding concurrent accesses through block device and fs

[Nick Piggin]

On Saturday 21 February 2009 01:10:24 Francis Moreau wrote:
> On Thu, Feb 19, 2009 at 2:44 PM, Nick Piggin <nickpiggin@xxxxxxxxxxxx> 
> > This is done for only newly allocated on-disk blocks, (which is what
> > buffer_new means, not new in-memory buffers). And it is only there to
> > synchronize buffercache access by the filesystem for its metadata, rather
> > than trying to make /dev/bdev access coherent with file access.
>
> Well I'm (still) confused by 2 things:
>
>   - the comments of unmap_underlying_metadata() doesn't sound that we're
>     dealing with meta data only:
>
>     " ... we don't want any output from any buffer-cache aliases starting
> ... "
>
>     note the word *any*. But I must admit that I don't understand the whole
>     comment.

Well buffer cache is (almost) always metadata from the filesystem pov. The
comment *could* be talking about access to /dev/bdev, but in that case the
code is doing the wrong thing WRT coherency anyway (clearing dirty bit), so
I don't see how it could be talking about that.


>   - looking at unmap_underlying_metadata(), there's no code to deal with
>     meta data buffers. It gets the buffer and unmap it whatever the type of
>     data it contains.

That's why I say it only really works for buffer cache used by the same
filesystem that is now known to be unused.


> But at least the name of this function is now more clear.
>
> > Basically what can happen is that a filesystem will have perhaps
> > allocated a block for an array of indirect pointers. The filesystem
> > manages this via the buffercache and writes a few pointers into it. Then
> > suppose the file is truncated and that block becomes unused so it can be
> > freed by the filesystem block allocator. And the filesystem may also call
> > bforget to prevent the now useless buffer from being written out in
> > future.
>
> ok, so now  the buffercache is discarded and its content is either
> discarded or is writing back.
>
> > Now suppose a new block required for *file* data, and the filesystem
> > happens to reallocate that block. So now we may still have that old
> > buffercache and buffer head around, but we also have this new pagecache
> > and buffer head for the file that points to the same block (buffer_new
> > will be set on this new buffer head, btw, to reflect that it is a newly
> > allocated block).
>
> ok
>
> > All fine so far.
> >
> > Now there is a potential problem because the old buffer can *still be
> > under writeback* dating back from when it was still good metadata and
> > before bforget was called. That's a problem because the new buffer is
> > expecting to be the owner and master of the block and its data.
>
> Now I don't see the problem.
>
> Even if the old meta data is under writeback process, the new buffer can
> still be used: since it's new there's no point to do IOs to read its
> content. If we need to write it to disk then the IOs will overwrite the old
> meta data, there's
> no risk that the old meta data overwrite the new data.
>
> What am I missing ?

That we might complete the write of the new buffer before the
old buffer is finished writing out?

Or, I suppose it also covers filesystems that do not always
discard old buffers with bforget, so they don't have dirty
bit cleared (but I don't know 100% sure if this is considered a
filesystem bug or not -- but at least unmap_underlying_metadata
protects against it).

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