On Tuesday 18 December 2007 09:54, David Howells wrote:
> Nick Piggin <[email protected]> wrote:
> > This reintroduces the fault vs truncate race window, which must be fixed.
>
> Hmmm... perhaps.
What do you mean by perhaps?
> I remember that cropped up in NFS, but I'm doing things
> a bit differently to NFS. Remind me again how that worked please.
How what worked? NFS is using invalidate inode pages quite frequently
so it ran into the race more often.
> > Also, it is adding a fair bit of complexity in an area where we should
> > instead be reducing it. I think your filesystem should not be doing
> > writeback caching of dirty data in the cases where it is so problematic
> > (or at least, disallow mmap and read on the dirty data until it has been
> > written back or failed).
>
> Eh? It's a stateless network filesystem. There's a gap between writing to
> a file (perhaps though an mmap) and the pagecache pages being written back
> in which someone may change the security on a file and block the writeback.
> There's nothing I can do to prevent it, so I have to instead deal with the
> consequences should they arise. See the description of patch 25 for
> examples.
>
> So you say I shouldn't do any writeback caching at all?
No, you could do writeback caching but disallow read of dirty data. But
yeah, a coherent mmap isn't possible then, I guess.
> > But otherwise I guess if you really want to discard the dirty data after
> > a failed writeback attempt, what's wrong with just
> > invalidate_inode_pages2?
>
> Erm... Because it deadlocks?
Why don't you call it after calling end_page_writeback?
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