On Wednesday 06 August 2008 19:44, tvrtko.ursulin@sophos.com wrote:
quoted text
> Nick Piggin wrote on 05/08/2008 19:08:05:
> > On Tuesday 05 August 2008 07:00, Eric Paris wrote:
> > > 5. Define which filesystems are cacheable and which are not
> >
> > This is practically impossible to do completely without rewriting a lot
> > of code (which will never be accepted). I don't see why it is needed
>
> though
>
> > as the filesystem cache is supposed to be kept coherent with disk.
>
> Problem is with network filesystems. So could it be a flag somewhere per
> filesystem which would say something like "this filesystem guarantees
> content of a file cannot change without get_write_access or
> file_update_time being called locally"? That doesn't sound like a lot of
> code so what am I missing?

Maybe... but that's not the same as what requirement 5 calls for.

But depending on exactly what semantics you really call for, it can get
tricky to account for all of pagecache. Writes can happen through page
tables or get_user_pages. True that a process has to at some point have
write permission to the file, but the cache itself could be modified
even after the file is closed and all mmaps disappear.
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