Re: [RFD] readonly/read-write semantics

[Bryan Henderson]

>>
>> 1) I want to see files open for write have nothing to do with it.  Unix
>> open/close is not a transaction, it's just a connection.  Some
applications
>> manage to use open/close as a transaction, but we're seeing less and
less
>> of that as more sophisticated facilities for transactions become
available.
>>
>> How many times have we all been frustrated trying to remount read only
when
>> some log file that hasn't been written to for hours is open for write?
>>
>> A file write is in progress when a write() system call hasn't returned,
not
>> when the file is open for write.
>
>Uh-oh...  How about shared mappings?

It's always shared mappings, isn't it?  :-)

Virtual memory access to the file is even easier, though.  A write in
progress is an individual store to virtual memory.  The only way you could
even see it is if a page fault is in progress.  So the most you would need
to wait for in going into the hard "read only" state I defined is for any
page I/O to complete.  And for the "no new writes" state, you just write
protect all the pages (and any new ones that fault in too).