On Fri, Feb 10, 2023 at 11:18:05AM -0800, Linus Torvalds via samba-technical wrote:
We should point the fingers at either the _user_ of splice - as Jeremy Allison has done a couple of times - or we should point it at the sink that cannot deal with unstable sources. .... - it sounds like the particular user in question (samba) already very much has a reasonable model for "I have exclusive access to this" that just wasn't used
Having said that, I just had a phone discussion with Ralph Boehme on the Samba Team, who has been following along with this in read-only mode, and he did point out one case I had missed. 1). Client opens file with a lease. Hurrah, we think we can use splice() ! 2). Client writes into file. 3). Client calls SMB_FLUSH to ensure data is on disk. 4). Client reads the data just wrtten to ensure it's good. 5). Client overwrites the previously written data. Now when client issues (4), the read request, if we zero-copy using splice() - I don't think theres a way we get notified when the data has finally left the system and the mapped splice memory in the buffer cache is safe to overwrite by the write (5). So the read in (4) could potentially return the data written in (5), if the buffer cache mapped memory has not yet been sent out over the network. That is certainly unexpected behavior for the client, even if the client leased the file. If that's the case, then splice() is unusable for Samba even in the leased file case.
Maybe this thread raised some awareness of it for some people, but more realistically - maybe we can really document this whole issue somewhere much more clearly
Complete comprehensive documentation on this would be extremely helpful (to say the least :-).
