Re: copy on write for splice() from file to pipe?

On Fri, Feb 10, 2023 at 03:06:26PM +1100, Dave Chinner wrote:

So while I was pondering the complexity of this and watching a great
big shiny rocket create lots of heat, light and noise, it occurred

That was kind of fun

to me that we already have a mechanism for preventing page cache
data from being changed while the folios are under IO:
SB_I_STABLE_WRITES and folio_wait_stable().

I thought about bringing that up, but it's not quite right.  That works
great for writeback, but it only works for writeback.  We'd need to track
another per-folio counter ... it'd be like the page pinning kerfuffle,
only worse.  And for such a rare thing it seems like a poor use of 32
bits of per-page state.  Not to mention that you can effectively block
all writes to a file for an indefinite time by splicing pages to a pipe
that you then never read from.