On Mon, Jun 14, 2021 at 02:37:12PM +0100, David Howells wrote: > Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > (1) If the page is not up to date, then we should just return 0 > > > (ie. indicating a zero-length copy). The loop in > > > generic_perform_write() will go around again, possibly breaking up the > > > iterator into discrete chunks. > > > > Does this actually work? What about the situation where you're reading > > the last page of a file and thus (almost) always reading fewer bytes > > than a PAGE_SIZE? > > Al Viro made such a change for Ceph - and we're writing, not reading. I'd feel better if you said "xfstests doesn't show any new problems" than arguing to authority. I know the operation which triggers this path is a call to write(), but if, say, the file is 32 bytes long, not in cache, and you write bytes 32-63, the client must READ bytes 0-31 from the server, which is less than a full page.
