On 5/7/20 12:31 PM, Jeremy Allison wrote: > On Thu, May 07, 2020 at 10:50:40AM -0600, Jens Axboe wrote: >> On 5/7/20 10:48 AM, Jeremy Allison wrote: >>> On Thu, May 07, 2020 at 10:43:17AM -0600, Jens Axboe wrote: >>>> >>>> Just like for regular system calls, applications must be able to deal >>>> with short IO. >>> >>> Thanks, that's a helpful definitive reply. Of course, the SMB3 >>> protocol is designed to deal with short IO replies as well, and >>> the Samba and linux kernel clients are well-enough written that >>> they do so. MacOS and Windows however.. >> >> I'm honestly surprised that such broken clients exists! Even being >> a somewhat old timer cynic... >> >>> Unfortunately they're the most popular clients on the planet, >>> so we'll probably have to fix Samba to never return short IOs. >> >> That does sound like the best way forward, short IOs is possible >> with regular system calls as well, but will definitely be a lot >> more frequent with io_uring depending on the access patterns, >> page cache, number of threads, and so on. > > OK, I just want to be *REALLY CLEAR* what you're telling me > (I've already written the pread/pwrite wrappers for Samba > that deal with short IO but want to ensure I understand > fully before making any changes to Samba). > > You're saying that on a bog-standard ext4 disk file: > > ret = pread(fd, buf, count, offset); > > can return *less* than count bytes if there's no IO > error and the file size is greater than offset+count > and no one else is in the middle of a truncate etc. ? > > And: > > ret = pwrite(fd, buf, count, offset); > > can return less* than count bytes if there's no IO > error and there's ample space on disk ? > > I have to say I've *never* seen that happen, and > Samba is widely enough used that IO corruption from > short reads/writes from MacOSX and Windows clients > would have been widely reported by now. > > Look at how quickly someone spotted disk corruption > because of the change in userspace-visible behavior > of the io_uring interface. We only shipped that code > 03 March 2020 and someone *already* found it. I _think_ that will only happen on regular files if you use RWF_NOWAIT or similar, for regular blocking it should not happen. So I don't think you're at risk there, though I do think that anyone should write applications with short IOs in mind or they will run into surprises down the line. Should have been more clear! -- Jens Axboe
