On 10/07/2020 17.57, Matthew Wilcox wrote: > On Fri, Jul 10, 2020 at 09:56:31AM +0200, Rasmus Villemoes wrote: >> The ability to check open file descriptions for equality (without >> resorting to unreliable fstat() and fcntl(F_GETFL) comparisons) can be >> useful outside of the checkpoint/restore use case - for example, >> systemd uses kcmp() to deduplicate the per-service file descriptor >> store. >> >> Make it possible to have the kcmp() syscall without the full >> CONFIG_CHECKPOINT_RESTORE. > > If systemd is using it, is it even worth making it conditional any more? > Maybe for CONFIG_EXPERT builds, it could be de-selectable. > [hm, I dropped the ball, sorry for the necromancy] Well, first, I don't want to change any defaults here, if that is to be done, it should be a separate patch. Second, yes, systemd uses it for the de-duplication, and for that reason recommends CONFIG_CHECKPOINT_RESTORE (at least, according to their README) - but I'm not aware of any daemons that actually make use of systemd's file descriptor store, so it's not really something essential to every systemd-based system out there. It would be nice if systemd could change its recommendation to just CONFIG_KCMP_SYSCALL. But it's also useful for others, e.g. I have some code that wants to temporarily replace stdin/stdout/stderr with some other file descriptors, but needs to preserve the '0 is a dup of 1, or not' state (i.e., is the same struct file) - that cannot reliably be determined from fstat()/lseek(SEEK_CUR)/F_GETFL or whatever else one could throw at an fd. Rasmus
