On 6/28/20 11:52 PM, Baoquan He wrote: > On 06/25/20 at 05:34pm, Dave Hansen wrote: >> >> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> >> >> I went to go add a new RECLAIM_* mode for the zone_reclaim_mode >> sysctl. Like a good kernel developer, I also went to go update the >> documentation. I noticed that the bits in the documentation didn't >> match the bits in the #defines. >> >> The VM evidently stopped caring about RECLAIM_ZONE at some point (or >> never cared) and the #define itself was later removed as a cleanup. > >>From git history, it seems to never care about the RECLAIM_ZONE bit. > > I think this patch is justified. I have one question about adding back > the RECLAIM_ZONE bit. Since we introduced RECLAIM_ZONE in the first > place, but never use it, removing it truly may fail some existing > script, does it mean we will never have chance to fix/clean up this kind > of mess? Our #1 rule is "don't break userspace". We only break userspace when we have no other choice. This case a bit fuzzier because we don't know if anyone is depending on the ignored bit to do anything. But, it *was* documented. To me, that means it might have been used, even though it would have been a "placebo" bit. > Do we have possibility to remove it in mainline tree, let distos or > stable kernel maintainer take care of the back porting? Like this, any > stable kernel after 5.8, or any distrols which chooses post v5.8 kenrel > as base won't have this confusion. I am not objecting this patch, just > be curious if we have a way to fix/clean up for this type of issue. The only way I can plausibly think of "cleaning up" the RECLAIM_ZONE bit would be to raise our confidence that it is truly unused. That takes time, and probably a warning if we see it being set. If we don't run into anybody setting it or depending on it being set in a few years, we can remove it. Backporting a _warning_ into the -stable trees might be an interesting way to find users of older kernels mucking with it.
