On Tue, Aug 27, 2024 at 08:58:39AM GMT, Michal Hocko wrote: > On Tue 27-08-24 02:40:16, Kent Overstreet wrote: > > On Tue, Aug 27, 2024 at 08:01:32AM GMT, Michal Hocko wrote: > > > You are not really answering the main concern I have brought up though. > > > I.e. GFP_NOFAIL being fundamentally incompatible with NORECLAIM semantic > > > because the page allocator doesn't and will not support this allocation > > > mode. Scoped noreclaim semantic makes such a use much less visible > > > because it can be deep in the scoped context there more error prone to > > > introduce thus making the code harder to maintain. > > > > You're too attached to GFP_NOFAIL. > > Unfortunatelly GFP_NOFAIL is there and we need to support it. We cannot > just close eyes and pretend it doesn't exist and hope for the best. You need to notice when you're trying to do something immpossible. > > GFP_NOFAIL is something we very rarely use, and it's not something we > > want to use. Furthermore, GFP_NOFAIL allocations can fail regardless of > > this patch - e.g. if it's more than 2 pages, it's not going to be > > GFP_NOFAIL. > > We can reasonably assume we do not have any of those users in the tree > though. We know that because we have a warning to tell us about that. > We still have legit GFP_NOFAIL users and we can safely assume we will > have some in the future though. And they have no way to handle the > failure. If they did they wouldn't have used GFP_NOFAIL in the first > place. So they do not check for NULL and they would either blow up or > worse fail in subtle and harder to detect way. No, because not all GFP_NOFAIL allocations are statically sized. And the problem of the dynamic context overriding GFP_NOFAIL is more general - if you use GFP_NOFAIL from nonblocking context (interrupt context or preemption disabled) - the allocation has to fail, or something even worse will happen. Just because we don't track that with PF_MEMALLOC flags doesn't mean the problem isn't htere.
