On Fri, Mar 01, 2024 at 04:54:55PM +1100, Dave Chinner wrote: > On Fri, Mar 01, 2024 at 01:16:18PM +1100, NeilBrown wrote: > > While we are considering revising mm rules, I would really like to > > revised the rule that GFP_KERNEL allocations are allowed to fail. > > I'm not at all sure that they ever do (except for large allocations - so > > maybe we could leave that exception in - or warn if large allocations > > are tried without a MAY_FAIL flag). > > > > Given that GFP_KERNEL can wait, and that the mm can kill off processes > > and clear cache to free memory, there should be no case where failure is > > needed or when simply waiting will eventually result in success. And if > > there is, the machine is a gonner anyway. > > Yes, please! > > XFS was designed and implemented on an OS that gave this exact > guarantee for kernel allocations back in the early 1990s. Memory > allocation simply blocked until it succeeded unless the caller > indicated they could handle failure. That's what __GFP_NOFAIL does > and XFS is still heavily dependent on this behaviour. I'm not saying we should get rid of __GFP_NOFAIL - actually, I'd say let's remove the underscores and get rid of the silly two page limit. GFP_NOFAIL|GFP_KERNEL is perfectly safe for larger allocations, as long as you don't mind possibly waiting a bit. But it can't be the default because, like I mentioned to Neal, there are a _lot_ of different places where we allocate memory in the kernel, and they have to be able to fail instead of shoving everything else out of memory. > This is the sort of thing I was thinking of in the "remove > GFP_NOFS" discussion thread when I said this to Kent: > > "We need to start designing our code in a way that doesn't require > extensive testing to validate it as correct. If the only way to > validate new code is correct is via stochastic coverage via error > injection, then that is a clear sign we've made poor design choices > along the way." > > https://lore.kernel.org/linux-fsdevel/ZcqWh3OyMGjEsdPz@xxxxxxxxxxxxxxxxxxx/ > > If memory allocation doesn't fail by default, then we can remove the > vast majority of allocation error handling from the kernel. Make the > common case just work - remove the need for all that code to handle > failures that is hard to exercise reliably and so are rarely tested. > > A simple change to make long standing behaviour an actual policy we > can rely on means we can remove both code and test matrix overhead - > it's a win-win IMO. We definitely don't want to make GFP_NOIO/GFP_NOFS allocations nofail by default - a great many of those allocations have mempools in front of them to avoid deadlocks, and if you do that you've made the mempools useless.
