Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Reclamation interactions with RCU

[Kent Overstreet <kent.overstreet@linux.dev>]

On Sat, Mar 02, 2024 at 10:47:59AM +1100, NeilBrown wrote:
> On Sat, 02 Mar 2024, Kent Overstreet wrote:
> > 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@dread.disaster.area/
> > > 
> > > 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.
> > 
> 
> Not strictly true.  mempool_alloc() adds __GFP_NORETRY so the allocation
> will certainly fail if that is appropriate.

*nod* 

> I suspect that most places where there is a non-error fallback already
> use NORETRY or RETRY_MAYFAIL or similar.

NORETRY and RETRY_MAYFAIL actually weren't on my radar, and I don't see
_tons_ of uses for either of them - more for NORETRY.

My go-to is NOWAIT in this scenario though; my common pattern is "try
nonblocking with locks held, then drop locks and retry GFP_KERNEL".
 
> But I agree that changing the meaning of GFP_KERNEL has a potential to
> cause problems.  I support promoting "GFP_NOFAIL" which should work at
> least up to PAGE_ALLOC_COSTLY_ORDER (8 pages).

I'd support this change.

> I'm unsure how it should be have in PF_MEMALLOC_NOFS and
> PF_MEMALLOC_NOIO context.  I suspect Dave would tell me it should work in
> these contexts, in which case I'm sure it should.
> 
> Maybe we could then deprecate GFP_KERNEL.

What do you have in mind?

Deprecating GFP_NOFS and GFP_NOIO would be wonderful - those should
really just be PF_MEMALLOC_NOFS and PF_MEMALLOC_NOIO, now that we're
pushing for memalloc_flags_(save|restore) more.

Getting rid of those would be a really nice cleanup beacuse then gfp
flags would mostly just be:
 - the type of memory to allocate (highmem, zeroed, etc.)
 - how hard to try (don't block at all, block some, block forever)