Re: [PATCH] MM: discard __GFP_ATOMIC

["NeilBrown" <neilb@suse.de>]

On Tue, 23 Nov 2021, Michal Hocko wrote:
> On Sat 20-11-21 21:51:20, Neil Brown wrote:
> > On Sat, 20 Nov 2021, Matthew Wilcox wrote:
> > > On Fri, Nov 19, 2021 at 10:14:38AM +1100, NeilBrown wrote:
> > > > On Thu, 18 Nov 2021, Matthew Wilcox wrote:
> > > > > Surely this should be gfpflags_allow_blocking() instead of poking about
> > > > > in the innards of gfp flags?
> > > > 
> > > > Possibly.  Didn't know about gfpflags_allow_blocking().  From a quick
> > > > grep in the kernel, a whole lot of other people don't know about it
> > > > either, though clearly some do.
> > > > 
> > > > Maybe we should reaname "__GFP_DIRECT_RECLAIM" to
> > > > "__GFP_ALLOW_BLOCKING", because that is what most users seems to care
> > > > about.
> > > 
> > > I tend towards the school of thought that the __GFP flags should make
> > > sense to the implementation and users should use either GFP_ or functions.
> > > When we see users adding or subtracting __GFP flags, that's a problem.
> > 
> > Except __GFP_NOWARN of course, or __GFP_ZERO, or __GFP_NOFAIL.
> > What about __GFP_HIGHMEM?  __GFP_DMA?  __GFP_HIGH?
> > 
> > They all seem to be quite meaningful to the caller - explicitly
> > specifying properties of the memory or properties of the service.
> > (But maybe you would prefer __GFP_HIGH be spelled "__GFP_LOW_WATERMARK"
> > so it would make more sense to the implementation).
> > 
> > __GFP_DIRECTRECLAIM seems to me to be more the exception than the rule -
> > specifying internal implementation details.
> 
> I do not think it is viable to fix up gfp flags to be consistent :/

You may be right :-)  Of course if we don't try - you'll definitely be right.

> Both __GFP_DIRECT_RECLAIM and __GFP_KSWAPD_RECLAIM are way too lowlevel
> but historically we've had requests to inhibit kswapd for a particular
> requests because that has led to problems - fun reading caf491916b1c1.

Unfortunately that commit doesn't provide any reasoning, just an
assertion.
The best reasoning I could find was in caf491916b1c1 which was the initial
revert.  There the primary reasoning was "there is a bug that we don't
have time for a proper fix before the next release, so let's just use
this quick fix".
...  and maybe "the quick fix" was "the right fix", but I cannot tell from
the commit logs :-(

> __GFP_ALLOW_BLOCKING would make a lot of sense but I am not sure it
> would be a good match to __GFP_KSWAPD_RECLAIM.

So? __GFP_ALLOW_BLOCKING makes it clear what is, or is not, acceptable
to the caller.  How much reclaim, or other activity, alloc_page()
engages in is largely irrelevant to the caller as lock as it doesn't
block if asked not to (and doesn't enter an FS if asked not to, etc).

> 
> > Actually ... I take it back about __GFP_NOWARN.  That probably shouldn't
> > exist at all.  Warnings should be based on how stressed the mm system is,
> > not on whether the caller wants thinks failure is manageable.
> 
> Unless we change the way when allocation warnings are triggered then we
> really need this. There are many opportunistic allocations with a
> fallback behavior which do not want to swamp kernel logs with failures
> that are of no use. Think of a THP allocation that really want to be
> just very quick and falls back to normal base pages otherwise. Deducing
> context which is just fine to not report failures is quite tricky and it
> can get wrong easily. Callers should know whether warning can be of any
> use in many cases.

"Unless" being the key work.
It makes sense to warn when a __GFP_HIGH or __GFP_MEMALLOC allocation
fails,  because they are clearly important.
It makes sense to warning if direct reclaim and retrying were enabled,
as then alloc_page() has tried really hard, but failed anyway.  Thought
maybe if COSTLY_ORDER is exceeded, then the warning is unlikely to be
interesting.
But does it ever make sense to warn if either of
__GFP_RETRY_MAYFAIL __GFP_NORETRY are present?

If we always suppressed warning when those flags were present, then many
(most?) uses for __GFP_NOWARN can be discarded.

I can see that some of the __GFP flags are designed to each perform a
single well-defined function and internally to mm/ that makes sense.
But exposing those flags to all users appears to be a recipe for
trouble.  Hiding them all behind "__" doesn't stop people from using and
misusing them.  Others are externally meaningful.  Making them visually
similar to the ones we want to hide isn't helping anyone.

When Willy wrote:
  > When we see users adding or subtracting __GFP flags, that's a problem.
the "problem" is not so much in the fact that they *do* but in the fact
that they *can*.

I would be greatly in favour of GFP flags which made sense to callers,
and a mapping to ALLOC_ flags in mm/ which makes sense to allocators.

I doubt anything outside mm/ cares about whether KSWAPD is woken or not.
IT probably should be for small-order allocations, and not so much for
large-order allocations.  but mm/khugepaged.c might make other decisions.

Thanks,
NeilBrown