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Date: Fri, 5 Apr 2024 11:53:27 +0200
From: Jan Kara <jack@suse.cz>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH 1/1] mm: change inlined allocation helpers to account at
 the call site
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On Thu 04-04-24 16:16:15, Suren Baghdasaryan wrote:
> On Thu, Apr 4, 2024 at 4:01 PM Kent Overstreet
> <kent.overstreet@linux.dev> wrote:
> >
> > On Thu, Apr 04, 2024 at 03:41:50PM -0700, Andrew Morton wrote:
> > > On Thu, 4 Apr 2024 18:38:39 -0400 Kent Overstreet <kent.overstreet@linux.dev> wrote:
> > >
> > > > On Thu, Apr 04, 2024 at 11:33:22PM +0100, Matthew Wilcox wrote:
> > > > > On Thu, Apr 04, 2024 at 03:17:43PM -0700, Suren Baghdasaryan wrote:
> > > > > > Ironically, checkpatch generates warnings for these type casts:
> > > > > >
> > > > > > WARNING: unnecessary cast may hide bugs, see
> > > > > > http://c-faq.com/malloc/mallocnocast.html
> > > > > > #425: FILE: include/linux/dma-fence-chain.h:90:
> > > > > > + ((struct dma_fence_chain *)kmalloc(sizeof(struct dma_fence_chain),
> > > > > > GFP_KERNEL))
> > > > > >
> > > > > > I guess I can safely ignore them in this case (since we cast to the
> > > > > > expected type)?
> > > > >
> > > > > I find ignoring checkpatch to be a solid move 99% of the time.
> > > > >
> > > > > I really don't like the codetags.  This is so much churn, and it could
> > > > > all be avoided by just passing in _RET_IP_ or _THIS_IP_ depending on
> > > > > whether we wanted to profile this function or its caller.  vmalloc
> > > > > has done it this way since 2008 (OK, using __builtin_return_address())
> > > > > and lockdep has used _THIS_IP_ / _RET_IP_ since 2006.
> > > >
> > > > Except you can't. We've been over this; using that approach for tracing
> > > > is one thing, using it for actual accounting isn't workable.
> > >
> > > I missed that.  There have been many emails.  Please remind us of the
> > > reasoning here.
> >
> > I think it's on the other people claiming 'oh this would be so easy if
> > you just do it this other way' to put up some code - or at least more
> > than hot takes.
> >
> > But, since you asked - one of the main goals of this patchset was to be
> > fast enough to run in production, and if you do it by return address
> > then you've added at minimum a hash table lookup to every allocate and
> > free; if you do that, running it in production is completely out of the
> > question.
> >
> > Besides that - the issues with annotating and tracking the correct
> > callsite really don't go away, they just shift around a bit. It's true
> > that the return address approach would be easier initially, but that's
> > not all we're concerned with; we're concerned with making sure
> > allocations get accounted to the _correct_ callsite so that we're giving
> > numbers that you can trust, and by making things less explicit you make
> > that harder.
> >
> > Additionally: the alloc_hooks() macro is for more than this. It's also
> > for more usable fault injection - remember every thread we have where
> > people are begging for every allocation to be __GFP_NOFAIL - "oh, error
> > paths are hard to test, let's just get rid of them" - never mind that
> > actually do have to have error paths - but _per callsite_ selectable
> > fault injection will actually make it practical to test memory error
> > paths.
> >
> > And Kees working on stuff that'll make use of the alloc_hooks() macro
> > for segregating kmem_caches.
> 
> Yeah, that pretty much summarizes it. Note that we don't have to make
> the conversions in this patch and accounting will still work but then
> all allocations from different callers will be accounted to the helper
> function and that's less useful than accounting at the call site.
> It's a sizable churn but the conversions are straight-forward and we
> do get accurate, performant and easy to use memory accounting.

OK, fair enough. I guess I can live with the allocation macros in jbd2 if
type safety is preserved. But please provide a short summary of why we need
these macros (e.g. instead of RET_IP approach) in the changelog (or at
least a link to some email explaining this if the explanation would get too
long). Because I was wondering about the same as Andrew (and yes, this is
because I wasn't really following the huge discussion last time).

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR