Received-SPF: pass (google.com: domain of linux-crypto+bounces-3371-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Precedence: bulk
MIME-Version: 1.0
References: <20240404165404.3805498-1-surenb@google.com> <Zg7dmp5VJkm1nLRM@casper.infradead.org>
 <CAJuCfpHbTCwDERz+Hh+aLZzNdtSFKA+Q7sW-xzvmFmtyHCqROg@mail.gmail.com>
 <CAJuCfpHy5Xo76S7h9rEuA3cQ1pVqurL=wmtQ2cx9-xN1aa_C_A@mail.gmail.com>
 <Zg8qstJNfK07siNn@casper.infradead.org> <jb25mtkveqf63bv74jhynf6ncxmums5s37esveqsv52yurh4z7@5q55ttv34bia>
 <20240404154150.c25ba3a0b98023c8c1eff3a4@linux-foundation.org>
 <jpaw4hdd73ngt7mvtcdryqscivx6m2ic76ikfkcopceb47becp@vox5czt5bec3> <Zg_yHGKpw4HJHdpb@casper.infradead.org>
In-Reply-To: <Zg_yHGKpw4HJHdpb@casper.infradead.org>
From: Suren Baghdasaryan <surenb@google.com>
Date: Fri, 5 Apr 2024 06:53:09 -0700
Message-ID: <CAJuCfpGMSHv7drSu7Veo5CVz3d_Upt8S6Rdx3isi7orct9-uNQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm: change inlined allocation helpers to account at
 the call site
To: Matthew Wilcox <willy@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>, Andrew Morton <akpm@linux-foundation.org>, 
	joro@8bytes.org, will@kernel.org, trond.myklebust@hammerspace.com, 
	anna@kernel.org, arnd@arndb.de, herbert@gondor.apana.org.au, 
	davem@davemloft.net, jikos@kernel.org, benjamin.tissoires@redhat.com, 
	tytso@mit.edu, jack@suse.com, dennis@kernel.org, tj@kernel.org, cl@linux.com, 
	jakub@cloudflare.com, penberg@kernel.org, rientjes@google.com, 
	iamjoonsoo.kim@lge.com, vbabka@suse.cz, edumazet@google.com, kuba@kernel.org, 
	pabeni@redhat.com, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, 
	linux-nfs@vger.kernel.org, linux-acpi@vger.kernel.org, 
	acpica-devel@lists.linux.dev, linux-arch@vger.kernel.org, 
	linux-crypto@vger.kernel.org, bpf@vger.kernel.org, 
	linux-input@vger.kernel.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org, 
	netdev@vger.kernel.org, linux-security-module@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, Apr 5, 2024 at 5:44=E2=80=AFAM Matthew Wilcox <willy@infradead.org>=
 wrote:
>
> On Thu, Apr 04, 2024 at 07:00:51PM -0400, Kent Overstreet 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@li=
nux.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 wrot=
e:
> > > > > > 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_ch=
ain),
> > > > > > 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.  vmallo=
c
> > > > > has done it this way since 2008 (OK, using __builtin_return_addre=
ss())
> > > > > and lockdep has used _THIS_IP_ / _RET_IP_ since 2006.
> > > >
> > > > Except you can't. We've been over this; using that approach for tra=
cing
> > > > 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.
>
> Well, /proc/vmallocinfo exists, and has existed since 2008, so this is
> slightly more than a "hot take".
>
> > 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.
>
> And yet vmalloc doesn't do that.
>
> > 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 givin=
g
> > numbers that you can trust, and by making things less explicit you make
> > that harder.
>
> I'm not convinced that _THIS_IP_ is less precise than a codetag.  They
> do essentially the same thing, except that codetags embed the source
> location in the file while _THIS_IP_ requires a tool like faddr2line
> to decode kernel_clone+0xc0/0x430 into a file + line number.
>
> > This is all stuff that I've explained before; let's please dial back on
> > the whining - or I'll just bookmark this for next time...
>
> Please stop mischaracterising serious thoughtful criticism as whining.
> I don't understand what value codetags bring over using _THIS_IP_ and
> _RET_IP_ and you need to explain that.

The conceptual difference between codetag and _THIS_IP_/_RET_IP_ is
that codetag injects counters at the call site, so you don't need to
spend time finding the appropriate counter to operate on during
allocation.