From: Kent Overstreet <kent.overstreet@linux.dev>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>,
Suren Baghdasaryan <surenb@google.com>,
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
Subject: Re: [PATCH 1/1] mm: change inlined allocation helpers to account at the call site
Date: Thu, 4 Apr 2024 19:00:51 -0400 [thread overview]
Message-ID: <jpaw4hdd73ngt7mvtcdryqscivx6m2ic76ikfkcopceb47becp@vox5czt5bec3> (raw)
In-Reply-To: <20240404154150.c25ba3a0b98023c8c1eff3a4@linux-foundation.org>
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.
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...
next prev parent reply other threads:[~2024-04-04 23:01 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-04 16:54 [PATCH 1/1] mm: change inlined allocation helpers to account at the call site Suren Baghdasaryan
2024-04-04 17:04 ` Matthew Wilcox
2024-04-04 17:08 ` Suren Baghdasaryan
2024-04-04 22:17 ` Suren Baghdasaryan
2024-04-04 22:28 ` Kent Overstreet
2024-04-04 22:33 ` Matthew Wilcox
2024-04-04 22:38 ` Kent Overstreet
2024-04-04 22:41 ` Andrew Morton
2024-04-04 23:00 ` Kent Overstreet [this message]
2024-04-04 23:16 ` Suren Baghdasaryan
2024-04-05 9:53 ` Jan Kara
2024-04-05 13:47 ` Suren Baghdasaryan
2024-04-05 12:44 ` Matthew Wilcox
2024-04-05 13:53 ` Suren Baghdasaryan
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