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Message-ID: <6596f8ef5fafe_8dc682949e@dwillia2-xfh.jf.intel.com.notmuch>
Date: Thu, 4 Jan 2024 10:29:03 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Lukas Wunner <lukas@...ner.de>, Peter Zijlstra <peterz@...radead.org>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Dan Williams
<dan.j.williams@...el.com>, <linux-coco@...ts.linux.dev>, Andrew Morton
<akpm@...ux-foundation.org>, <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] mm/slab: Add __free() support for kvfree
Lukas Wunner wrote:
> On Mon, Aug 14, 2023 at 06:17:31PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 14, 2023 at 05:31:27PM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Aug 14, 2023 at 12:43:32AM -0700, Dan Williams wrote:
> > > > Allow for the declaration of variables that trigger kvfree() when they
> > > > go out of scope.
> > > >
> > > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > > > ---
> > > > include/linux/slab.h | 2 ++
> > > > 1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > > > index 848c7c82ad5a..241025367943 100644
> > > > --- a/include/linux/slab.h
> > > > +++ b/include/linux/slab.h
> > > > @@ -746,6 +746,8 @@ static inline __alloc_size(1, 2) void *kvcalloc(size_t n, size_t size, gfp_t fla
> > > > extern void *kvrealloc(const void *p, size_t oldsize, size_t newsize, gfp_t flags)
> > > > __realloc_size(3);
> > > > extern void kvfree(const void *addr);
> > > > +DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> > >
> > > No need to check _T before calling this, right (as was also pointed out
> > > earlier).
> >
> > Well, that does mean you get an unconditional call to kvfree() in the
> > success case. Linus argued against this.
> >
> > This way the compiler sees:
> >
> > buf = NULL;
> > if (buf)
> > kvfree(buf);
> >
> > and goes: 'let me clean that up for you'. And all is well.
>
> Have you actually verified that assumption in the generated Assembler code?
>
> The kernel is compiled with -fno-delete-null-pointer-checks since commit
> a3ca86aea507 ("Add '-fno-delete-null-pointer-checks' to gcc CFLAGS").
>
> So NULL pointer checks are *not* optimized away even if the compiler
> knows that a pointer is NULL.
Interesting, I am not sure how -fno-delete-null-pointer-checks plays
into this, but I can confirm that Peter's expectations are being met in
a routine with:
DEFINE_FREE(pci_dev_put, struct pci_dev *, if (_T) pci_dev_put(_T))
...without that conditional the assembly is:
0xffffffff819ad129 <+41>: call 0xffffffff81800840 <pci_get_domain_bus_and_slot>
0xffffffff819ad12e <+46>: mov %rax,%r12
0xffffffff819ad131 <+49>: test %rax,%rax
0xffffffff819ad134 <+52>: je 0xffffffff819ad154 <cxl_cper_event_call+84>
0xffffffff819ad136 <+54>: mov %rax,%rdi
0xffffffff819ad139 <+57>: call 0xffffffff817f5f10 <pci_dev_lock>
0xffffffff819ad13e <+62>: cmpq $0xffffffff82c681c0,0x80(%r12)
0xffffffff819ad14a <+74>: je 0xffffffff819ad160 <cxl_cper_event_call+96>
0xffffffff819ad14c <+76>: mov %r12,%rdi
0xffffffff819ad14f <+79>: call 0xffffffff817f5fa0 <pci_dev_unlock>
0xffffffff819ad154 <+84>: pop %rbx
0xffffffff819ad155 <+85>: mov %r12,%rdi
0xffffffff819ad158 <+88>: pop %rbp
0xffffffff819ad159 <+89>: pop %r12
0xffffffff819ad15b <+91>: jmp 0xffffffff817fe1e0 <pci_dev_put>
...i.e. the check for NULL at 0xffffffff819ad134 jumps to do an
unnecessary pci_dev_put(). With the conditional in the macro the
sequence is:
0xffffffff819ad129 <+41>: call 0xffffffff81800840 <pci_get_domain_bus_and_slot>
0xffffffff819ad12e <+46>: test %rax,%rax
0xffffffff819ad131 <+49>: je 0xffffffff819ad18c <cxl_cper_event_call+140>
0xffffffff819ad133 <+51>: mov %rax,%r12
0xffffffff819ad136 <+54>: mov %rax,%rdi
0xffffffff819ad139 <+57>: call 0xffffffff817f5f10 <pci_dev_lock>
0xffffffff819ad13e <+62>: cmpq $0xffffffff82c681c0,0x80(%r12)
0xffffffff819ad14a <+74>: je 0xffffffff819ad160 <cxl_cper_event_call+96>
0xffffffff819ad14c <+76>: mov %r12,%rdi
0xffffffff819ad14f <+79>: call 0xffffffff817f5fa0 <pci_dev_unlock>
...
0xffffffff819ad18c <+140>: pop %rbx
0xffffffff819ad18d <+141>: pop %rbp
0xffffffff819ad18e <+142>: pop %r12
0xffffffff819ad190 <+144>: jmp 0xffffffff81efc6a0 <__x86_return_thunk>
...i.e. optimize away the pci_dev_put() and return directly when @pdev
is already known to be NULL. So empirically
-fno-delete-null-pointer-checks still allows for redundant NULL checks
to be optimized.
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