From: Joe Perches <joe@perches.com>
To: David Hildenbrand <david@redhat.com>,
Bartosz Golaszewski <brgl@bgdev.pl>,
Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Bartosz Golaszewski <bgolaszewski@baylibre.com>
Subject: Re: [PATCH] mm: util: update the kerneldoc for kstrdup_const()
Date: Tue, 30 Jun 2020 07:51:27 -0700 [thread overview]
Message-ID: <c0be072fee83719c74249d603cccf7fa59f9d3cf.camel@perches.com> (raw)
In-Reply-To: <9dfad174-4e8b-c733-b529-5c86a34333d4@redhat.com>
On Tue, 2020-06-30 at 16:36 +0200, David Hildenbrand wrote:
> On 30.06.20 16:14, Joe Perches wrote:
> > On Tue, 2020-06-30 at 10:57 +0200, David Hildenbrand wrote:
> > > On 29.06.20 21:21, Joe Perches wrote:
> > > > On Mon, 2020-06-29 at 12:54 +0200, David Hildenbrand wrote:
> > > > > On 28.06.20 19:37, Joe Perches wrote:
> > > > > > On Sun, 2020-06-28 at 17:25 +0200, Bartosz Golaszewski wrote:
> > > > > > > From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> > > > > > >
> > > > > > > Memory allocated with kstrdup_const() must not be passed to regular
> > > > > > > krealloc() as it is not aware of the possibility of the chunk residing
> > > > > > > in .rodata. Since there are no potential users of krealloc_const()
> > > > > > > at the moment, let's just update the doc to make it explicit.
> > > > > >
> > > > > > Another option would be to return NULL if it's
> > > > > > used from krealloc with a pointer into rodata
> > > > []
> > > > > > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > > > []
> > > > > > @@ -1683,6 +1683,9 @@ static __always_inline void *__do_krealloc(const void *p, size_t new_size,
> > > > > > * @new_size: how many bytes of memory are required.
> > > > > > * @flags: the type of memory to allocate.
> > > > > > *
> > > > > > + * If the object pointed to is in rodata (likely from kstrdup_const)
> > > > > > + * %NULL is returned.
> > > > > > + *
> > > > []
> > > > > Won't we have similar issues if somebody would do a kfree() instead of a
> > > > > kfree_const()? So I think the original patch makes sense.
> > > >
> > > > Which is why I also suggested making kfree work for
> > > > more types of memory freeing earlier this month.
> > > >
> > > > https://lore.kernel.org/lkml/573b3fbd5927c643920e1364230c296b23e7584d.camel@perches.com/
> > []
> > > what's the real benefit that is worth spending extra runtime cycles?
> >
> > I very much doubt there is an actual instance
> > where the runtime cycles matter. Where could
> > there be a fast-path instance of free?
>
> Well, looking at kfree() I can directly spot "unlikely()", which sounds
> like somebody cares about branch prediction in the slab.
Or is telling the compiler of a 95%+ likely case.
> Once you have cases that can happen equally likely it most certainly
> degrades performance. The question is if we care.
Right.
Does 4 additional tests in what appears to be almost
exclusively non-fast paths matter?
> Coming back to my question, so the major benefit you see is coding
> simplicity, correct?
Yes.
prev parent reply other threads:[~2020-06-30 14:51 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-28 15:25 [PATCH] mm: util: update the kerneldoc for kstrdup_const() Bartosz Golaszewski
2020-06-28 17:37 ` Joe Perches
2020-06-28 18:06 ` Bartosz Golaszewski
2020-06-28 18:20 ` Joe Perches
2020-06-29 10:54 ` David Hildenbrand
2020-06-29 19:21 ` Joe Perches
2020-06-30 8:57 ` David Hildenbrand
2020-06-30 14:14 ` Joe Perches
2020-06-30 14:36 ` David Hildenbrand
2020-06-30 14:51 ` Joe Perches [this message]
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