linux-kernel - Re: [PATCH] mm: util: update the kerneldoc for kstrdup

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Message-ID: <c0be072fee83719c74249d603cccf7fa59f9d3cf.camel@perches.com>
Date:   Tue, 30 Jun 2020 07:51:27 -0700
From:   Joe Perches <joe@...ches.com>
To:     David Hildenbrand <david@...hat.com>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH] mm: util: update the kerneldoc for kstrdup_const()

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@...libre.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.