linux-kernel - Re: [PATCH] devres: zero the memory in devm

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Message-ID: <CAMpxmJWxsNY_Eepq2fx0diDr96prBZKGtyb2j43PLe4_vGZAwA@mail.gmail.com>
Date:   Fri, 30 Oct 2020 12:03:35 +0100
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devres: zero the memory in devm_krealloc() if needed

On Fri, Oct 30, 2020 at 11:56 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>

[snip]

> > >
> > > Any use case? Because to me it sounds contradictory to the whole idea of [k]realloc().
> >
> > This is kind of a gray area in original krealloc() too and I want to
> > submit a patch for mm too. Right now krealloc ignores the __GFP_ZERO
> > flag if new_size <= old_size but zeroes the memory if new_size >
> > old_size.
>
> > This should be consistent - either ignore __GFP_ZERO or
> > don't ignore it in both cases. I think that not ignoring it is better
> > - if user passes it then it's for a reason.
>
> Sorry, but I consider in these two choices the best is the former one, i.e.
> ignoring, because non-ignoring for sizes less than current is counter the
> REalloc() by definition.
>
> Reading realloc(3):
>
> "If the new size is larger than the old size, the added memory will not be
> initialized."
>
> So, supports my choice over yours.

Kernel memory management API is not really orthogonal to the one in
user-space. For example: kmalloc() takes the gfp parameter and if you
pass __GFP_ZERO to it, it zeroes the memory even if user-space
malloc() never does.

On Fri, Oct 30, 2020 at 11:57 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>

[snip]

>
> Two notes:
>  - perhaps kzrealloc() for what you want

It could be used as a helper wrapper around krealloc() but kzalloc()
is already a simple kmalloc() with additional __GFP_ZERO flag passed.
This is why I think krealloc() should honor __GFP_ZERO.

>  - there is a library call reallocarray() which supports your idea about
>    krealloc_array() API in kernel.
>

reallocarray() is a bsd extension. I'd stick to krealloc_array()
naming in the kernel.

Bartosz