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Tue, 30 Jun 2020 10:36:39 -0400 X-MC-Unique: L4jfmIcgO7axwsI04ka3hg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B98BE18FE867; Tue, 30 Jun 2020 14:36:37 +0000 (UTC) Received: from [10.36.114.56] (ovpn-114-56.ams2.redhat.com [10.36.114.56]) by smtp.corp.redhat.com (Postfix) with ESMTP id C5CF25C1B0; Tue, 30 Jun 2020 14:36:35 +0000 (UTC) Subject: Re: [PATCH] mm: util: update the kerneldoc for kstrdup_const() To: Joe Perches , Bartosz Golaszewski , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Bartosz Golaszewski References: <20200628152500.17916-1-brgl@bgdev.pl> <693db5a878ac09994e2a27c64cb14c0e552f3f50.camel@perches.com> <7f1439be-75c4-3a07-ab7b-f4505bf30c48@redhat.com> <644c67ae316bde28669f660aa5aade274d19a2d0.camel@perches.com> <98163576-c98e-77f7-17a7-efd04dc2e86f@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Rspamd-Queue-Id: CE25A100ED9C4 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 >>>>>> >>>>>> Memory allocated with kstrdup_const() must not be passed to regula= r >>>>>> krealloc() as it is not aware of the possibility of the chunk resi= ding >>>>>> 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(co= nst 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_cons= t) >>>>> + * %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? >=20 > 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. Once you have cases that can happen equally likely it most certainly degrades performance. The question is if we care. Coming back to my question, so the major benefit you see is coding simplicity, correct? --=20 Thanks, David / dhildenb