From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5EA4C26D4C3; Wed, 2 Sep 2026 07:50:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788335448; cv=none; b=qJGW2gsde5g/wi5OzDBt1f99f2LAEgq8OBh2rtcVTkF90K2vaUBfpynE5nAfUx9o/zxJHWwgj2Gf7JuuspoLoR6MIN6kPGLGD70OyWHE0xaS5mP61dxVs8LFGCv7pnx2YCokllawn+VScWw/aBbGm67Ba+36vgSaWP/9EtOo++A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788335448; c=relaxed/simple; bh=ZGSvZBUM8lYkI+KW5XRg5aaz1bZaxRqVwiUWm2jql1A=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=h5ZRcRZHxQSn8ozEmo5sRUyNQIaLaO1u1rjD46D43PQbmT/Jw8sBcn1P06o9z76cmoTUkjCGo/GNBROas5FYNvKjZFzBnJ9bK8f72UsnGQP94prCj7bTeBv9BLaBy8KAC7EPh52tmSCLLymAfAH+NLB/iHAgjeUxbQ5LWL4DN3Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Ytq25x9Z; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Ytq25x9Z" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6BA171F000E9; Wed, 2 Sep 2026 07:50:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788335446; bh=HQKQi6BjlLM/tFqlWQgDt+Hy+RAahli1Ac/ry6Tye0s=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Ytq25x9ZypLI7pvIaWHby9YN/2xI3l2sCJNs7/09WI9M8Ijbrbm45iedYNdUZdYwJ YztvJIkDFUF1FiiaArXjCDsuESH6MV+5J6IIL5xBG8TvYOr3lw4MRTy8RexGYD3lzk ln7QNhswSQEXCWNIDEIeM/JMOGUVQ0/q2P4Ejmak4GMDG5NhoII6UUeBEOBfyHcbe2 NLurCHI6pSVyhUALB5kApFa1C13Ypo0/vMZm6cV5lND6j2RuKWh0dXlzkRlUlzfOGB vukMqtASOWsB19/gDNEMKMBN+2kVygD42YfageoaNr2sjyvPzdVxNdL6VsMwzRiK3U 3NijN9UwqG6vA== Message-ID: Date: Wed, 2 Sep 2026 09:50:42 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc To: Mike Rapoport Cc: Jonathan Corbet , Andrew Morton , David Hildenbrand , "Liam R. Howlett" , Lorenzo Stoakes , Michal Hocko , Randy Dunlap , Shuah Khan , Suren Baghdasaryan , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20260831-docs-memalloc-guide-v1-0-547718c274c1@kernel.org> <20260831-docs-memalloc-guide-v1-1-547718c274c1@kernel.org> <23db34a1-ba1b-495d-91ea-18419e0dc8df@kernel.org> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/1/26 18:10, Mike Rapoport wrote: > On Tue, Sep 01, 2026 at 12:17:02PM +0200, Vlastimil Babka (SUSE) wrote: >> On 8/31/26 17:13, Mike Rapoport (Microsoft) wrote: >> > Since v7.0 the most used memory allocation function is kzalloc_obj(). >> > >> > Update hte memory-allocation guide to describe k[mz]alloc_obj() family >> > and make kzalloc_obj() the first answer to "How should I allocate >> > memory?" question. >> > >> > While on it, clarify description of kmalloc() size limitations. >> > >> > Signed-off-by: Mike Rapoport (Microsoft) >> >> Acked-by: Vlastimil Babka (SUSE) > > Thanks! > >> Suggestion below: >> >> > --- >> > Documentation/core-api/memory-allocation.rst | 30 +++++++++++++++++++++------- >> > 1 file changed, 23 insertions(+), 7 deletions(-) >> > >> > -For large allocations you can use vmalloc() and vzalloc(), or directly >> > -request pages from the page allocator. The memory allocated by `vmalloc` >> > -and related functions is not physically contiguous. >> > +`kmalloc` always allocates physically contiguous memory and the maximal size of >> > +a chunk that can be allocated with `kmalloc` is limited by `MAX_ORDER`, the >> > +same limit also applies to the page allocator. >> >> Would it be better to say the maximal size is KMALLOC_MAX_SIZE which is >> derived from page allocator's MAX_PAGE_ORDER? > > My point was that whenever you allocate with kmalloc() the limit is > MAX_PAGE_ORDER, not necessarily for actual slab allocations. The actual slab allocations limit is KMALLOC_MAX_CACHE_SIZE and that's indeed not the point and hence I wouldn't mention it here. KMALLOC_MAX_SIZE is literally a conversion of MAX_PAGE_ORDER to bytes, and since kmalloc() takes bytes and not order as an argument, it seems more natural to me, e.g.: `kmalloc` always allocates physically contiguous memory and the maximal size of a chunk that can be allocated with `kmalloc` is limited by `KMALLOC_MAX_SIZE`, which matches the page allocator's MAX_PAGE_ORDER limit. >> (Also `MAX_ORDER` doesn't exist anymore) > > Will fix. > >> > +Internally, slab allocator differentiates allocations of different orders and >> > +delegates larger allocations to the page allocator, but for the users of >> > +`kmalloc` family it is entirely transparent. >> > + >> > +For large allocations that do not require physically contiguous memory you can >> > +use vmalloc() and vzalloc() family. >> > >> > If you are not sure whether the allocation size is too large for >> > `kmalloc`, it is possible to use kvmalloc() and its derivatives. It will >> > >> >