Re: [RFC PATCH 00/15] Make MAX_ORDER adjustable as a kernel boot time parameter.

[David Hildenbrand]

On 05.08.21 21:02, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
>
> Hi all,
>
> This patchset add support for kernel boot time adjustable MAX_ORDER, so that
> user can change the largest size of pages obtained from buddy allocator. It also
> removes the restriction on MAX_ORDER based on SECTION_SIZE_BITS, so that
> buddy allocator can merge PFNs across memory sections when SPARSEMEM_VMEMMAP is
> set. It is on top of v5.14-rc4-mmotm-2021-08-02-18-51.
>
> Motivation
> ===
>
> This enables kernel to allocate 1GB pages and is necessary for my ongoing work
> on adding support for 1GB PUD THP[1]. This is also the conclusion I came up with
> after some discussion with David Hildenbrand on what methods should be used for
> allocating gigantic pages[2], since other approaches like using CMA allocator or
> alloc_contig_pages() are regarded as suboptimal.
>
> This also prevents increasing SECTION_SIZE_BITS when increasing MAX_ORDER, since
> increasing SECTION_SIZE_BITS is not desirable as memory hotadd/hotremove chunk
> size will be increased as well, causing memory management difficulty for VMs.
>
> In addition, make MAX_ORDER a kernel boot time parameter can enable user to
> adjust buddy allocator without recompiling the kernel for their own needs, so
> that one can still have a small MAX_ORDER if he/she does not need to allocate
> gigantic pages like 1GB PUD THPs.
>
> Background
> ===
>
> At the moment, kernel imposes MAX_ORDER - 1 + PAGE_SHFIT < SECTION_SIZE_BITS
> restriction. This prevents buddy allocator merging pages across memory sections,
> as PFNs might not be contiguous and code like page++ would fail. But this would
> not be an issue when SPARSEMEM_VMEMMAP is set, since all struct page are
> virtually contiguous. In addition, as long as buddy allocator checks the PFN
> validity during buddy page merging (done in Patch 3), pages allocated from
> buddy allocator can be manipulated by code like page++.
>
>
> Description
> ===
>
> I tested the patchset on both x86_64 and ARM64 at 4KB, 16KB, and 64KB base
> pages. The systems boot and ltp mm test suite finished without issue. Also
> memory hotplug worked on x86_64 when I tested. It definitely needs more tests
> and reviews for other architectures.
>
> In terms of the concerns on performance degradation if MAX_ORDER is increased,
> I did some initial performance tests comparing MAX_ORDER=11 and MAX_ORDER=20 on
> x86_64 machines and saw no performance difference[3].
>
> Patch 1 excludes MAX_ORDER check from 32bit vdso compilation. The check uses
> irrelevant 32bit SECTION_SIZE_BITS during 64bit kernel compilation. The
> exclusion does not break the check in 32bit kernel, since the check will still
> be performed during other kernel component compilation.
>
> Patch 2 gives FORCE_MAX_ZONEORDER a better name.
>
> Patch 3 restores the pfn_valid_within() check when buddy allocator can merge
> pages across memory sections. The check was removed when ARM64 gets rid of holes
> in zones, but holes can appear in zones again after this patchset.
>
> Patch 4-11 convert the use of MAX_ORDER to SECTION_SIZE_BITS or its derivative
> constants, since these code places use MAX_ORDER as boundary check for
> physically contiguous pages, where SECTION_SIZE_BITS should be used. After this
> patchset, MAX_ORDER can go beyond SECTION_SIZE_BITS, the code can break.
> I separate changes to different patches for easy review and can merge them into
> a single one if that works better.
>
> Patch 12 adds a new Kconfig option SET_MAX_ORDER to allow specifying MAX_ORDER
> when ARCH_FORCE_MAX_ORDER is not used by the arch, like x86_64.
>
> Patch 13 converts statically allocated arrays with MAX_ORDER length to dynamic
> ones if possible and prepares for making MAX_ORDER a boot time parameter.
>
> Patch 14 adds a new MIN_MAX_ORDER constant to replace soon-to-be-dynamic
> MAX_ORDER for places where converting static array to dynamic one is causing
> hassle and not necessary, i.e., ARM64 hypervisor page allocation and SLAB.
>
> Patch 15 finally changes MAX_ORDER to be a kernel boot time parameter.
>
>
> Any suggestion and/or comment is welcome. Thanks.
>
>
> TODO
> ===
>
> 1. Redo the performance comparison tests using this patchset to understand the
>     performance implication of changing MAX_ORDER.


2. Make alloc_contig_range() cleanly deal with pageblock_order instead 
of MAX_ORDER - 1 to not force the minimal CMA area size/alignment to be 
e.g., 1 GiB instead of 4 MiB and to keep virtio-mem working as expected.

virtio-mem short term would mean disallowing initialization when an 
incompatible setup (MAX_ORDER_NR_PAGE > SECTION_NR_PAGES) is detected 
and bailing out loud that the admin has to fix that on the command line. 
I have optimizing alloc_contig_range() on my todo list, to get rid of 
the MAX_ORDER -1 dependency in virtio-mem; but I have no idea when I'll 
have time to work on that.


--
Thanks,

David / dhildenb