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Subject: Re: [PATCH 1/3] mm,memory_hotplug: export mhp min alignment
To:     David Hildenbrand <david@redhat.com>,
        <linux-nvme@lists.infradead.org>, <dan.j.williams@intel.com>,
        <logang@deltatee.com>, <linux-mm@kvack.org>, <hch@lst.de>
CC:     <sagi@grimberg.me>, <oren@nvidia.com>,
        <linux-kernel@vger.kernel.org>, <akpm@linux-foundation.org>
References: <20210602111055.10480-1-mgurtovoy@nvidia.com>
 <20210602111055.10480-2-mgurtovoy@nvidia.com>
 <283740c3-db3f-3c9a-2954-f1c037a13e86@redhat.com>
From:   Max Gurtovoy <mgurtovoy@nvidia.com>
Message-ID: <46524325-8e2a-9395-e0d0-7d559c753c67@nvidia.com>
Date:   Thu, 3 Jun 2021 13:52:59 +0300
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On 6/2/2021 3:14 PM, David Hildenbrand wrote:
> On 02.06.21 13:10, Max Gurtovoy wrote:
>> Hotplugged memory has alignmet restrictions. E.g, it disallows all
>> operations smaller than a sub-section and only allow operations smaller
>> than a section for SPARSEMEM_VMEMMAP. Export the alignment restrictions
>> for mhp users.
>>
>> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
>> ---
>>   include/linux/memory_hotplug.h |  5 +++++
>>   mm/memory_hotplug.c            | 33 +++++++++++++++++++--------------
>>   2 files changed, 24 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/memory_hotplug.h 
>> b/include/linux/memory_hotplug.h
>> index 28f32fd00fe9..c55a9049b11e 100644
>> --- a/include/linux/memory_hotplug.h
>> +++ b/include/linux/memory_hotplug.h
>> @@ -76,6 +76,7 @@ struct mhp_params {
>>     bool mhp_range_allowed(u64 start, u64 size, bool need_mapping);
>>   struct range mhp_get_pluggable_range(bool need_mapping);
>> +unsigned long mhp_get_min_align(void);
>>     /*
>>    * Zone resizing functions
>> @@ -248,6 +249,10 @@ void mem_hotplug_done(void);
>>       ___page;                \
>>    })
>>   +static inline unsigned long mhp_get_min_align(void)
>> +{
>> +    return 0;
>> +}
>>   static inline unsigned zone_span_seqbegin(struct zone *zone)
>>   {
>>       return 0;
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 9e86e9ee0a10..161bb6704a9b 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -270,24 +270,29 @@ void __init 
>> register_page_bootmem_info_node(struct pglist_data *pgdat)
>>   }
>>   #endif /* CONFIG_HAVE_BOOTMEM_INFO_NODE */
>>   +/*
>> + * Disallow all operations smaller than a sub-section and only
>> + * allow operations smaller than a section for
>> + * SPARSEMEM_VMEMMAP. Note that check_hotplug_memory_range()
>> + * enforces a larger memory_block_size_bytes() granularity for
>> + * memory that will be marked online, so this check should only
>> + * fire for direct arch_{add,remove}_memory() users outside of
>> + * add_memory_resource().
>> + */
>> +unsigned long mhp_get_min_align(void)
>> +{
>> +    if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
>> +        return PAGES_PER_SUBSECTION;
>> +    return PAGES_PER_SECTION;
>> +}
>> +EXPORT_SYMBOL_GPL(mhp_get_min_align);
>
> We have to main interfaces to "hotplug" memory:
>
> a) add_memory() and friends for System RAM, which have memory block 
> alignment requirements.
>
> b) memremap_pages(), which has the alignemnt requirements you mention 
> here.
>
> I feel like what you need would better be exposed in mm/memremap.c, 
> for example, via "memremap_min_alignment" so it matches the 
> "memremap_pages" semantics.
>
> And then, memremap_pages() is only available with CONFIG_ZONE_DEVICE, 
> which depends on SPARSEMEM_VMEMMAP. So you'll always have 
> PAGES_PER_SUBSECTION.
>
> I can already spot "memremap_compat_align", maybe you can reuse that 
> or handle it accordingly in there?

Yes I think that since subsection is aligned to PAGE_SIZE I can do:

size_t pci_p2pdma_align_size(size_t size)
{
         unsigned long min_align;

         min_align = memremap_compat_align();
         if (!IS_ALIGNED(size, min_align))
                 return ALIGN_DOWN(size, min_align);

         return size;
}


thoughts ?

>