Re: [PATCH v8 06/12] mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB page

[David Hildenbrand <david@redhat.com>]

> Am 11.12.2020 um 10:35 schrieb Oscar Salvador <osalvador@suse.de>:
> 
> On Thu, Dec 10, 2020 at 11:55:20AM +0800, Muchun Song wrote:
>> When we free a HugeTLB page to the buddy allocator, we should allocate the
>> vmemmap pages associated with it. We can do that in the __free_hugepage()
> "vmemmap pages that describe the range" would look better to me, but it is ok.
> 
>> +#define GFP_VMEMMAP_PAGE        \
>> +    (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_HIGH | __GFP_NOWARN)
>> 
>> #ifndef VMEMMAP_HPAGE_SHIFT
>> #define VMEMMAP_HPAGE_SHIFT        HPAGE_SHIFT
>> @@ -197,6 +200,11 @@
>>    (__boundary - 1 < (end) - 1) ? __boundary : (end);         \
>> })
>> 
>> +typedef void (*vmemmap_remap_pte_func_t)(struct page *reuse, pte_t *pte,
>> +                     unsigned long start, unsigned long end,
>> +                     void *priv);
> 
> Any reason to not have defined GFP_VMEMMAP_PAGE and the new typedef into
> hugetlb_vmemmap.h?
> 
> 
>> +static void vmemmap_restore_pte_range(struct page *reuse, pte_t *pte,
>> +                      unsigned long start, unsigned long end,
>> +                      void *priv)
>> +{
>> +    pgprot_t pgprot = PAGE_KERNEL;
>> +    void *from = page_to_virt(reuse);
>> +    unsigned long addr;
>> +    struct list_head *pages = priv;
> [...]
>> +
>> +        /*
>> +         * Make sure that any data that writes to the @to is made
>> +         * visible to the physical page.
>> +         */
>> +        flush_kernel_vmap_range(to, PAGE_SIZE);
> 
> Correct me if I am wrong, but flush_kernel_vmap_range is a NOOP under arches which
> do not have ARCH_HAS_FLUSH_KERNEL_DCACHE_PAGE.
> Since we only enable support for x86_64, and x86_64 is one of those arches,
> could we remove this, and introduced later on in case we enable this feature
> on an arch that needs it?
> 
> I am not sure if you need to flush the range somehow, as you did in
> vmemmap_remap_range.
> 
>> +retry:
>> +        page = alloc_page(GFP_VMEMMAP_PAGE);
>> +        if (unlikely(!page)) {
>> +            msleep(100);
>> +            /*
>> +             * We should retry infinitely, because we cannot
>> +             * handle allocation failures. Once we allocate
>> +             * vmemmap pages successfully, then we can free
>> +             * a HugeTLB page.
>> +             */
>> +            goto retry;
> 
> I think this is the trickiest part.
> With 2MB HugeTLB pages we only need 6 pages, but with 1GB, the number of pages
> we need to allocate increases significantly (4088 pages IIRC).
> And you are using __GFP_HIGH, which will allow us to use more memory (by
> cutting down the watermark), but it might lead to putting the system
> on its knees wrt. memory.
> And yes, I know that once we allocate the 4088 pages, 1GB gets freed, but
> still.

Similar to memory hotplug, no? I don‘t think this is really an issue that cannot be mitigated. Yeah, we might want to tweak allocation flags.

> 
> I would like to hear Michal's thoughts on this one, but I wonder if it makes
> sense to not let 1GB-HugeTLB pages be freed.
> 
> -- 
> Oscar Salvador
> SUSE L3
>