linux-kernel - Re: [RFC PATCH 4/7] drm/ttm: Support huge pagefaults

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Message-ID: <83b83746-7d54-e56b-3753-0b29b01074a2@shipmail.org>
Date:   Wed, 27 Nov 2019 13:24:52 +0100
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Christian König <christian.koenig@....com>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-graphics-maintainer@...are.com
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [RFC PATCH 4/7] drm/ttm: Support huge pagefaults

On 11/27/19 10:12 AM, Christian König wrote:
> Am 27.11.19 um 09:31 schrieb Thomas Hellström (VMware):
>> From: Thomas Hellstrom <thellstrom@...are.com>
>>
>> Support huge (PMD-size and PUD-size) page-table entries by providing a
>> huge_fault() callback.
>> We still support private mappings and write-notify by splitting the huge
>> page-table entries on write-access.
>>
>> Note that for huge page-faults to occur, either the kernel needs to be
>> compiled with trans-huge-pages always enabled, or the kernel needs to be
>> compiled with trans-huge-pages enabled using madvise, and the user-space
>> app needs to call madvise() to enable trans-huge pages on a per-mapping
>> basis.
>>
>> Furthermore huge page-faults will not occur unless buffer objects and
>> user-space addresses are aligned on huge page size boundaries.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>> Cc: Ralph Campbell <rcampbell@...dia.com>
>> Cc: "Jérôme Glisse" <jglisse@...hat.com>
>> Cc: "Christian König" <christian.koenig@....com>
>> Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 139 +++++++++++++++++++++++++++++++-
>>   include/drm/ttm/ttm_bo_api.h    |   3 +-
>>   2 files changed, 138 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c 
>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> index 2098f8d4dfc5..8d6089880e39 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>> @@ -150,6 +150,84 @@ vm_fault_t ttm_bo_vm_reserve(struct 
>> ttm_buffer_object *bo,
>>   }
>>   EXPORT_SYMBOL(ttm_bo_vm_reserve);
>>   +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +/**
>> + * ttm_bo_vm_insert_huge - Insert a pfn for PUD or PMD faults
>> + * @vmf: Fault data
>> + * @bo: The buffer object
>> + * @page_offset: Page offset from bo start
>> + * @fault_page_size: The size of the fault in pages.
>> + * @pgprot: The page protections.
>> + * Does additional checking whether it's possible to insert a PUD or 
>> PMD
>> + * pfn and performs the insertion.
>> + *
>> + * Return: VM_FAULT_NOPAGE on successful insertion, 
>> VM_FAULT_FALLBACK if
>> + * a huge fault was not possible, and a VM_FAULT_ERROR code otherwise.
>> + */
>> +static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
>> +                    struct ttm_buffer_object *bo,
>> +                    pgoff_t page_offset,
>> +                    pgoff_t fault_page_size,
>> +                    pgprot_t pgprot)
>> +{
>> +    pgoff_t i;
>> +    vm_fault_t ret;
>> +    unsigned long pfn;
>> +    pfn_t pfnt;
>> +    struct ttm_tt *ttm = bo->ttm;
>> +    bool write = vmf->flags & FAULT_FLAG_WRITE;
>> +
>> +
>> +    /* Fault should not cross bo boundary */
>> +    page_offset &= ~(fault_page_size - 1);
>> +    if (page_offset + fault_page_size > bo->num_pages)
>> +        goto out_fallback;
>> +
>> +    if (bo->mem.bus.is_iomem)
>> +        pfn = ttm_bo_io_mem_pfn(bo, page_offset);
>> +    else
>> +        pfn = page_to_pfn(ttm->pages[page_offset]);
>> +
>> +    /* pfn must be fault_page_size aligned. */
>> +    if ((pfn & (fault_page_size - 1)) != 0)
>> +        goto out_fallback;
>> +
>> +    /* IO memory is OK now, TT memory must be contigous. */
>
> That won't work correctly, IO mem might not be contiguous either.
>
> We either need to call ttm_bo_io_mem_pfn() multiple times and check 
> that the addresses are linear or return the length additional to the pfn.

Yes, you're right. Will fix that up.

Thanks,

Thomas



>
> Regards,
> Christian.