Re: [PATCH 6/8] drm: Add a drm_get_unmapped_area() helper

["Christian König" <christian.koenig@amd.com>]

Am 04.12.19 um 12:36 schrieb Thomas Hellström (VMware):
> On 12/4/19 12:11 PM, Christian König wrote:
>> Am 03.12.19 um 14:22 schrieb Thomas Hellström (VMware):
>>> From: Thomas Hellstrom <thellstrom@vmware.com>
>>>
>>> This helper is used to align user-space buffer object addresses to
>>> huge page boundaries, minimizing the chance of alignment mismatch
>>> between user-space addresses and physical addresses.
>>
>> Mhm, I'm wondering if that is really such a good idea.
>
> Could you elaborate? What drawbacks do you see?

Main problem for me seems to be that I don't fully understand what the 
get_unmapped_area callback is doing.

For example why do we need to use drm_vma_offset_lookup_locked() to 
adjust the pgoff?

The mapped offset should be completely irrelevant for finding some piece 
of userspace address space or am I totally off here?

> Note that this is the way other subsystems are doing it. Take a look 
> at shmem_get_unmapped_area() for instance.
>
>>
>> Wouldn't it be sufficient if userspace uses MAP_HUGETLB?
>
> MAP_HUGETLB is something different and appears to be tied to the 
> kernel persistent huge page mechanism, whereas the TTM huge pages is 
> tided to the THP functionality (although skipped by khugepaged).

Ok, that makes sense. Over all we want to be transparent here.

Regards,
Christian.

>
> Thanks,
>
> Thomas
>
>
>
>>
>> Regards,
>> Christian.
>>
>>>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>>> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>>> Cc: "Jérôme Glisse" <jglisse@redhat.com>
>>> Cc: "Christian König" <christian.koenig@amd.com>
>>> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
>>> ---
>>>   drivers/gpu/drm/drm_file.c | 130 
>>> +++++++++++++++++++++++++++++++++++++
>>>   include/drm/drm_file.h     |   5 ++
>>>   2 files changed, 135 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
>>> index ea34bc991858..e5b4024cd397 100644
>>> --- a/drivers/gpu/drm/drm_file.c
>>> +++ b/drivers/gpu/drm/drm_file.c
>>> @@ -31,6 +31,8 @@
>>>    * OTHER DEALINGS IN THE SOFTWARE.
>>>    */
>>>   +#include <uapi/asm/mman.h>
>>> +
>>>   #include <linux/dma-fence.h>
>>>   #include <linux/module.h>
>>>   #include <linux/pci.h>
>>> @@ -41,6 +43,7 @@
>>>   #include <drm/drm_drv.h>
>>>   #include <drm/drm_file.h>
>>>   #include <drm/drm_print.h>
>>> +#include <drm/drm_vma_manager.h>
>>>     #include "drm_crtc_internal.h"
>>>   #include "drm_internal.h"
>>> @@ -754,3 +757,130 @@ void drm_send_event(struct drm_device *dev, 
>>> struct drm_pending_event *e)
>>>       spin_unlock_irqrestore(&dev->event_lock, irqflags);
>>>   }
>>>   EXPORT_SYMBOL(drm_send_event);
>>> +
>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>> +/*
>>> + * drm_addr_inflate() attempts to construct an aligned area by 
>>> inflating
>>> + * the area size and skipping the unaligned start of the area.
>>> + * adapted from shmem_get_unmapped_area()
>>> + */
>>> +static unsigned long drm_addr_inflate(unsigned long addr,
>>> +                      unsigned long len,
>>> +                      unsigned long pgoff,
>>> +                      unsigned long flags,
>>> +                      unsigned long huge_size)
>>> +{
>>> +    unsigned long offset, inflated_len;
>>> +    unsigned long inflated_addr;
>>> +    unsigned long inflated_offset;
>>> +
>>> +    offset = (pgoff << PAGE_SHIFT) & (huge_size - 1);
>>> +    if (offset && offset + len < 2 * huge_size)
>>> +        return addr;
>>> +    if ((addr & (huge_size - 1)) == offset)
>>> +        return addr;
>>> +
>>> +    inflated_len = len + huge_size - PAGE_SIZE;
>>> +    if (inflated_len > TASK_SIZE)
>>> +        return addr;
>>> +    if (inflated_len < len)
>>> +        return addr;
>>> +
>>> +    inflated_addr = current->mm->get_unmapped_area(NULL, 0, 
>>> inflated_len,
>>> +                               0, flags);
>>> +    if (IS_ERR_VALUE(inflated_addr))
>>> +        return addr;
>>> +    if (inflated_addr & ~PAGE_MASK)
>>> +        return addr;
>>> +
>>> +    inflated_offset = inflated_addr & (huge_size - 1);
>>> +    inflated_addr += offset - inflated_offset;
>>> +    if (inflated_offset > offset)
>>> +        inflated_addr += huge_size;
>>> +
>>> +    if (inflated_addr > TASK_SIZE - len)
>>> +        return addr;
>>> +
>>> +    return inflated_addr;
>>> +}
>>> +
>>> +/**
>>> + * drm_get_unmapped_area() - Get an unused user-space virtual 
>>> memory area
>>> + * suitable for huge page table entries.
>>> + * @file: The struct file representing the address space being 
>>> mmap()'d.
>>> + * @uaddr: Start address suggested by user-space.
>>> + * @len: Length of the area.
>>> + * @pgoff: The page offset into the address space.
>>> + * @flags: mmap flags
>>> + * @mgr: The address space manager used by the drm driver. This 
>>> argument can
>>> + * probably be removed at some point when all drivers use the same
>>> + * address space manager.
>>> + *
>>> + * This function attempts to find an unused user-space virtual 
>>> memory area
>>> + * that can accommodate the size we want to map, and that is properly
>>> + * aligned to facilitate huge page table entries matching actual
>>> + * huge pages or huge page aligned memory in buffer objects. Buffer 
>>> objects
>>> + * are assumed to start at huge page boundary pfns (io memory) or be
>>> + * populated by huge pages aligned to the start of the buffer object
>>> + * (system- or coherent memory). Adapted from shmem_get_unmapped_area.
>>> + *
>>> + * Return: aligned user-space address.
>>> + */
>>> +unsigned long drm_get_unmapped_area(struct file *file,
>>> +                    unsigned long uaddr, unsigned long len,
>>> +                    unsigned long pgoff, unsigned long flags,
>>> +                    struct drm_vma_offset_manager *mgr)
>>> +{
>>> +    unsigned long addr;
>>> +    unsigned long inflated_addr;
>>> +    struct drm_vma_offset_node *node;
>>> +
>>> +    if (len > TASK_SIZE)
>>> +        return -ENOMEM;
>>> +
>>> +    /* Adjust mapping offset to be zero at bo start */
>>> +    drm_vma_offset_lock_lookup(mgr);
>>> +    node = drm_vma_offset_lookup_locked(mgr, pgoff, 1);
>>> +    if (node)
>>> +        pgoff -= node->vm_node.start;
>>> +    drm_vma_offset_unlock_lookup(mgr);
>>> +
>>> +    addr = current->mm->get_unmapped_area(file, uaddr, len, pgoff, 
>>> flags);
>>> +    if (IS_ERR_VALUE(addr))
>>> +        return addr;
>>> +    if (addr & ~PAGE_MASK)
>>> +        return addr;
>>> +    if (addr > TASK_SIZE - len)
>>> +        return addr;
>>> +
>>> +    if (len < HPAGE_PMD_SIZE)
>>> +        return addr;
>>> +    if (flags & MAP_FIXED)
>>> +        return addr;
>>> +    /*
>>> +     * Our priority is to support MAP_SHARED mapped hugely;
>>> +     * and support MAP_PRIVATE mapped hugely too, until it is COWed.
>>> +     * But if caller specified an address hint, respect that as 
>>> before.
>>> +     */
>>> +    if (uaddr)
>>> +        return addr;
>>> +
>>> +    inflated_addr = drm_addr_inflate(addr, len, pgoff, flags,
>>> +                     HPAGE_PMD_SIZE);
>>> +
>>> +    if (IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) &&
>>> +        len >= HPAGE_PUD_SIZE)
>>> +        inflated_addr = drm_addr_inflate(inflated_addr, len, pgoff,
>>> +                         flags, HPAGE_PUD_SIZE);
>>> +    return inflated_addr;
>>> +}
>>> +#else /* CONFIG_TRANSPARENT_HUGEPAGE */
>>> +unsigned long drm_get_unmapped_area(struct file *file,
>>> +                    unsigned long uaddr, unsigned long len,
>>> +                    unsigned long pgoff, unsigned long flags,
>>> +                    struct drm_vma_offset_manager *mgr)
>>> +{
>>> +    return current->mm->get_unmapped_area(file, uaddr, len, pgoff, 
>>> flags);
>>> +}
>>> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>> +EXPORT_SYMBOL_GPL(drm_get_unmapped_area);
>>> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
>>> index 67af60bb527a..4719cc80d547 100644
>>> --- a/include/drm/drm_file.h
>>> +++ b/include/drm/drm_file.h
>>> @@ -386,5 +386,10 @@ void drm_event_cancel_free(struct drm_device *dev,
>>>                  struct drm_pending_event *p);
>>>   void drm_send_event_locked(struct drm_device *dev, struct 
>>> drm_pending_event *e);
>>>   void drm_send_event(struct drm_device *dev, struct 
>>> drm_pending_event *e);
>>> +struct drm_vma_offset_manager;
>>> +unsigned long drm_get_unmapped_area(struct file *file,
>>> +                    unsigned long uaddr, unsigned long len,
>>> +                    unsigned long pgoff, unsigned long flags,
>>> +                    struct drm_vma_offset_manager *mgr);
>>>     #endif /* _DRM_FILE_H_ */
>
>