[PATCH v4 0/9] Huge page-table entries for TTM

[Thomas Hellström (VMware)]

Andrew, Michal

I'm wondering what's the best way here to get the patches touching mm 
reviewed and accepted?
While drm people and VMware internal people have looked at them, I think 
the huge_fault() fallback splitting and the introduction of 
vma_is_special_huge() needs looking at more thoroughly.

Apart from that, if possible, I think the best way to merge this series 
is also through a DRM tree.

Thanks,
Thomas


On 2/20/20 1:27 PM, Thomas Hellström (VMware) wrote:
> In order to reduce TLB misses and CPU usage this patchset enables huge-
> and giant page-table entries for TTM and TTM-enabled graphics drivers.
>
> Patch 1 and 2 introduce a vma_is_special_huge() function to make the mm code
> take the same path as DAX when splitting huge- and giant page table entries,
> (which currently means zapping the page-table entry and rely on re-faulting).
>
> Patch 3 makes the mm code split existing huge page-table entries
> on huge_fault fallbacks. Typically on COW or on buffer-objects that want
> write-notify. COW and write-notification is always done on the lowest
> page-table level. See the patch log message for additional considerations.
>
> Patch 4 introduces functions to allow the graphics drivers to manipulate
> the caching- and encryption flags of huge page-table entries without ugly
> hacks.
>
> Patch 5 implements the huge_fault handler in TTM.
> This enables huge page-table entries, provided that the kernel is configured
> to support transhuge pages, either by default or using madvise().
> However, they are unlikely to be inserted unless the kernel buffer object
> pfns and user-space addresses align perfectly. There are various options
> here, but since buffer objects that reside in system pages typically start
> at huge page boundaries if they are backed by huge pages, we try to enforce
> buffer object starting pfns and user-space addresses to be huge page-size
> aligned if their size exceeds a huge page-size. If pud-size transhuge
> ("giant") pages are enabled by the arch, the same holds for those.
>
> Patch 6 implements a specialized huge_fault handler for vmwgfx.
> The vmwgfx driver may perform dirty-tracking and needs some special code
> to handle that correctly.
>
> Patch 7 implements a drm helper to align user-space addresses according
> to the above scheme, if possible.
>
> Patch 8 implements a TTM range manager for vmwgfx that does the same for
> graphics IO memory. This may later be reused by other graphics drivers
> if necessary.
>
> Patch 9 finally hooks up the helpers of patch 7 and 8 to the vmwgfx driver.
> A similar change is needed for graphics drivers that want a reasonable
> likelyhood of actually using huge page-table entries.
>
> If a buffer object size is not huge-page or giant-page aligned,
> its size will NOT be inflated by this patchset. This means that the buffer
> object tail will use smaller size page-table entries and thus no memory
> overhead occurs. Drivers that want to pay the memory overhead price need to
> implement their own scheme to inflate buffer-object sizes.
>
> PMD size huge page-table-entries have been tested with vmwgfx and found to
> work well both with system memory backed and IO memory backed buffer objects.
>
> PUD size giant page-table-entries have seen limited (fault and COW) testing
> using a modified kernel (to support 1GB page allocations) and a fake vmwgfx
> TTM memory type. The vmwgfx driver does otherwise not support 1GB-size IO
> memory resources.
>
> Comments and suggestions welcome.
> Thomas
>
> Changes since RFC:
> * Check for buffer objects present in contigous IO Memory (Christian König)
> * Rebased on the vmwgfx emulated coherent memory functionality. That rebase
>    adds patch 5.
> Changes since v1:
> * Make the new TTM range manager vmwgfx-specific. (Christian König)
> * Minor fixes for configs that don't support or only partially support
>    transhuge pages.
> Changes since v2:
> * Minor coding style and doc fixes in patch 5/9 (Christian König)
> * Patch 5/9 doesn't touch mm. Remove from the patch title.
> Changes since v3:
> * Added reviews and acks
> * Implemented ugly but generic ttm_pgprot_is_wrprotecting() instead of arch
>    specific code.
>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Michal Hocko <mhocko at suse.com>
> Cc: "Matthew Wilcox (Oracle)" <willy at infradead.org>
> Cc: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> Cc: Ralph Campbell <rcampbell at nvidia.com>
> Cc: "Jérôme Glisse" <jglisse at redhat.com>
> Cc: "Christian König" <christian.koenig at amd.com>
> Cc: Dan Williams <dan.j.williams at intel.com>
>
>
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