Re: [PATCH v6 1/2] drm/ttm: Add a flag to allow drivers to skip clear-on-free

[Nirmoy Das <nirmoy.das@xxxxxxxxx>]

On 8/20/2024 3:33 PM, Thomas Hellström wrote:
> Hi, Nirmoy, Christian
>
> On Fri, 2024-08-16 at 15:51 +0200, Nirmoy Das wrote:
> > Add TTM_TT_FLAG_CLEARED_ON_FREE, which DRM drivers can set before
> > releasing backing stores if they want to skip clear-on-free.
> >
> > Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> > Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > Suggested-by: Christian König <christian.koenig@xxxxxxx>
> > Signed-off-by: Nirmoy Das <nirmoy.das@xxxxxxxxx>
> > Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> What happens if two devices share the same global TTM pool
>   type and one that does its own clearing. Wouldn't there be a pretty
> high chance that the the device that doesn't clear its own pages
> allocate non-cleared memory from the pool?


You are right, mixing such devices will poison the global pool. 
Unfortunately, I fully concentrated on single device

use-case. This is problematic mainly because on XE, we are doing clear 
on alloc so from ttm prospective the flag is correct.


A quick option would be to limit this for non-pooled allocations on XE 
or even create a separate pool for XE when have this 
gpu-system-page-clear is enabled.

Thanks for catching this, I will test above options and send patches to 
fix the xe commit.


Regards,

Nirmoy


> /Thomas
>
> > ---
> >   drivers/gpu/drm/ttm/ttm_pool.c | 18 +++++++++++-------
> >   include/drm/ttm/ttm_tt.h       |  6 +++++-
> >   2 files changed, 16 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_pool.c
> > b/drivers/gpu/drm/ttm/ttm_pool.c
> > index 8504dbe19c1a..935ab3cfd046 100644
> > --- a/drivers/gpu/drm/ttm/ttm_pool.c
> > +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> > @@ -222,15 +222,18 @@ static void ttm_pool_unmap(struct ttm_pool
> > *pool, dma_addr_t dma_addr,
> >   }
> >   
> >   /* Give pages into a specific pool_type */
> > -static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page
> > *p)
> > +static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page
> > *p,
> > +			       bool cleared)
> >   {
> >   	unsigned int i, num_pages = 1 << pt->order;
> >   
> > -	for (i = 0; i < num_pages; ++i) {
> > -		if (PageHighMem(p))
> > -			clear_highpage(p + i);
> > -		else
> > -			clear_page(page_address(p + i));
> > +	if (!cleared) {
> > +		for (i = 0; i < num_pages; ++i) {
> > +			if (PageHighMem(p))
> > +				clear_highpage(p + i);
> > +			else
> > +				clear_page(page_address(p + i));
> > +		}
> >   	}
> >   
> >   	spin_lock(&pt->lock);
> > @@ -394,6 +397,7 @@ static void ttm_pool_free_range(struct ttm_pool
> > *pool, struct ttm_tt *tt,
> >   				pgoff_t start_page, pgoff_t
> > end_page)
> >   {
> >   	struct page **pages = &tt->pages[start_page];
> > +	bool cleared = tt->page_flags & TTM_TT_FLAG_CLEARED_ON_FREE;
> >   	unsigned int order;
> >   	pgoff_t i, nr;
> >   
> > @@ -407,7 +411,7 @@ static void ttm_pool_free_range(struct ttm_pool
> > *pool, struct ttm_tt *tt,
> >   
> >   		pt = ttm_pool_select_type(pool, caching, order);
> >   		if (pt)
> > -			ttm_pool_type_give(pt, *pages);
> > +			ttm_pool_type_give(pt, *pages, cleared);
> >   		else
> >   			ttm_pool_free_page(pool, caching, order,
> > *pages);
> >   	}
> > diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
> > index 2b9d856ff388..cfaf49de2419 100644
> > --- a/include/drm/ttm/ttm_tt.h
> > +++ b/include/drm/ttm/ttm_tt.h
> > @@ -85,6 +85,9 @@ struct ttm_tt {
> >   	 * fault handling abuses the DMA api a bit and dma_map_attrs
> > can't be
> >   	 * used to assure pgprot always matches.
> >   	 *
> > +	 * TTM_TT_FLAG_CLEARED_ON_FREE: Set this if a drm driver
> > handles
> > +	 * clearing backing store
> > +	 *
> >   	 * TTM_TT_FLAG_PRIV_POPULATED: TTM internal only. DO NOT
> > USE. This is
> >   	 * set by TTM after ttm_tt_populate() has successfully
> > returned, and is
> >   	 * then unset when TTM calls ttm_tt_unpopulate().
> > @@ -94,8 +97,9 @@ struct ttm_tt {
> >   #define TTM_TT_FLAG_EXTERNAL		BIT(2)
> >   #define TTM_TT_FLAG_EXTERNAL_MAPPABLE	BIT(3)
> >   #define TTM_TT_FLAG_DECRYPTED		BIT(4)
> > +#define TTM_TT_FLAG_CLEARED_ON_FREE	BIT(5)
> >   
> > -#define TTM_TT_FLAG_PRIV_POPULATED	BIT(5)
> > +#define TTM_TT_FLAG_PRIV_POPULATED	BIT(6)
> >   	uint32_t page_flags;
> >   	/** @num_pages: Number of pages in the page array. */
> >   	uint32_t num_pages;