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

[Nirmoy Das <nirmoy.das@xxxxxxxxx>]

Hi Thomas, Christian,

On 8/20/2024 5:47 PM, Christian König wrote:
> Am 20.08.24 um 17:45 schrieb Thomas Hellström:
> > On Tue, 2024-08-20 at 17:30 +0200, Christian König wrote:
> > > Am 20.08.24 um 15:33 schrieb Thomas Hellström:
> > > > 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?
> > > That's completely unproblematic. The flag indicates that the released
> > > pages are already cleared, if that isn't the case then the flag
> > > shouldn't be set on the TT object.
> > Yeah, this patch is OK, but the way the follow-up xe patch uses it is
> > problematic since, AFAICT, xe dma clears on alloc, meaning the pool
> > pages are not cleared after use.
>
> Yeah that is clearly invalid behavior.


I was only thinking about one device use-case which won't leak any data 
though I am now miss-using the flag.

If I skip dma clear for pooled BO then this flag  is not really needed. 
Shall I revert the this and usage of TTM_TT_FLAG_CLEARED_ON_FREE

and re-introduce it after I get a working clear on free implementation 
for XE?


Regards,

Nirmoy


> Regards,
> Christian.
>
> > /Thomas
> >
> > > If one device clear it's pages and another device doesn't clear it's
> > > pages then we would just clear the pages of the device which doesn't
> > > do
> > > it with a hardware DMA.
> > >
> > > Regards,
> > > Christian.
> > >
> > > > /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;