[PATCH v4 10/15] drm/ttm, drm/amdgpu: Allow the driver some control over swapping
Thomas Hellström
thomas.hellstrom at linux.intel.com
Thu May 27 14:21:02 UTC 2021
On Thu, 2021-05-27 at 15:52 +0200, Thomas Hellström wrote:
> On Thu, 2021-05-27 at 14:36 +0200, Christian König wrote:
> > Am 27.05.21 um 09:33 schrieb Thomas Hellström (Intel):
> > > Hi, Christian,
> > >
> > > Thanks for reviewing.
> > >
> > > On 5/26/21 3:26 PM, Christian König wrote:
> > > > Am 26.05.21 um 13:32 schrieb Thomas Hellström:
> > > > > We are calling the eviction_valuable driver callback at
> > > > > eviction
> > > > > time to
> > > > > determine whether we actually can evict a buffer object.
> > > > > The upcoming i915 TTM backend needs the same functionality
> > > > > for
> > > > > swapout,
> > > > > and that might actually be beneficial to other drivers as
> > > > > well.
> > > > >
> > > > > Add an eviction_valuable call also in the swapout path. Try
> > > > > to
> > > > > keep the
> > > > > current behaviour for all drivers by returning true if the
> > > > > buffer
> > > > > object
> > > > > is already in the TTM_PL_SYSTEM placement. We change
> > > > > behaviour
> > > > > for the
> > > > > case where a buffer object is in a TT backed placement when
> > > > > swapped
> > > > > out,
> > > > > in which case the drivers normal eviction_valuable path is
> > > > > run.
> > > > >
> > > > > Finally make sure we don't try to swapout a bo that was
> > > > > recently
> > > > > purged
> > > > > and therefore unpopulated.
> > > > >
> > > > > Reviewed-by: Maarten Lankhorst <
> > > > > maarten.lankhorst at linux.intel.com>
> > > > > Cc: Christian König <christian.koenig at amd.com>
> > > > > Signed-off-by: Thomas Hellström <
> > > > > thomas.hellstrom at linux.intel.com>
> > > > > ---
> > > > > v3:
> > > > > - Don't export ttm_tt_unpopulate
> > > > > - Fix confusion reading the locked pointer instead of the
> > > > > value
> > > > > pointed to in ttm_bo_evict_swapout_allowable (Reported by
> > > > > Maarten Lankhorst)
> > > > > ---
> > > > > drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 4 +++
> > > > > drivers/gpu/drm/ttm/ttm_bo.c | 43
> > > > > ++++++++++++++++---------
> > > > > drivers/gpu/drm/ttm/ttm_tt.c | 3 ++
> > > > > 3 files changed, 34 insertions(+), 16 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > > > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > > > > index 3bc3aebfef7c..45d194bffc3f 100644
> > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> > > > > @@ -1348,6 +1348,10 @@ static bool
> > > > > amdgpu_ttm_bo_eviction_valuable(struct ttm_buffer_object *bo,
> > > > > struct dma_fence *f;
> > > > > int i;
> > > > > + /* Swapout? */
> > > > > + if (bo->mem.mem_type == TTM_PL_SYSTEM)
> > > > > + return true;
> > > > > +
> > > > > if (bo->type == ttm_bo_type_kernel &&
> > > > > !amdgpu_vm_evictable(ttm_to_amdgpu_bo(bo)))
> > > > > return false;
> > > > > diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
> > > > > b/drivers/gpu/drm/ttm/ttm_bo.c
> > > > > index be0406466460..1b2d062266ed 100644
> > > > > --- a/drivers/gpu/drm/ttm/ttm_bo.c
> > > > > +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> > > > > @@ -536,6 +536,10 @@ static int ttm_bo_evict(struct
> > > > > ttm_buffer_object *bo,
> > > > > bool ttm_bo_eviction_valuable(struct ttm_buffer_object *bo,
> > > > > const struct ttm_place *place)
> > > > > {
> > > > > + dma_resv_assert_held(bo->base.resv);
> > > > > + if (bo->mem.mem_type == TTM_PL_SYSTEM)
> > > > > + return true;
> > > > > +
> > > > > /* Don't evict this BO if it's outside of the
> > > > > * requested placement range
> > > > > */
> > > > > @@ -558,7 +562,9 @@ EXPORT_SYMBOL(ttm_bo_eviction_valuable);
> > > > > * b. Otherwise, trylock it.
> > > > > */
> > > > > static bool ttm_bo_evict_swapout_allowable(struct
> > > > > ttm_buffer_object *bo,
> > > > > - struct ttm_operation_ctx *ctx, bool *locked,
> > > > > bool
> > > > > *busy)
> > > > > + struct ttm_operation_ctx *ctx,
> > > > > + const struct ttm_place *place,
> > > > > + bool *locked, bool *busy)
> > > > > {
> > > > > bool ret = false;
> > > > > @@ -576,6 +582,14 @@ static bool
> > > > > ttm_bo_evict_swapout_allowable(struct ttm_buffer_object *bo,
> > > > > *busy = !ret;
> > > > > }
> > > > > + if (ret && place && !bo->bdev->funcs-
> > > > > > eviction_valuable(bo,
> > > > > place)) {
> > > > > + ret = false;
> > > > > + if (*locked) {
> > > > > + dma_resv_unlock(bo->base.resv);
> > > > > + *locked = false;
> > > > > + }
> > > > > + }
> > > > > +
> > > > > return ret;
> > > > > }
> > > > > @@ -630,20 +644,14 @@ int ttm_mem_evict_first(struct
> > > > > ttm_device
> > > > > *bdev,
> > > > > list_for_each_entry(bo, &man->lru[i], lru) {
> > > > > bool busy;
> > > > > - if (!ttm_bo_evict_swapout_allowable(bo, ctx,
> > > > > &locked,
> > > > > - &busy)) {
> > > > > + if (!ttm_bo_evict_swapout_allowable(bo, ctx,
> > > > > place,
> > > > > + &locked, &busy)) {
> > > > > if (busy && !busy_bo && ticket !=
> > > > > dma_resv_locking_ctx(bo->base.resv))
> > > > > busy_bo = bo;
> > > > > continue;
> > > > > }
> > > > > - if (place && !bdev->funcs-
> > > > > >eviction_valuable(bo,
> > > > > - place)) {
> > > > > - if (locked)
> > > > > - dma_resv_unlock(bo->base.resv);
> > > > > - continue;
> > > > > - }
> > > > > if (!ttm_bo_get_unless_zero(bo)) {
> > > > > if (locked)
> > > > > dma_resv_unlock(bo->base.resv);
> > > > > @@ -1140,10 +1148,18 @@ EXPORT_SYMBOL(ttm_bo_wait);
> > > > > int ttm_bo_swapout(struct ttm_buffer_object *bo, struct
> > > > > ttm_operation_ctx *ctx,
> > > > > gfp_t gfp_flags)
> > > > > {
> > > > > + struct ttm_place place = {};
> > > > > bool locked;
> > > > > int ret;
> > > > > - if (!ttm_bo_evict_swapout_allowable(bo, ctx, &locked,
> > > > > NULL))
> > > > > + /*
> > > > > + * While the bo may already reside in SYSTEM placement,
> > > > > set
> > > > > + * SYSTEM as new placement to cover also the move
> > > > > further
> > > > > below.
> > > > > + * The driver may use the fact that we're moving from
> > > > > SYSTEM
> > > > > + * as an indication that we're about to swap out.
> > > > > + */
> > > > > + place.mem_type = TTM_PL_SYSTEM;
> > > > > + if (!ttm_bo_evict_swapout_allowable(bo, ctx, &place,
> > > > > &locked,
> > > > > NULL))
> > > > > return -EBUSY;
> > > > > if (!ttm_bo_get_unless_zero(bo)) {
> > > > > @@ -1168,12 +1184,7 @@ int ttm_bo_swapout(struct
> > > > > ttm_buffer_object
> > > > > *bo, struct ttm_operation_ctx *ctx,
> > > > > if (bo->mem.mem_type != TTM_PL_SYSTEM) {
> > > > > struct ttm_operation_ctx ctx = { false, false };
> > > > > struct ttm_resource evict_mem;
> > > > > - struct ttm_place place, hop;
> > > > > -
> > > > > - memset(&place, 0, sizeof(place));
> > > > > - memset(&hop, 0, sizeof(hop));
> > > > > -
> > > > > - place.mem_type = TTM_PL_SYSTEM;
> > > > > + struct ttm_place hop = {};
> > > >
> > > > I would stick with memset because of the padding reasons.
> > > >
> > > > > ret = ttm_resource_alloc(bo, &place, &evict_mem);
> > > > > if (unlikely(ret))
> > > > > diff --git a/drivers/gpu/drm/ttm/ttm_tt.c
> > > > > b/drivers/gpu/drm/ttm/ttm_tt.c
> > > > > index 913b330a234b..d9793cbb6d13 100644
> > > > > --- a/drivers/gpu/drm/ttm/ttm_tt.c
> > > > > +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> > > > > @@ -263,6 +263,9 @@ int ttm_tt_swapout(struct ttm_device
> > > > > *bdev,
> > > > > struct ttm_tt *ttm,
> > > > > struct page *to_page;
> > > > > int i, ret;
> > > > > + if (!ttm_tt_is_populated(ttm))
> > > > > + return 0;
> > > > > +
> > > >
> > > > This here is just because of a bug in the higher level
> > > > function.
> > > >
> > > > I've just pushed the fix for that to drm-misc-fixes, so maybe
> > > > drop
> > > > that here as soon as this is backmerged.
> > > >
> > > That code doesn't look correct to me. In ttm_device_swapout only
> > > the
> > > lru lock is held, and the bo->ttm pointer is protected by the
> > > resv
> > > lock, meaning that bo->ttm can disappear at any time in that
> > > function,
> > > so while an advisory reading bo->ttm using READ_ONCE() is ok,
> > > dereferencing the bo->ttm pointer without reservation held is
> > > illegal
> > > and may send you into recently freed memory.
> > >
> > > For an example, consider
> > >
> > > thread A. Selects bo for eviction, moves to system lru, creates
> > > ttm
> > > Thread B locks lru in swapout code. finds bo->ttm NON_NULL,
> > > thread A tries to evict bo, fails, destroys the ttm.
> > > Thread B derefs freed memory.
> > >
> > > But even relying on that there were no such example in the ttm
> > > core
> > > itself, not adhering to the protection of bo->ttm makes the code
> > > extremely fragile and IMHO needs fixing.
> > >
> > > Also as a secondary note, a driver is in principle free to do
> > > things
> > > in the swap notifier that may result in an unpopulated ttm so
> > > IMHO a
> > > late check is needed here.
> > >
> > > So ack to keep the above?
> >
> > Oh, really good point. Haven't thought about that for the quick
> > fix.
> >
> > spinlock.
>
> To avoid more locking complexity,
> I think it would be easy to just defer the code that derefs the ttm
> (except the new unpopulated check) to ttm_bo_swapout() after we've
> taken the resv trylock, but before taking the kref, returning -EBUSY
> if
> conditions for swapping are not met.
>
> >
> > The problem doing it here is that you end up in an endless loop
> > currently.
> >
> > E.g. you trylock and inspect the same BO over and over again.
> >
> > Need to double check the code to see if that can somehow be
> > avoided.
>
> Well ttm_tt_swapout happily returns 0, so the BO gets pulled off the
> LRU anyway so I think that shouldn't happen.
>
> The only thing that becomes incorrect is the num_pages return in
> ttm_device_swapout(). OTOH, the caller shouldn't care whether we
> actually swapped out or whether the bo losts its pages in
> swap_notify.
>
> /Thomas
>
Sent a quick patch that might do the trick. Completely untested.
/Thomas
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