On Wed, Jul 01, 2026 at 01:57:28PM +0200, Thomas Hellström wrote:
> Hi, Matt
>
> On Fri, 2026-06-26 at 00:46 -0700, Matthew Brost wrote:
> > Pages in ttm_pool_backup can be NULL, and set_pages_array_wb() cannot
> > handle NULL entries. Switch to set_pages_wb() after checking for NULL
> > pages.
> >
> > Fixes the following oops:
> >
> > Oops: general protection fault, kernel NULL pointer dereference 0x0:
> > 0000 [#1] SMP NOPTI
> > RIP: 0010:__cpa_process_fault+0xf8/0x770
> > RSP: 0018:ffffc90000a87718 EFLAGS: 00010287
> > RAX: 0000000000000000 RBX: ffffc90000a87868 RCX: 0000000000000000
> > RDX: 0000000000001000 RSI: 0005088000000000 RDI: ffffffff827c5f34
> > RBP: 0005088000000000 R08: ffffc90000a877cb R09: ffffc90000a877d0
> > R10: 0000000000000000 R11: 000000000000001b R12: 000ffffffffff000
> > R13: ffffc90000a87868 R14: ffffc90000a87868 R15: ffff88815b882ae0
> > FS: 0000000000000000(0000) GS:ffff8884ec840000(0000)
> > knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f930b844000 CR3: 000000000262e003 CR4: 0000000008f70ef0
> > PKRU: 55555554
> > Call Trace:
> > <TASK>
> > __change_page_attr_set_clr+0x989/0xe90
> > ? __purge_vmap_area_lazy+0x6c/0x3a0
> > ? _vm_unmap_aliases+0x250/0x2a0
> > set_pages_array_wb+0x7f/0x120
> > ttm_pool_backup+0x4c9/0x5b0 [ttm]
> > ? dma_resv_wait_timeout+0x3b/0xf0
> > ttm_tt_backup+0x32/0x60 [ttm]
> > ttm_bo_shrink+0x66/0x110 [ttm]
> > xe_bo_shrink_purge+0x12b/0x1b0 [xe]
> > xe_bo_shrink+0xbb/0x270 [xe]
> > __xe_shrinker_walk+0xf7/0x160 [xe]
> > xe_shrinker_walk+0x9d/0xc0 [xe]
> > xe_shrinker_scan+0x11f/0x210 [xe]
> > do_shrink_slab+0x13b/0x270
> > shrink_slab+0xf1/0x400
> > shrink_node+0x352/0x8a0
> > balance_pgdat+0x32c/0x700
> > kswapd+0x205/0x2f0
> > ? __pfx_autoremove_wake_function+0x10/0x10
> > ? __pfx_kswapd+0x10/0x10
> > kthread+0xd1/0x110
> > ? __pfx_kthread+0x10/0x10
> > ret_from_fork+0x1b1/0x200
> > ? __pfx_kthread+0x10/0x10
> > ret_from_fork_asm+0x1a/0x30
> > </TASK>
> >
> > Cc: Christian Koenig <[email protected]>
> > Cc: Huang Rui <[email protected]>
> > Cc: Matthew Auld <[email protected]>
> > Cc: Matthew Brost <[email protected]>
> > Cc: Maarten Lankhorst <[email protected]>
> > Cc: Maxime Ripard <[email protected]>
> > Cc: Thomas Zimmermann <[email protected]>
> > Cc: David Airlie <[email protected]>
> > Cc: Simona Vetter <[email protected]>
> > Cc: Thomas Hellström <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > Cc: [email protected]
> > Fixes: b63d715b8090 ("drm/ttm/pool, drm/ttm/tt: Provide a helper to
> > shrink pages")
> > Signed-off-by: Matthew Brost <[email protected]>
> > ---
> > drivers/gpu/drm/ttm/ttm_pool.c | 49 +++++++++++++++++---------------
> > --
> > 1 file changed, 24 insertions(+), 25 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_pool.c
> > b/drivers/gpu/drm/ttm/ttm_pool.c
> > index 682ae4f40424..ea14447411a6 100644
> > --- a/drivers/gpu/drm/ttm/ttm_pool.c
> > +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> > @@ -1064,34 +1064,33 @@ long ttm_pool_backup(struct ttm_pool *pool,
> > struct ttm_tt *tt,
> > ttm_pool_uses_dma_alloc(pool) ||
> > ttm_tt_is_backed_up(tt))
> > return -EBUSY;
> >
> > -#ifdef CONFIG_X86
> > - /* Anything returned to the system needs to be cached. */
> > - if (tt->caching != ttm_cached)
> > - set_pages_array_wb(tt->pages, tt->num_pages);
> > -#endif
> > + for (i = 0; i < tt->num_pages; i += num_pages) {
> > + unsigned int order;
> >
> > - if (tt->dma_address || flags->purge) {
> > - for (i = 0; i < tt->num_pages; i += num_pages) {
> > - unsigned int order;
> > + page = tt->pages[i];
> > + if (unlikely(!page)) {
> > + num_pages = 1;
> > + continue;
> > + }
> >
> > - page = tt->pages[i];
> > - if (unlikely(!page)) {
> > - num_pages = 1;
> > - continue;
> > - }
> > + order = ttm_pool_page_order(pool, page);
> > + num_pages = 1UL << order;
> >
> > - order = ttm_pool_page_order(pool, page);
> > - num_pages = 1UL << order;
> > - if (tt->dma_address)
> > - ttm_pool_unmap(pool, tt-
> > >dma_address[i],
> > - num_pages);
> > - if (flags->purge) {
> > - shrunken += num_pages;
> > - page->private = 0;
> > - __free_pages_gpu_account(page,
> > order, false);
> > - memset(tt->pages + i, 0,
> > - num_pages * sizeof(*tt-
> > >pages));
> > - }
> > +#ifdef CONFIG_X86
> > + /* Anything returned to the system needs to be
> > cached. */
> > + if (tt->caching != ttm_cached)
> > + set_pages_wb(page, 1 << order);
> > +#endif
>
> As discussed otherwise, this causes one IPI per page when TLB flushing,
> whereas set_pages_array_wb() causes one per array. IPIs are quite
> costly, and
> set_pages_array_wb() has been tailor-made to only issue one per array,
> so we would want to call it on subarrays in case there are NULL
> pointers and always call it in case there are no NULL pointers.
>
Thanks, fixing this another rev. Also is possible for a page to be a
backup ptr, so folding that fix as well into the following revs.
Matt
> /Thomas
>
>
> > +
> > + if (tt->dma_address)
> > + ttm_pool_unmap(pool, tt->dma_address[i],
> > + num_pages);
> > + if (flags->purge) {
> > + shrunken += num_pages;
> > + page->private = 0;
> > + __free_pages_gpu_account(page, order,
> > false);
> > + memset(tt->pages + i, 0,
> > + num_pages * sizeof(*tt->pages));
> > }
> > }
> >
