[PATCH v6 00/22] Add generic memory shrinker to VirtIO-GPU and Panfrost DRM drivers

[Rob Clark]

On Tue, Jun 28, 2022 at 5:51 AM Dmitry Osipenko
<dmitry.osipenko at collabora.com> wrote:
>
> On 6/28/22 15:31, Robin Murphy wrote:
> > ----->8-----
> > [   68.295951] ======================================================
> > [   68.295956] WARNING: possible circular locking dependency detected
> > [   68.295963] 5.19.0-rc3+ #400 Not tainted
> > [   68.295972] ------------------------------------------------------
> > [   68.295977] cc1/295 is trying to acquire lock:
> > [   68.295986] ffff000008d7f1a0
> > (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_shmem_free+0x7c/0x198
> > [   68.296036]
> > [   68.296036] but task is already holding lock:
> > [   68.296041] ffff80000c14b820 (fs_reclaim){+.+.}-{0:0}, at:
> > __alloc_pages_slowpath.constprop.0+0x4d8/0x1470
> > [   68.296080]
> > [   68.296080] which lock already depends on the new lock.
> > [   68.296080]
> > [   68.296085]
> > [   68.296085] the existing dependency chain (in reverse order) is:
> > [   68.296090]
> > [   68.296090] -> #1 (fs_reclaim){+.+.}-{0:0}:
> > [   68.296111]        fs_reclaim_acquire+0xb8/0x150
> > [   68.296130]        dma_resv_lockdep+0x298/0x3fc
> > [   68.296148]        do_one_initcall+0xe4/0x5f8
> > [   68.296163]        kernel_init_freeable+0x414/0x49c
> > [   68.296180]        kernel_init+0x2c/0x148
> > [   68.296195]        ret_from_fork+0x10/0x20
> > [   68.296207]
> > [   68.296207] -> #0 (reservation_ww_class_mutex){+.+.}-{3:3}:
> > [   68.296229]        __lock_acquire+0x1724/0x2398
> > [   68.296246]        lock_acquire+0x218/0x5b0
> > [   68.296260]        __ww_mutex_lock.constprop.0+0x158/0x2378
> > [   68.296277]        ww_mutex_lock+0x7c/0x4d8
> > [   68.296291]        drm_gem_shmem_free+0x7c/0x198
> > [   68.296304]        panfrost_gem_free_object+0x118/0x138
> > [   68.296318]        drm_gem_object_free+0x40/0x68
> > [   68.296334]        drm_gem_shmem_shrinker_run_objects_scan+0x42c/0x5b8
> > [   68.296352]        drm_gem_shmem_shrinker_scan_objects+0xa4/0x170
> > [   68.296368]        do_shrink_slab+0x220/0x808
> > [   68.296381]        shrink_slab+0x11c/0x408
> > [   68.296392]        shrink_node+0x6ac/0xb90
> > [   68.296403]        do_try_to_free_pages+0x1dc/0x8d0
> > [   68.296416]        try_to_free_pages+0x1ec/0x5b0
> > [   68.296429]        __alloc_pages_slowpath.constprop.0+0x528/0x1470
> > [   68.296444]        __alloc_pages+0x4e0/0x5b8
> > [   68.296455]        __folio_alloc+0x24/0x60
> > [   68.296467]        vma_alloc_folio+0xb8/0x2f8
> > [   68.296483]        alloc_zeroed_user_highpage_movable+0x58/0x68
> > [   68.296498]        __handle_mm_fault+0x918/0x12a8
> > [   68.296513]        handle_mm_fault+0x130/0x300
> > [   68.296527]        do_page_fault+0x1d0/0x568
> > [   68.296539]        do_translation_fault+0xa0/0xb8
> > [   68.296551]        do_mem_abort+0x68/0xf8
> > [   68.296562]        el0_da+0x74/0x100
> > [   68.296572]        el0t_64_sync_handler+0x68/0xc0
> > [   68.296585]        el0t_64_sync+0x18c/0x190
> > [   68.296596]
> > [   68.296596] other info that might help us debug this:
> > [   68.296596]
> > [   68.296601]  Possible unsafe locking scenario:
> > [   68.296601]
> > [   68.296604]        CPU0                    CPU1
> > [   68.296608]        ----                    ----
> > [   68.296612]   lock(fs_reclaim);
> > [   68.296622] lock(reservation_ww_class_mutex);
> > [   68.296633]                                lock(fs_reclaim);
> > [   68.296644]   lock(reservation_ww_class_mutex);
> > [   68.296654]
> > [   68.296654]  *** DEADLOCK ***
>
> This splat could be ignored for now. I'm aware about it, although
> haven't looked closely at how to fix it since it's a kind of a lockdep
> misreporting.

The lockdep splat could be fixed with something similar to what I've
done in msm, ie. basically just not acquire the lock in the finalizer:

https://patchwork.freedesktop.org/patch/489364/

There is one gotcha to watch for, as danvet pointed out
(scan_objects() could still see the obj in the LRU before the
finalizer removes it), but if scan_objects() does the
kref_get_unless_zero() trick, it is safe.

BR,
-R