Hi, I have a reproducible, quantified kernel memory leak in vmwgfx on a headless VMware guest. It is attributed to a specific call site with slab_debug, and I have a clean causal test that stops it. I could not find this reported anywhere - I searched the dri-devel archives, the mainline and 6.12.y git logs, and the distro trackers.
Summary ======= On a headless VMware guest with the text console bound to the vmwgfx framebuffer, the fbdev-emulation damage worker issues a full atomic commit for every console damage event, and vmwgfx leaks a command buffer on each one. The result is unreclaimable kernel slab growing linearly at ~103 MiB/day, never reclaimed under memory pressure, released only by reboot. Environment =========== Kernel: 6.12.100+deb13-amd64 (Debian 13 stock, tracks 6.12.y) Guest: VMware Virtual Platform, BIOS 6.00; vmwgfx drives the virtual display Console: headless, administered over SSH; a single agetty on the console fb0: vmwgfxdrmfb vtcon0: "(M) frame buffer device", bind=1 Config: CONFIG_HZ=250, CONFIG_SLUB_DEBUG=y, CONFIG_STACKDEPOT=y (CONFIG_DEBUG_KMEMLEAK is not set in Debian's kernel) i915 is also loaded on this host (a passed-through GPU used for video transcoding) but is unrelated - the console framebuffer is vmwgfx. Symptom ======= SUnreclaim grows linearly at ~103 MiB/day and is never reclaimed under pressure. The growth is confined to the plain kmalloc-256 cache - notably *not* kmalloc-cg-256, so it is plain GFP_KERNEL from an in-kernel allocator rather than anything cgroup-accounted. Attribution =========== Booted with slab_debug=U,kmalloc-256, then read /sys/kernel/debug/slab/kmalloc-256/alloc_traces, which ranks by live object count: 41025 vmw_cmdbuf_alloc+0x40/0x200 [vmwgfx] waste=328200/8 age=254700/1301825/2351179 pid=9-208926 cpus=0-5 vmw_cmdbuf_alloc+0x40/0x200 [vmwgfx] vmw_cmdbuf_reserve+0x142/0x170 [vmwgfx] vmw_cmd_ctx_reserve+0x24/0x60 [vmwgfx] vmw_stdu_bind_st+0x6b/0x100 [vmwgfx] vmw_stdu_primary_plane_atomic_update+0xa8/0x1d0 [vmwgfx] drm_atomic_helper_commit_planes+0x112/0x320 [drm_kms_helper] drm_atomic_helper_commit_tail+0x48/0x90 [drm_kms_helper] vmw_atomic_commit_tail+0x1b/0x80 [vmwgfx] commit_tail+0x91/0x130 [drm_kms_helper] drm_atomic_helper_commit+0x11a/0x140 [drm_kms_helper] drm_atomic_commit+0xa9/0xe0 [drm] drm_atomic_helper_dirtyfb+0x1c6/0x2a0 [drm_kms_helper] drm_fbdev_ttm_helper_fb_dirty+0x216/0x310 [drm_ttm_helper] drm_fb_helper_damage_work+0x94/0x170 [drm_kms_helper] process_one_work+0x174/0x330 worker_thread+0x191/0x2f0 The next-largest entry in that cache held 1056 objects - this one is ~30x everything else combined. Quantitative confirmation ========================= With CONFIG_HZ=250, from the age=min/avg/max field above: - max age 2,351,179 jiffies = 9405 s = the full uptime at time of reading - avg age is almost exactly half of max, which is the signature of uniform allocation with zero frees - 41,025 objects over the 8386 s during which allocation was active = 4.89 objects/sec = 4.30 MiB/h = ~103 MiB/day That matches the independently measured SUnreclaim growth rate to within noise. Causal test =========== Unbinding the text console from the framebuffer stops it instantly: # echo 0 > /sys/class/vtconsole/vtcon0/bind After that, the minimum object age jumped from 26 jiffies to 254,700 and kept climbing - i.e. no new allocations at all. SUnreclaim also dropped ~93 MiB immediately as the fbdev shadow buffer was released, and has been flat-to- falling since. Two things I checked so you don't have to ========================================= This is not a recent regression. drivers/gpu/drm/vmwgfx/vmwgfx_cmdbuf.c has had no functional change between v6.12 and current mainline - only the command buffer ID addition (922f9de) and the treewide kmalloc -> kmalloc_obj conversions. vmwgfx_stdu.c is similarly untouched in the relevant paths, and nothing in the 6.12.y stable series touches cmdbuf, damage or dirtyfb handling. So this looks long-standing rather than newly introduced. It is also not caused by the fbdev-ttm conversion, which is the obvious suspect given the stack. drm_fbdev_ttm was a pure rename of drm_fbdev_generic (aae4682e5d66, v6.10), and vmwgfx has used the generic DRM fbdev emulation since v6.2 (df42523c12f8, which deleted vmwgfx_fb.c). What did change at v6.2 is that console damage now goes through a full atomic commit, which the old vmwgfx_fb.c did not do - but the leak itself is on the vmwgfx side. Possibly related ================ There is an open, unresolved report of a different vmwgfx leak in the same atomic-commit path - Yuma Kakei, 12 Mar 2026, "[BUG] vmwgfx: Severe Slab memory leak (radix_tree_node) with Wayland Atomic KMS" (kernels 6.17 / 7.0-rc3, KWin Wayland with Atomic KMS, stops with KWIN_DRM_NO_AMS=1). Ian Forbes proposed a patch; the reporter confirmed on 15 Mar 2026 that it did not fix it. I do *not* claim these are the same bug - different slab caches, different trigger, different kernels. But both are vmwgfx leaking on every atomic commit and stopping dead when the atomic path stops, so they may share a root cause. Workarounds =========== - echo 0 > /sys/class/vtconsole/vtcon0/bind runtime, reversible, stops it immediately - drm_kms_helper.fbdev_emulation=0 on the kernel command line cleaner for a headless host; disables fbdev emulation outright so no damage worker exists. Must be set at boot. - modprobe.blacklist=vmwgfx heavy-handed; loses all console output All three cost the virtual console, which is fine headless but is not a general fix. Happy to test patches, run instrumented kernels, or provide further data - the system reproduces this continuously and I can rebind the console at will to toggle it on and off. Thanks, Dmitry
