Re: [PATCH v2 1/2] mm: multi-gen LRU: fix NULL lruvec->pgdat deref with memcg disabled on cmdline

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/17/26 1:09 PM, Siddharth Chintamaneni wrote:
> When CONFIG_MEMCG=y, lruvec_pgdat() returns lruvec->pgdat, which for the
> per-node pgdat->__lruvec is left NULL by lruvec_init() and only filled in
> lazily by mem_cgroup_lruvec(). Normally that happens early, e.g. on the
> first folio_add_lru().
> 
> With cgroup_disable=memory, lru_gen_shrink_node() passes &pgdat->__lruvec
> directly to shrink_one(). If direct reclaim runs before anything has
> called mem_cgroup_lruvec() for the node, which can happen during early
> boot under memory pressure (e.g. a kdump kernel), get_swappiness() ends
> up dereferencing a NULL pgdat:
> 
>    Unable to handle kernel NULL pointer dereference at virtual address 0000000000005040
>    pc : get_swappiness+0x68/0xb0
>    lr : try_to_shrink_lruvec+0x54/0x2c0
>    Call trace:
>     get_swappiness+0x68/0xb0 (P)
>     try_to_shrink_lruvec+0x54/0x2c0
>     shrink_one+0x8c/0x240
>     shrink_node+0xc6c/0xe08
>     do_try_to_free_pages+0xdc/0x570
>     try_to_free_pages+0x124/0x278
>     __alloc_frozen_pages_noprof+0x6e8/0xf78
>     alloc_pages_mpol+0xc4/0x1c0
>     alloc_frozen_pages_noprof+0x54/0xe8
>     alloc_pages_noprof+0x1c/0x40
>     atomic_pool_expand+0x98/0x300
>     __dma_atomic_pool_init+0x58/0xd8
>     dma_atomic_pool_init+0x154/0x170
>     do_one_initcall+0x64/0x350
>     kernel_init_freeable+0x214/0x430
>     kernel_init+0x2c/0x208
>     ret_from_fork+0x10/0x20
> 
> The classic LRU path and MGLRU before per-node lru_gen_folio lists
> obtained the lruvec through mem_cgroup_lruvec(), which sets
> lruvec->pgdat.
> 
> The above was hit on an arm64 kdump kernel (6.18-based) booted with
> cgroup_disable=memory nr_cpus=4. It also reproduces on mainline on an
> arm64 VM with CONFIG_ARM64_64K_PAGES=y, CONFIG_MEMCG=y,
> CONFIG_LRU_GEN_ENABLED=y and:
> 
>    mem=160M nr_cpus=4 coherent_pool=512M cgroup_disable=memory
> 
> These options only serve to force direct reclaim early in boot; with
> 160M the boot eventually ends in OOM regardless. On mainline the first
> LRU reclaim happens while unpacking the initramfs, and faults the same
> way:
> 
>    Unable to handle kernel NULL pointer dereference at virtual address 0000000000005040
>    pc : get_swappiness+0xd4/0x150
>    Call trace:
>     get_swappiness+0xd4/0x150 (P)
>     try_to_shrink_lruvec+0x44/0x2b8
>     shrink_one+0x80/0x1a0
>     shrink_node+0xd98/0xea8
>     do_try_to_free_pages+0xf4/0x4b0
>     try_to_free_pages+0x110/0x360
>     __alloc_frozen_pages_noprof+0x8b0/0x1718
>     ...
>     __filemap_get_folio_mpol+0x2ac/0x548
>     simple_write_begin+0x44/0xf8
>     page_symlink+0x178/0x198
>     ...
>     do_populate_rootfs+0x13c/0x200
> 
> Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
> Cc: stable@vger.kernel.org # 6.3+
> Assisted-by: LLM
> Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
> Signed-off-by: Hardik Garg <hargar@linux.microsoft.com>
> ---

Good catch. LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>