Re: Potential Linux Crash: WARNING in ext4_dirty_folio in Linux kernel v6.13-rc5

[Vlastimil Babka]

On 4/3/25 14:29, Matt Fleming wrote:
> On Wed, Mar 26, 2025 at 10:59 AM Matt Fleming <matt@xxxxxxxxxxxxxxxx> wrote:
>>
>> Hi there,

+ Cc also Michal

>> I'm also seeing this PF_MEMALLOC WARN triggered from kswapd in 6.12.19.

We're talking about __alloc_pages_slowpath() doing WARN_ON_ONCE(current-
>flags & PF_MEMALLOC); for __GFP_NOFAIL allocations.

kswapd() sets:

tsk->flags |= PF_MEMALLOC | PF_KSWAPD;

so any __GFP_NOFAIL allocation done in the kswapd context risks this
warning. It's also objectively bad IMHO because for direct reclaim we can
loop and hope kswapd rescues us, but kswapd would then have to rely on
direct reclaimers to get unstuck. I don't see an easy generic solution?

>> Does overlayfs need some kind of background inode reclaim support?
> 
> Hey everyone, I know there was some off-list discussion last week at
> LSFMM, but I don't think a definite solution has been proposed for the
> below stacktrace.
> 
> What is the shrinker API policy wrt memory allocation and I/O? Should
> overlayfs do something more like XFS and background reclaim to avoid
> GFP_NOFAIL
> allocations when kswapd is shrinking caches?
> 
>>   Call Trace:
>>    <TASK>
>>    __alloc_pages_noprof+0x31c/0x330
>>    alloc_pages_mpol_noprof+0xe3/0x1d0
>>    folio_alloc_noprof+0x5b/0xa0
>>    __filemap_get_folio+0x1f3/0x380
>>    __getblk_slow+0xa3/0x1e0
>>    __ext4_get_inode_loc+0x121/0x4b0
>>    ext4_get_inode_loc+0x40/0xa0
>>    ext4_reserve_inode_write+0x39/0xc0
>>    __ext4_mark_inode_dirty+0x5b/0x220
>>    ext4_evict_inode+0x26d/0x690
>>    evict+0x112/0x2a0
>>    __dentry_kill+0x71/0x180
>>    dput+0xeb/0x1b0
>>    ovl_stack_put+0x2e/0x50 [overlay]
>>    ovl_destroy_inode+0x3a/0x60 [overlay]
>>    destroy_inode+0x3b/0x70
>>    __dentry_kill+0x71/0x180
>>    shrink_dentry_list+0x6b/0xe0
>>    prune_dcache_sb+0x56/0x80
>>    super_cache_scan+0x12c/0x1e0
>>    do_shrink_slab+0x13b/0x350
>>    shrink_slab+0x278/0x3a0
>>    shrink_node+0x328/0x880
>>    balance_pgdat+0x36d/0x740
>>    kswapd+0x1f0/0x380
>>    kthread+0xd2/0x100
>>    ret_from_fork+0x34/0x50
>>    ret_from_fork_asm+0x1a/0x30
>>    </TASK>
>