Re: [BUG] BUG: scheduling while atomic in throttle_direct_reclaim

[Harry Yoo <harry.yoo@oracle.com>]

On Mon, May 26, 2025 at 11:49:30PM +0800, Xianying Wang wrote:
> Hi,
> 
> I discovered a kernel crash described as "BUG: scheduling while atomic
> in throttle_direct_reclaim." This issue occurs in the memory reclaim
> path, specifically in the throttle_direct_reclaim function
> (mm/vmscan.c), where the kernel attempts to perform a potentially
> blocking operation (schedule_timeout) while still in an atomic or
> non-preemptible context, leading to an invalid scheduling state and
> triggering __schedule_bug().
> 
> The crash trace shows that this condition can occur when the kernel
> mounts a specially crafted ISO9660 image via syz_mount_image$iso9660.
> During image parsing, the VFS initiates page readahead through
> read_pages, which issues block I/O backed by a loop device. This leads
> to a SCSI read path where scsi_alloc_sgtables
> (drivers/scsi/scsi_lib.c) attempts to allocate memory for a
> scatterlist using mempool_alloc. If memory pressure is present,
> mempool_alloc triggers try_to_free_pages, and subsequently
> throttle_direct_reclaim.
> 
> At this point, the kernel is likely in an atomic context due to
> earlier direct reclaim or preemption disabling within the block layer
> or SCSI stack. As a result, schedule_timeout is not allowed and
> triggers a BUG.
> 
> I recommend reviewing the reclaim context propagation in:
> 
> scsi_alloc_sgtables and sg_alloc_table_chained
> mempool_alloc in SCSI I/O paths
> throttle_direct_reclaim to ensure blocking calls are not made from
> atomic contexts
>
> This can be reproduced on:
> 
> HEAD commit:
> 
> commit e8f897f4afef0031fe618a8e94127a0934896aba

Well, that's Linux v6.8, which is already end of life.
Please DO NOT REPORT bugs from kernels that are past their EOL.

I spent an hour only to realize this had already been fixed.

https://lore.kernel.org/all/20240614143238.60323-1-andrey.konovalov@linux.dev/T/#u

This is KASAN passing incorrect gfp flag to stackdepot, triggering
memory reclamation while mempool is holding a spinlock.

> report: https://pastebin.com/raw/bxuLHCgu
> 
> console output : https://pastebin.com/raw/mCZ4Ap8Q
> 
> kernel config : https://pastebin.com/raw/aJ9rUnhG
> 
> C reproducer : https://pastebin.com/raw/1dku01DG
> 
> Best regards,
> 
> Xianying

-- 
Cheers,
Harry / Hyeonggon