Re: Fix potential data loss and corruption due to Incorrect BIO Chain Handling

[Ming Lei <ming.lei@redhat.com>]

On Fri, Nov 21, 2025 at 04:17:39PM +0800, zhangshida wrote:
> From: Shida Zhang <zhangshida@kylinos.cn>
> 
> Hello everyone,
> 
> We have recently encountered a severe data loss issue on kernel version 4.19,
> and we suspect the same underlying problem may exist in the latest kernel versions.
> 
> Environment:
> *   **Architecture:** arm64
> *   **Page Size:** 64KB
> *   **Filesystem:** XFS with a 4KB block size
> 
> Scenario:
> The issue occurs while running a MySQL instance where one thread appends data
> to a log file, and a separate thread concurrently reads that file to perform
> CRC checks on its contents.
> 
> Problem Description:
> Occasionally, the reading thread detects data corruption. Specifically, it finds
> that stale data has been exposed in the middle of the file.
> 
> We have captured four instances of this corruption in our production environment.
> In each case, we observed a distinct pattern:
>     The corruption starts at an offset that aligns with the beginning of an XFS extent.
>     The corruption ends at an offset that is aligned to the system's `PAGE_SIZE` (64KB in our case).
> 
> Corruption Instances:
> 1.  Start:`0x73be000`, **End:** `0x73c0000` (Length: 8KB)
> 2.  Start:`0x10791a000`, **End:** `0x107920000` (Length: 24KB)
> 3.  Start:`0x14535a000`, **End:** `0x145b70000` (Length: 8280KB)
> 4.  Start:`0x370d000`, **End:** `0x3710000` (Length: 12KB)
> 
> After analysis, we believe the root cause is in the handling of chained bios, specifically
> related to out-of-order io completion.
> 
> Consider a bio chain where `bi_remaining` is decremented as each bio in the chain completes.
> For example,
> if a chain consists of three bios (bio1 -> bio2 -> bio3) with
> bi_remaining count:
> 1->2->2

Right.

> if the bio completes in the reverse order, there will be a problem. 
> if bio 3 completes first, it will become:
> 1->2->1

Yes.

> then bio 2 completes:
> 1->1->0

No, it is supposed to be 1->1->1.

When bio 1 completes, it will become 0->0->0

bio3's `__bi_remaining` won't drop to zero until bio2's reaches
zero, and bio2 won't be done until bio1 is ended.

Please look at bio_endio():

void bio_endio(struct bio *bio)
{
again:
	if (!bio_remaining_done(bio))
		return;
	...
	if (bio->bi_end_io == bio_chain_endio) {
		bio = __bio_chain_endio(bio);
        goto again;
	}
	...
}


Thanks,
Ming