Re: [RFC 1/1] ext4: fail fast on repeated metadata reads after IO failure

["Diangang Li" <lidiangang@bytedance.com>]

Hi Andreas,

BH_Read_EIO is cleared on successful read or write.

In practice bad blocks are typically repaired/remapped on write, so we
expect recovery after a successful rewrite. If the block is never
rewritten, repeatedly issuing the same failing read does not help.

We clear the flag on successful reads so the buffer can recover 
immediately if the error was transient. Since read-ahead reads are not 
blocked, a later successful read-ahead will clear the flag and allow 
subsequent synchronous readers to proceed normally.

Best,
Diangang

On 3/25/26 6:15 PM, Andreas Dilger wrote:
> On Mar 25, 2026, at 03:33, Diangang Li <diangangli@gmail.com> wrote:
>>
>> From: Diangang Li <lidiangang@bytedance.com>
>>
>> ext4 metadata reads serialize on BH_Lock (lock_buffer). If the read fails,
>> the buffer remains !Uptodate. With concurrent callers, each waiter can
>> retry the same failing read after the previous holder drops BH_Lock. This
>> amplifies device retry latency and may trigger hung tasks.
>>
>> In the normal read path the block driver already performs its own retries.
>> Once the retries keep failing, re-submitting the same metadata read from
>> the filesystem just amplifies the latency by serializing waiters on
>> BH_Lock.
>>
>> Remember read failures on buffer_head and fail fast for ext4 metadata reads
>> once a buffer has already failed to read. Clear the flag on successful
>> read/write completion so the buffer can recover. ext4 read-ahead uses
>> ext4_read_bh_nowait(), so it does not set the failure flag and remains
>> best-effort.
> 
> Not that the patch is bad, but if the BH_Read_EIO flag is set on a buffer
> and it prevents other tasks from reading that block again, how would the
> buffer ever become Uptodate to clear the flag?  There isn't enough state
> in a 1-bit flag to have any kind of expiry and later retry.
> 
> Cheers, Andreas