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

[Zhang Yi <yizhang089@gmail.com>]

Hi, Diangang,

On 3/25/2026 7:13 PM, Diangang Li wrote:
> Hi Andreas,
> 
> BH_Read_EIO is cleared on successful read or write.

I think what Andreas means is, since you modified the ext4_read_bh() 
interface, if the bh to be read already has the Read_EIO flag set, then 
subsequent read operations through this interface will directly return 
failure without issuing a read I/O. At the same time, because its state 
is also not uptodate, for an existing block, a write request will not be 
issued either. How can we clear this Read_EIO flag? IIRC, relying solely 
on ext4_read_bh_nowait() doesn't seem sufficient to achieve this.

Thanks,
Yi.

> 
> 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
>