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

[Andreas Dilger]

On Mar 25, 2026, at 03:33, Diangang Li <diangangli@xxxxxxxxx> wrote:
> 
> From: Diangang Li <lidiangang@xxxxxxxxxxxxx>
> 
> 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