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

[changfengnan]

> From: "Zhang Yi"<yizhang089@xxxxxxxxx>
> Date:  Wed, Mar 25, 2026, 22:27
> Subject:  Re: [RFC 1/1] ext4: fail fast on repeated metadata reads after IO failure
> To: "Diangang Li"<lidiangang@xxxxxxxxxxxxx>, "Andreas Dilger"<adilger@xxxxxxxxx>, "Diangang Li"<diangangli@xxxxxxxxx>
> Cc: <tytso@xxxxxxx>, <linux-ext4@xxxxxxxxxxxxxxx>, <linux-fsdevel@xxxxxxxxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>, <changfengnan@xxxxxxxxxxxxx>
> 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

IMO, we first need to reach a consensus on whether we can expect a
retry to succeed after a read failure. 
Given that current SCSI and NVMe drivers already perform multiple
retries for I/O errors.
IMO, this depends on the specific error. If the block layer returns
BLK_STS_RESOURCE or BLK_STS_AGAIN, we can retry; however, if
it returns BLK_STS_MEDIUM or BLK_STS_IOERR, there is no need to retry.
For scenarios requiring a retry, we should also wait for a certain time
window before retrying.

Thanks.
Fengnan.

> 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@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
> >
>