linux-kernel - Re: [PATCH RFC 0/6] bio_split() error handling rework

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Message-ID: <b909783a-51d8-49c6-a9a0-a23d51fb6d29@oracle.com>
Date: Mon, 23 Sep 2024 11:21:20 +0100
From: John Garry <john.g.garry@...cle.com>
To: Hannes Reinecke <hare@...e.de>, axboe@...nel.dk, hch@....de
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org, martin.petersen@...cle.com
Subject: Re: [PATCH RFC 0/6] bio_split() error handling rework

On 23/09/2024 10:43, Hannes Reinecke wrote:
> On 9/23/24 09:19, John Garry wrote:
>> On 23/09/2024 06:53, Hannes Reinecke wrote:
>>> On 9/19/24 11:22, John Garry wrote:
>>>> bio_split() error handling could be improved as follows:
>>>> - Instead of returning NULL for an error - which is vague - return a
>>>>    PTR_ERR, which may hint what went wrong.
>>>> - Remove BUG_ON() calls - which are generally not preferred - and 
>>>> instead
>>>>    WARN and pass an error code back to the caller. Many callers of
>>>>    bio_split() don't check the return code. As such, for an error we 
>>>> would
>>>>    be getting a crash still from an invalid pointer dereference.
>>>>
>>>> Most bio_split() callers don't check the return value. However, it 
>>>> could
>>>> be argued the bio_split() calls should not fail. So far I have just
>>>> fixed up the md RAID code to handle these errors, as that is my 
>>>> interest
>>>> now.
>>>>
>>>> Sending as an RFC as unsure if this is the right direction.
>>>>
>>>> The motivator for this series was initial md RAID atomic write 
>>>> support in
>>>> https://lore.kernel.org/linux-block/21f19b4b-4b83-4ca2- 
>>>> a93b-0a433741fd26@...cle.com/
>>>>
>>>> There I wanted to ensure that we don't split an atomic write bio, 
>>>> and it
>>>> made more sense to handle this in bio_split() (instead of the 
>>>> bio_split()
>>>> caller).
>>>>
>>>> John Garry (6):
>>>>    block: Rework bio_split() return value
>>>>    block: Error an attempt to split an atomic write in bio_split()
>>>>    block: Handle bio_split() errors in bio_submit_split()
>>>>    md/raid0: Handle bio_split() errors
>>>>    md/raid1: Handle bio_split() errors
>>>>    md/raid10: Handle bio_split() errors
>>>>
>>>>   block/bio.c                 | 14 ++++++++++----
>>>>   block/blk-crypto-fallback.c |  2 +-
>>>>   block/blk-merge.c           |  5 +++++
>>>>   drivers/md/raid0.c          | 10 ++++++++++
>>>>   drivers/md/raid1.c          |  8 ++++++++
>>>>   drivers/md/raid10.c         | 18 ++++++++++++++++++
>>>>   6 files changed, 52 insertions(+), 5 deletions(-)
>>>>
>>> You are missing '__bio_split_to_limits()' which looks as it would 
>>> need to be modified, too.
>>>
>>
>> In __bio_split_to_limits(), for REQ_OP_DISCARD, REQ_OP_SECURE_ERASE, 
>> and REQ_OP_WRITE_ZEROES, we indirectly call bio_split(). And 
>> bio_split() might error. But functions like bio_split_discard() can 
>> return NULL for cases where a split is not required. So I suppose we 
>> need to check IS_ERR(split) for those request types mentioned. For 
>> NULL being returned, we would still have the __bio_split_to_limits() 
>> is "if (split)" check.
>>

hold on a moment - were you looking at latest code on Jens' branch? 
There __bio_split_to_limits() will not return a ERR_PTR() (from changes 
in this series) - it will still just return NULL or a bio.

In all cases there, __bio_split_to_limits() calls bio_submit_rw(), and 
still bio_submit_rw() will return NULL or a proper bio. This is because 
we translate a ERR_PTR() from bio_split() to NULL.

Christoph changed this bio splitting in 
https://lore.kernel.org/linux-block/20240826173820.1690925-1-hch@lst.de/

I think that if my changes were based on v6.11, you were right.

Thanks,
John