linux-kernel - Re: [PATCH V3 5/6] f2fs: ignore discard return value

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Message-ID: <9079ffb8-3b66-415f-bb2a-4d3f79dc9cb4@kernel.org>
Date: Wed, 26 Nov 2025 11:58:29 +0800
From: Chao Yu <chao@...nel.org>
To: Chaitanya Kulkarni <chaitanyak@...dia.com>
Cc: chao@...nel.org, "linux-block@...r.kernel.org"
 <linux-block@...r.kernel.org>, "jaegeuk@...nel.org" <jaegeuk@...nel.org>,
 "hch@....de" <hch@....de>, "song@...nel.org" <song@...nel.org>,
 "axboe@...nel.dk" <axboe@...nel.dk>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "dm-devel@...ts.linux.dev" <dm-devel@...ts.linux.dev>,
 "linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
 "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
 "linux-f2fs-devel@...ts.sourceforge.net"
 <linux-f2fs-devel@...ts.sourceforge.net>, "cem@...nel.org" <cem@...nel.org>,
 "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
 "sagi@...mberg.me" <sagi@...mberg.me>, "yukuai@...as.com"
 <yukuai@...as.com>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
 "mpatocka@...hat.com" <mpatocka@...hat.com>,
 "Martin K . Petersen" <martin.petersen@...cle.com>,
 "agk@...hat.com" <agk@...hat.com>,
 Johannes Thumshirn <johannes.thumshirn@....com>,
 Chaitanya Kulkarni <ckulkarnilinux@...il.com>,
 "snitzer@...nel.org" <snitzer@...nel.org>
Subject: Re: [PATCH V3 5/6] f2fs: ignore discard return value

On 11/26/25 11:37, Chaitanya Kulkarni wrote:
> On 11/25/25 18:47, Chao Yu wrote:
>> On 11/25/25 07:48, Chaitanya Kulkarni wrote:
>>> __blkdev_issue_discard() always returns 0, making the error assignment
>>> in __submit_discard_cmd() dead code.
>>>
>>> Initialize err to 0 and remove the error assignment from the
>>> __blkdev_issue_discard() call to err. Move fault injection code into
>>> already present if branch where err is set to -EIO.
>>>
>>> This preserves the fault injection behavior while removing dead error
>>> handling.
>>>
>>> Reviewed-by: Martin K. Petersen <martin.petersen@...cle.com>
>>> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@....com>
>>> Reviewed-by: Christoph Hellwig <hch@....de>
>>> Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@...il.com>
>>> ---
>>>   fs/f2fs/segment.c | 10 +++-------
>>>   1 file changed, 3 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index b45eace879d7..22b736ec9c51 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -1343,15 +1343,9 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>>>   
>>>   		dc->di.len += len;
>>>   
>>> +		err = 0;
>>>   		if (time_to_inject(sbi, FAULT_DISCARD)) {
>>>   			err = -EIO;
>>> -		} else {
>>> -			err = __blkdev_issue_discard(bdev,
>>> -					SECTOR_FROM_BLOCK(start),
>>> -					SECTOR_FROM_BLOCK(len),
>>> -					GFP_NOFS, &bio);
>>> -		}
>>> -		if (err) {
>>>   			spin_lock_irqsave(&dc->lock, flags);
>>>   			if (dc->state == D_PARTIAL)
>>>   				dc->state = D_SUBMIT;
>>> @@ -1360,6 +1354,8 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>>>   			break;
>>>   		}
>>>   
>>> +		__blkdev_issue_discard(bdev, SECTOR_FROM_BLOCK(start),
>>> +				SECTOR_FROM_BLOCK(len), GFP_NOFS, &bio);
>> Oh, wait, bio can be NULL? Then below f2fs_bug_on() will trigger panic or warning.
>>
>> Thanks,
> 
> That will happen without this patch also or not ?
> 
> Since __blkdev_issue_discard() is always returning 0 irrespective of bio
> is null or not.
> 
> The following condition in original code will only execute when err is set to
> -EIO and that will only happen when time_to_inject() -> true.
> Original call to __blkdev_issue_discard() without this patch will always
> return 0 even for bio == NULL after __blkdev_issue_discard().
> 
> This is what we are trying to fix so caller should not rely on
> __blkdev_issue_discard() return value  :-
> 
> 354                 if (err) {
> 1355                         spin_lock_irqsave(&dc->lock, flags);
> 1356                         if (dc->state == D_PARTIAL)
> 1357                                 dc->state = D_SUBMIT;
> 1358                         spin_unlock_irqrestore(&dc->lock, flags);
> 1359
> 1360                         break;
> 1361                 }
> 
> which will lead f2fs_bug_on() for bio == NULL even without this patch.
> 
> This patch is not changing exiting behavior, correct me if I'm wrong.

Yes, I think you're right, thanks for the explanation.

So it's fine to leave this cleanup patch as it is, and let's fix this bug in
a separated patch.

Thanks,

> 
> 
>>
>>>   		f2fs_bug_on(sbi, !bio);
>>>   
>>>   		/*
> 
> -ck
> 
>