Re: [PATCH] block: Bail out iteration functions upon SIGKILL.

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2019/11/13 10:54, Damien Le Moal wrote:
> On 2019/11/12 23:48, Tetsuo Handa wrote:
> [...]
>>>> +static int blk_should_abort(struct bio *bio)
>>>> +{
>>>> +	int ret;
>>>> +
>>>> +	cond_resched();
>>>> +	if (!fatal_signal_pending(current))
>>>> +		return 0;
>>>> +	ret = submit_bio_wait(bio);
>>>
>>> This will change the behavior of __blkdev_issue_discard() to a sync IO
>>> execution instead of the current async execution since submit_bio_wait()
>>> call is the responsibility of the caller (e.g. blkdev_issue_discard()).
>>> Have you checked if users of __blkdev_issue_discard() are OK with that ?
>>> f2fs, ext4, xfs, dm and nvme use this function.
>>
>> I'm not sure...
>>
>>>
>>> Looking at f2fs, this does not look like it is going to work as expected
>>> since the bio setup, including end_io callback, is done after this
>>> function is called and a regular submit_bio() execution is being used.
>>
>> Then, just breaking the iteration like below?
>> nvmet_bdev_execute_write_zeroes() ignores -EINTR if "*biop = bio;" is done. Is that no problem?
>>
>> --- a/block/blk-lib.c
>> +++ b/block/blk-lib.c
>> @@ -7,6 +7,7 @@
>>  #include <linux/bio.h>
>>  #include <linux/blkdev.h>
>>  #include <linux/scatterlist.h>
>> +#include <linux/sched/signal.h>
>>  
>>  #include "blk.h"
>>  
>> @@ -30,6 +31,7 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>  	struct bio *bio = *biop;
>>  	unsigned int op;
>>  	sector_t bs_mask;
>> +	int ret = 0;
>>  
>>  	if (!q)
>>  		return -ENXIO;
>> @@ -76,10 +78,14 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>  		 * is disabled.
>>  		 */
>>  		cond_resched();
>> +		if (fatal_signal_pending(current)) {
>> +			ret = -EINTR;
>> +			break;
>> +		}
>>  	}
>>  
>>  	*biop = bio;
>> -	return 0;
>> +	return ret;
> 
> This will leak a bio as blkdev_issue_discard() executes the bio only in
> the case "if (!ret && bio)". So that does not work as is, unless all
> callers of __blkdev_issue_discard() are also changed. Same problem for
> the other __blkdev_issue_xxx() functions.
> 
> Looking more into this, if an error is returned here, no bio should be
> returned and we need to make sure that all started bios are also
> completed. So your helper blk_should_abort() did the right thing calling
> submit_bio_wait(). However, I Think it would be better to fail
> immediately the current loop bio instead of executing it and then
> reporting the -EINTR error, unconditionally, regardless of what the
> started bios completion status is.
> 
> This could be done with the help of a function like this, very similar
> to submit_bio_wait().
> 
> void bio_chain_end_wait(struct bio *bio)
> {
> 	DECLARE_COMPLETION_ONSTACK_MAP(done, bio->bi_disk->lockdep_map);
> 
> 	bio->bi_private = &done;
> 	bio->bi_end_io = submit_bio_wait_endio;
> 	bio->bi_opf |= REQ_SYNC;
> 	bio_endio(bio);
> 	wait_for_completion_io(&done);
> }
> 
> And then your helper function becomes something like this:
> 
> static int blk_should_abort(struct bio *bio)
> {
> 	int ret;
> 
> 	cond_resched();
> 	if (!fatal_signal_pending(current))
> 		return 0;
> 
> 	if (bio_flagged(bio, BIO_CHAIN))
> 		bio_chain_end_wait(bio);

I don't know about block layer, but I feel this is bad because bio_put()
will be called without submit_bio_wait() when bio_flagged() == false.
Who calls submit_bio_wait() if bio_flagged() == false ?

> 	bio_put(bio);
> 
> 	return -EINTR;
> }
> 
> Thoughts ?
>