linux-kernel - Re: [PATCH] md: raid10: remove VLAIS

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Date:   Fri, 06 Oct 2017 13:22:12 +1100
From:   NeilBrown <neilb@...e.com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Shaohua Li <shli@...nel.org>, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Michael Davidson <md@...gle.com>,
        Guenter Roeck <groeck@...omium.org>
Subject: Re: [PATCH] md: raid10: remove VLAIS

On Thu, Oct 05 2017, Matthias Kaehlcke wrote:

> Hi Neil,
>
> El Fri, Oct 06, 2017 at 10:58:59AM +1100 NeilBrown ha dit:
>
>> On Thu, Oct 05 2017, Matthias Kaehlcke wrote:
>> 
>> > The raid10 driver can't be built with clang since it uses a variable
>> > length array in a structure (VLAIS):
>> >
>> > drivers/md/raid10.c:4583:17: error: fields must have a constant size:
>> >   'variable length array in structure' extension will never be supported
>> >
>> > Allocate the r10bio struct with kmalloc instead of using the VLAIS
>> > construct.
>> >
>> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
>> > ---
>> >  drivers/md/raid10.c | 13 ++++++++-----
>> >  1 file changed, 8 insertions(+), 5 deletions(-)
>> >
>> > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>> > index 374df5796649..9616163eaf8c 100644
>> > --- a/drivers/md/raid10.c
>> > +++ b/drivers/md/raid10.c
>> > @@ -4578,15 +4578,16 @@ static int handle_reshape_read_error(struct mddev *mddev,
>> >  	/* Use sync reads to get the blocks from somewhere else */
>> >  	int sectors = r10_bio->sectors;
>> >  	struct r10conf *conf = mddev->private;
>> > -	struct {
>> > -		struct r10bio r10_bio;
>> > -		struct r10dev devs[conf->copies];
>> > -	} on_stack;
>> > -	struct r10bio *r10b = &on_stack.r10_bio;
>> > +	struct r10bio *r10b;
>> >  	int slot = 0;
>> >  	int idx = 0;
>> >  	struct page **pages;
>> >  
>> > +	r10b = kmalloc(sizeof(*r10b) +
>> > +	       sizeof(struct r10dev) * conf->copies, GFP_KERNEL);
>> 
>> GFP_KERNEL isn't a good idea here.
>> This could wait for writeback, and if writeback tries to write to the
>> region of the array which is being reshaped, it might deadlock.
>> 
>> GFP_NOIO is safer.
>
> Good point, thanks!
>
>> given that conf->copies is almost always 2 it might be nicer to
>> have
>> 
>> 	struct {
>> 		struct r10bio r10_bio;
>> 		struct r10dev devs[2];
>> 	} on_stack;
>> 
>>         struct r10bio *r10b;
>> 
>> 	if (conf->copies <= ARRAY_SIZE(on_stack.devs))
>>         	r10b = &on_stack.r10_bio;
>>         else
>>         	r10b = kmalloc(sizeof(*r10b) +
>> 			       sizeof(struct r10dev) * conf->copies, GFP_NOIO);
>
> It would add also add an extra condition to determine if r10b needs to
> be freed or not.

True.

>
> Given that array reshaping is a rare operation and an error during
> this operation is an exceptional condition I think the simpler code
> with always dynamic allocation is preferable. That said I'm fine with
> reworking the patch according to your suggestion if you or Shaohua
> prefer it.

I don't feel strongly about it.  As long as the GFP_KERNEL->GFP_NOIO
change happens I'm OK with this patch.

Thanks,
NeilBrown

>
> Matthias
>
>> > +	if (!r10b)
>> > +		return -ENOMEM;
>> > +
>> >  	/* reshape IOs share pages from .devs[0].bio */
>> >  	pages = get_resync_pages(r10_bio->devs[0].bio)->pages;
>> >  
>> > @@ -4635,11 +4636,13 @@ static int handle_reshape_read_error(struct mddev *mddev,
>> >  			/* couldn't read this block, must give up */
>> >  			set_bit(MD_RECOVERY_INTR,
>> >  				&mddev->recovery);
>> > +			kfree(r10b);
>> >  			return -EIO;
>> >  		}
>> >  		sectors -= s;
>> >  		idx++;
>> >  	}
>> > +	kfree(r10b);
>> >  	return 0;
>> >  }
>> >  

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