Re: [PATCH] md: raid10: remove VLAIS

[Shaohua Li]

On Fri, Oct 06, 2017 at 01:22:12PM +1100, Neil Brown wrote:
> 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@xxxxxxxxxxxx>
> >> > ---
> >> >  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.

Let's use GFP_NOIO then, should not be big deal. I updated the patch.

Thanks,
Shaohua