Re: [PATCH] scsi: eata: drop VLA in reorder()

[Salvatore Mesoraca]

2018-03-12 4:08 GMT+01:00 Tobin C. Harding <tobin@xxxxxxxxxxxx>:
> Adding kernel newbies to CC because I pose a few noob questions :)
> Adding Linus to CC because I quoted him.
>
> On Sun, Mar 11, 2018 at 10:06:58PM +0100, Salvatore Mesoraca wrote:
>> n_ready will always be less than or equal to MAX_MAILBOXES.
>> So we avoid a VLA[1] and use fixed-length arrays instead.
>>
>> [1] https://lkml.org/lkml/2018/3/7/621
>>
>> Signed-off-by: Salvatore Mesoraca <s.mesoraca16@xxxxxxxxx>
>> ---
>>  drivers/scsi/eata.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/scsi/eata.c b/drivers/scsi/eata.c
>> index 6501c33..202cd17 100644
>> --- a/drivers/scsi/eata.c
>> +++ b/drivers/scsi/eata.c
>> @@ -2096,7 +2096,7 @@ static int reorder(struct hostdata *ha, unsigned long cursec,
>>       unsigned int k, n;
>>       unsigned int rev = 0, s = 1, r = 1;
>>       unsigned int input_only = 1, overlap = 0;
>> -     unsigned long sl[n_ready], pl[n_ready], ll[n_ready];
>> +     unsigned long sl[MAX_MAILBOXES], pl[MAX_MAILBOXES], ll[MAX_MAILBOXES];
>
> I think we are going to see a recurring theme here.  MAX_MAILBOXES==64
> so this patch adds 1536 bytes to the stack on a 64 bit machine or 768
> bytes on a 32 bit machine.  Linus already commented on another VLA
> removal patch that 768 was a lot of stack space.  That comment did,
> however say 'deep in some transfer call chain'.  I don't know what a
> 'transfer call chain' (the transfer bit) is but is there some heuristic
> we can use to know how deep is deep?  Or more to the point, is there some
> heuristic we can use to know what is an acceptable amount of stack space
> to use?
>
> As far as this patch is concerned wouldn't a kmalloc (with GFP_ATOMIC)
> be ok?  We are in an interrupt handler, can we assume that since IO has
> just occurred that the IO will be so slow comparatively that a memory
> allocation will be quick.  (assuming IO since eata.c only requests a
> single irq line.)

Yes, I think you are right. I'll change it in v2.
Thank you very much,

Salvatore