Re: scsi: address leak in the error path of discard page allocation

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Jul 01 2010 at  9:03am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Thu, Jul 01 2010 at  6:49am -0400,
> FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
> 
> > This fixes discard page leak by using q->unprep_rq_fn facility.
> > 
> > q->unprep_rq_fn is called when all the data buffer (req->bio and
> > scsi_data_buffer) in the request is freed.
> > 
> > sd_unprep() uses rq->buffer to free discard page allocated in
> > sd_prepare_discard().
> > 
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> 
> Thanks for sorting this out Tomo, all 3 patches work great!
> 
> BTW, there is one remaining (rare) leak in the allocation path.
> 
> The following patch serves to fix it but I'm not sure if there is a more
> elegant way to address this.

I've continued to look at this to arrive at alternative implementation.
Here is a summary of the problem:

A 'scsi_setup_discard_cmnd' return other than BLKPREP_OK will not cause
a discard request to get completely stripped down ('blk_finish_request'
isn't calling 'blk_unprep_request' because REQ_DONTPREP is not set by
'scsi_prep_return' for none BLKPREP_OK return).  Therefore the discard
request's page will _not_ get cleaned up.

Aside from code inspection, I confirmed this by adding some test code to
force a one-time initial BLKPREP_DEFER return from
'scsi_setup_discard_cmnd'.

> An alternative would be to check if the page is already allocated
> (before allocating the page in scsi_setup_discard_cmnd)?

Unfortunatey this "alternative" won't work because it completely ignores
the case where BLKPREP_KILL is returned from scsi_setup_discard_cmnd'.
 
> Please advise, thanks.

In short, I'm not too happy that the following patch doesn't allow for
centralized cleanup of the discard request's page (via sd_unprep_fn).
But in order to do that we'd likely have to:
1) relax blk_finish_request's REQ_DONTPREP constraint
2) add other weird conditionals within blk_unprep_request because
   the discard request wasn't _really_ prepared?

So given this I'm inclined to stick with the following patch.

Jens and/or James, what do you think?

Mike

> From: Mike Snitzer <snitzer@redhat.com>
> Subject: scsi: address leak in the error path of discard page allocation
> 
> Be sure to free the discard page if scsi_setup_blk_pc_cmnd fails.
> E.g. Returning BLKPREP_DEFER from scsi_setup_blk_pc_cmnd will not cause
> the request to be processed by sd_unprep_fn before the request is
> retried (preparation included).
> 
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> 
> ---
>  block/blk-core.c       |   23 +++++++++++++++++++++++
>  drivers/scsi/sd.c      |    6 +++++-
>  include/linux/blkdev.h |    1 +
>  3 files changed, 29 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/drivers/scsi/sd.c
> ===================================================================
> --- linux-2.6.orig/drivers/scsi/sd.c
> +++ linux-2.6/drivers/scsi/sd.c
> @@ -466,7 +466,11 @@ static int scsi_setup_discard_cmnd(struc
>  
>  	blk_add_request_payload(rq, page, len);
>  	ret = scsi_setup_blk_pc_cmnd(sdp, rq);
> -	rq->buffer = page_address(page);
> +	if (ret != BLKPREP_OK) {
> +		blk_clear_request_payload(rq);
> +		__free_page(page);
> +	} else
> +		rq->buffer = page_address(page);
>  	return ret;
>  }
>  
> Index: linux-2.6/block/blk-core.c
> ===================================================================
> --- linux-2.6.orig/block/blk-core.c
> +++ linux-2.6/block/blk-core.c
> @@ -1164,6 +1164,29 @@ void blk_add_request_payload(struct requ
>  }
>  EXPORT_SYMBOL_GPL(blk_add_request_payload);
>  
> +/**
> + * blk_clear_request_payload - clear a request's payload
> + * @rq: request to update
> + *
> + * The driver needs to take care of freeing the payload itself.
> + */
> +void blk_clear_request_payload(struct request *rq)
> +{
> +	struct bio *bio = rq->bio;
> +
> +	rq->__data_len = rq->resid_len = 0;
> +	rq->nr_phys_segments = 0;
> +	rq->buffer = NULL;
> +
> +	bio->bi_size = 0;
> +	bio->bi_vcnt = 0;
> +	bio->bi_phys_segments = 0;
> +
> +	bio->bi_io_vec->bv_page = NULL;
> +	bio->bi_io_vec->bv_len = 0;
> +}
> +EXPORT_SYMBOL_GPL(blk_clear_request_payload);
> +
>  void init_request_from_bio(struct request *req, struct bio *bio)
>  {
>  	req->cpu = bio->bi_comp_cpu;
> Index: linux-2.6/include/linux/blkdev.h
> ===================================================================
> --- linux-2.6.orig/include/linux/blkdev.h
> +++ linux-2.6/include/linux/blkdev.h
> @@ -781,6 +781,7 @@ extern void blk_insert_request(struct re
>  extern void blk_requeue_request(struct request_queue *, struct request *);
>  extern void blk_add_request_payload(struct request *rq, struct page *page,
>  		unsigned int len);
> +extern void blk_clear_request_payload(struct request *rq);
>  extern int blk_rq_check_limits(struct request_queue *q, struct request *rq);
>  extern int blk_lld_busy(struct request_queue *q);
>  extern int blk_rq_prep_clone(struct request *rq, struct request *rq_src,