Re: [PATCH] lib/scatterlist: Fix chaining support in sgl_alloc_order()

[Laurence Oberman <loberman@redhat.com>]

On Fri, 2018-01-19 at 11:00 -0800, Bart Van Assche wrote:
> This patch avoids that workloads with large block sizes (megabytes)
> can trigger the following call stack with the ib_srpt driver (that
> driver is the only driver that chains scatterlists allocated by
> sgl_alloc_order()):
> 
> BUG: Bad page state in process kworker/0:1H  pfn:2423a78
> page:fffffb03d08e9e00 count:-3 mapcount:0 mapping:          (null)
> index:0x0
> flags: 0x57ffffc0000000()
> raw: 0057ffffc0000000 0000000000000000 0000000000000000
> fffffffdffffffff
> raw: dead000000000100 dead000000000200 0000000000000000
> 0000000000000000
> page dumped because: nonzero _count
> CPU: 0 PID: 733 Comm: kworker/0:1H Tainted: G          I      4.15.0-
> rc7.bart+ #1
> Hardware name: HP ProLiant DL380 G7, BIOS P67 08/16/2015
> Workqueue: ib-comp-wq ib_cq_poll_work [ib_core]
> Call Trace:
>  dump_stack+0x5c/0x83
>  bad_page+0xf5/0x10f
>  get_page_from_freelist+0xa46/0x11b0
>  __alloc_pages_nodemask+0x103/0x290
>  sgl_alloc_order+0x101/0x180
>  target_alloc_sgl+0x2c/0x40 [target_core_mod]
>  srpt_alloc_rw_ctxs+0x173/0x2d0 [ib_srpt]
>  srpt_handle_new_iu+0x61e/0x7f0 [ib_srpt]
>  __ib_process_cq+0x55/0xa0 [ib_core]
>  ib_cq_poll_work+0x1b/0x60 [ib_core]
>  process_one_work+0x141/0x340
>  worker_thread+0x47/0x3e0
>  kthread+0xf5/0x130
>  ret_from_fork+0x1f/0x30
> 
> Fixes: e80a0af4759a ("lib/scatterlist: Introduce sgl_alloc() and
> sgl_free()")
> Reported-by: Laurence Oberman <loberman@redhat.com>
> Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
> Cc: Nicholas A. Bellinger <nab@linux-iscsi.org>
> Cc: Laurence Oberman <loberman@redhat.com>
> ---
>  drivers/target/target_core_transport.c |  2 +-
>  include/linux/scatterlist.h            |  1 +
>  lib/scatterlist.c                      | 32
> +++++++++++++++++++++++++++-----
>  3 files changed, 29 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/target/target_core_transport.c
> b/drivers/target/target_core_transport.c
> index a001ba711cca..c03a78ee26cd 100644
> --- a/drivers/target/target_core_transport.c
> +++ b/drivers/target/target_core_transport.c
> @@ -2300,7 +2300,7 @@ static void target_complete_ok_work(struct
> work_struct *work)
>  
>  void target_free_sgl(struct scatterlist *sgl, int nents)
>  {
> -	sgl_free(sgl);
> +	sgl_free_n_order(sgl, nents, 0);
>  }
>  EXPORT_SYMBOL(target_free_sgl);
>  
> diff --git a/include/linux/scatterlist.h
> b/include/linux/scatterlist.h
> index b8a7c1d1dbe3..22b2131bcdcd 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -282,6 +282,7 @@ struct scatterlist *sgl_alloc_order(unsigned long
> long length,
>  				    gfp_t gfp, unsigned int
> *nent_p);
>  struct scatterlist *sgl_alloc(unsigned long long length, gfp_t gfp,
>  			      unsigned int *nent_p);
> +void sgl_free_n_order(struct scatterlist *sgl, int nents, int
> order);
>  void sgl_free_order(struct scatterlist *sgl, int order);
>  void sgl_free(struct scatterlist *sgl);
>  #endif /* CONFIG_SGL_ALLOC */
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 9afc9b432083..53728d391d3a 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -512,7 +512,7 @@ struct scatterlist *sgl_alloc_order(unsigned long
> long length,
>  	if (!sgl)
>  		return NULL;
>  
> -	sg_init_table(sgl, nent);
> +	sg_init_table(sgl, nalloc);
>  	sg = sgl;
>  	while (length) {
>  		elem_len = min_t(u64, length, PAGE_SIZE << order);
> @@ -526,7 +526,7 @@ struct scatterlist *sgl_alloc_order(unsigned long
> long length,
>  		length -= elem_len;
>  		sg = sg_next(sg);
>  	}
> -	WARN_ON_ONCE(sg);
> +	WARN_ONCE(length, "length = %lld\n", length);
>  	if (nent_p)
>  		*nent_p = nent;
>  	return sgl;
> @@ -549,22 +549,44 @@ struct scatterlist *sgl_alloc(unsigned long
> long length, gfp_t gfp,
>  EXPORT_SYMBOL(sgl_alloc);
>  
>  /**
> - * sgl_free_order - free a scatterlist and its pages
> + * sgl_free_n_order - free a scatterlist and its pages
>   * @sgl: Scatterlist with one or more elements
> + * @nents: Maximum number of elements to free
>   * @order: Second argument for __free_pages()
> + *
> + * Notes:
> + * - If several scatterlists have been chained and each chain
> element is
> + *   freed separately then it's essential to set nents correctly to
> avoid that a
> + *   page would get freed twice.
> + * - All pages in a chained scatterlist can be freed at once by
> setting @nents
> + *   to a high number.
>   */
> -void sgl_free_order(struct scatterlist *sgl, int order)
> +void sgl_free_n_order(struct scatterlist *sgl, int nents, int order)
>  {
>  	struct scatterlist *sg;
>  	struct page *page;
> +	int i;
>  
> -	for (sg = sgl; sg; sg = sg_next(sg)) {
> +	for_each_sg(sgl, sg, nents, i) {
> +		if (!sg)
> +			break;
>  		page = sg_page(sg);
>  		if (page)
>  			__free_pages(page, order);
>  	}
>  	kfree(sgl);
>  }
> +EXPORT_SYMBOL(sgl_free_n_order);
> +
> +/**
> + * sgl_free_order - free a scatterlist and its pages
> + * @sgl: Scatterlist with one or more elements
> + * @order: Second argument for __free_pages()
> + */
> +void sgl_free_order(struct scatterlist *sgl, int order)
> +{
> +	sgl_free_n_order(sgl, INT_MAX, order);
> +}
>  EXPORT_SYMBOL(sgl_free_order);
>  
>  /**

Spent a good few days working on this with Bart, 
The issue was very familiar to me so I know its now fixed.
Thank you Bart!

Tested-by: Laurence Oberman <loberman@redhat.com>