Re: [PATCH rdma-next v2 1/2] lib/scatterlist: Fix wrong update of orig_nents

On Sun, Jul 18, 2021 at 02:09:12PM +0300, Leon Romanovsky wrote:

@@ -386,12 +414,14 @@ static struct scatterlist *get_next_sg(struct sg_table *table, return ERR_PTR(-ENOMEM); sg_init_table(new_sg, alloc_size); if (cur) {

• if (total_nents)
  

• 	*total_nents += alloc_size - 1;
  

  __sg_chain(next_sg, new_sg);

• table->orig_nents += alloc_size - 1;
  

  } else { table->sgl = new_sg;

• table->orig_nents = alloc_size;
  

  table->nents = 0;

Why does this still touch nents?

@@ -515,6 +548,7 @@ struct scatterlist *__sg_alloc_table_from_pages(struct sg_table *sgt, cur_page = j; } sgt->nents += added_nents;

• sgt->orig_nents = sgt->nents;

And here too?

nents should only be set by the dma mapper, right?

I'm also trying to understand why it is OK to pass in NULL for total_nents?

Any situation where _sg_alloc_table_from_pages() returns with sgt->orig_nents != total_nents requires the use of sg_free_table_entries()

It looks like there is some trouble here:

for (i = 0; i < chunks; i++) { s = get_next_sg(sgt, s, chunks - i + left_pages, gfp_mask, total_nents); if (IS_ERR(s)) {

This will update total_nents but after a few loops it can exit without synchronizing sgt->orig_nents - thus any caller handling an error return from __sg_alloc_table_from_pages() must not pass in NULL and must use sg_free_table_entries()

So I would see two options:

1) Remove the possiblity to return NULL and fix all callers to use sg_free_table_entries() on error

2) Once __sg_alloc_table_from_pages() fails the sg_table is corrupted and the user must call sg_free_table_entries(). ie forcibly store total_nents in the orig_nents and thus destroy the ability to continue to use the sg_table.

This is what sg_alloc_table_from_pages() already has to do

Further upon success of __sg_alloc_table_from_pages() it should be true that sgt->orig_nents == total_nents so the ib_umem change is confusing. total_nents should be removed from the struct and only the failure paths in the function calling __sg_alloc_table_from_pages() need a stack local variable and sg_free_table_entries()

IMHO this API may have become unwieldly and complicated, I wonder if this is better:

struct sg_append_table state;

sg_append_init(&state, sgt, gfp_mask);

while (..) ret = sg_append_pages(&state, pages, n_pages, ..) if (ret) sg_append_abort(&state); // Frees the sgt and puts it to NULL sg_append_complete(&state)

Which allows sg_alloc_table_from_pages() to be written as

struct sg_append_table state; sg_append_init(&state, sgt, gfp_mask); ret = sg_append_pages(&state,pages, n_pages, offset, size, UINT_MAX) if (ret) { sg_append_abort(&state); return ret; } sg_append_complete(&state); return 0;

And then the API can manage all of this in some sane and understandable way.

Jason