Re: [PATCH v3 4/20] PCI/P2PDMA: introduce helpers for dma_map_sg implementations

[Jason Gunthorpe]

On Wed, Sep 29, 2021 at 03:30:42PM -0600, Logan Gunthorpe wrote:
> 
> 
> 
> On 2021-09-28 4:05 p.m., Jason Gunthorpe wrote:
> > On Thu, Sep 16, 2021 at 05:40:44PM -0600, Logan Gunthorpe wrote:
> > 
> >> +enum pci_p2pdma_map_type
> >> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev,
> >> +		       struct scatterlist *sg)
> >> +{
> >> +	if (state->pgmap != sg_page(sg)->pgmap) {
> >> +		state->pgmap = sg_page(sg)->pgmap;
> > 
> > This has built into it an assumption that every page in the sg element
> > has the same pgmap, but AFAIK nothing enforces this rule? There is no
> > requirement that the HW has pfn gaps between the pgmaps linux decides
> > to create over it.
> 
> No, that's not a correct reading of the code. Every time there is a new
> pagemap, this code calculates the mapping type and bus offset. If a page
> comes along with a different page map,f it recalculates. This just
> reduces the overhead so that the calculation is done only every time a
> page with a different pgmap comes along and not doing it for every
> single page.

Each 'struct scatterlist *sg' refers to a range of contiguous pfns
starting at page_to_pfn(sg_page()) and going for approx sg->length/PAGE_SIZE
pfns long.

sg_page() returns the first page, but nothing says that sg_page()+1
has the same pgmap.

The code in this patch does check the first page of each sg in a
larger sgl.

> > At least sg_alloc_append_table_from_pages() and probably something in
> > the block world should be updated to not combine struct pages with
> > different pgmaps, and this should be documented in scatterlist.*
> > someplace.
> 
> There's no sane place to do this check. The code is designed to support
> mappings with different pgmaps.

All places that generate compound sg's by aggregating multiple pages
need to include this check along side the check for physical
contiguity. There are not that many places but
sg_alloc_append_table_from_pages() is one of them:

@@ -470,7 +470,8 @@ int sg_alloc_append_table_from_pages(struct sg_append_table *sgt_append,
 
                /* Merge contiguous pages into the last SG */
                prv_len = sgt_append->prv->length;
-               while (n_pages && page_to_pfn(pages[0]) == paddr) {
+               while (n_pages && page_to_pfn(pages[0]) == paddr &&
+                      sg_page(sgt_append->prv)->pgmap == pages[0]->pgmap) {
                        if (sgt_append->prv->length + PAGE_SIZE > max_segment)
                                break;
                        sgt_append->prv->length += PAGE_SIZE;
@@ -488,7 +489,8 @@ int sg_alloc_append_table_from_pages(struct sg_append_table *sgt_append,
        for (i = 1; i < n_pages; i++) {
                seg_len += PAGE_SIZE;
                if (seg_len >= max_segment ||
-                   page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1) {
+                   page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1 ||
+                   pages[i]->pgmap != pages[i - 1]->pgmap) {
                        chunks++;
                        seg_len = 0;
                }
@@ -505,9 +507,10 @@ int sg_alloc_append_table_from_pages(struct sg_append_table *sgt_append,
                        seg_len += PAGE_SIZE;
                        if (seg_len >= max_segment ||
                            page_to_pfn(pages[j]) !=
-                           page_to_pfn(pages[j - 1]) + 1)
+                                   page_to_pfn(pages[j - 1]) + 1 ||
+                           pages[i]->pgmap != pages[i - 1]->pgmap) {
                                break;
-               }
+                       }
 
                /* Pass how many chunks might be left */
                s = get_next_sg(sgt_append, s, chunks - i + left_pages,