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

On 2021-09-29 4:46 p.m., Jason Gunthorpe wrote:

On Wed, Sep 29, 2021 at 03:30:42PM -0600, Logan Gunthorpe wrote:

quoted

On 2021-09-28 4:05 p.m., Jason Gunthorpe wrote:
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.

Ugh, right. A bit contrived for consecutive pages to have different
pgmaps and still be next to each other in a DMA transaction. But I guess
it is technically possible and should be protected against.

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.

quoted

quoted

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:

Yes. The block layer also does this. I believe a check in
page_is_mergable() will be sufficient there.

I don't think it's correct to use pgmap without first checking if it is
a zone device page. But your point is taken. I'll try to address this.

Logan