Re: [PATCH] lib/scatterlist: Fix to merge contiguous pages into the last SG properly

[Yishai Hadas]

On 05/01/2023 15:36, Jason Gunthorpe wrote:
> On Thu, Jan 05, 2023 at 01:23:39PM +0200, Yishai Hadas wrote:
> > When sg_alloc_append_table_from_pages() calls to pages_are_mergeable()
> > in its 'sgt_append->prv' flow to check whether it can merge contiguous
> > pages into the last SG, it passes the page arguments in the wrong order.
> >
> > The first parameter should be the next candidate page to be merged to
> > the last page and not the opposite.
> >
> > The current code leads to a corrupted SG which resulted in OOPs and
> > unexpected errors when non-contiguous pages are merged wrongly.
> >
> > Fix to pass the page parameters in the right order.
> >
> > Fixes: 1567b49d1a40 ("lib/scatterlist: add check when merging zone device pages")
> > Signed-off-by: Yishai Hadas <yishaih@xxxxxxxxxx>
> > ---
> >   lib/scatterlist.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Thanks Jason

> Also, I'm looking more closely at '156 and this is not right either:
>
> -               unsigned long paddr =
> -                       (page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
> -                        sgt_append->prv->offset + sgt_append->prv->length) /
> -                       PAGE_SIZE;
> -
> -               while (n_pages && page_to_pfn(pages[0]) == paddr) {
> +               last_pg = sg_page(sgt_append->prv);
> +               while (n_pages && pages_are_mergeable(last_pg, pages[0])) {
>
> This change will break things like multi-page combining, sub page
> scenarios and maybe more.
>
> The contiguity test here has to be done a phys, it should go back to
> struct page to check if the pgmap is OK.
>
> Can you fix it as well?


Yes, I have locally some candidate patch as you asked, on top of this one.

I would like to run some extra testing on, then may send it.

Yishai