Re: [PATCH] lib/scatterlist: Fix to calculate the last_pg properly

[Yishai Hadas]

On 10/01/2023 15:53, Jason Gunthorpe wrote:
> On Tue, Jan 10, 2023 at 10:01:53AM +0200, Yishai Hadas wrote:
> > On 10/01/2023 7:08, Matthew Wilcox wrote:
> > > On Mon, Jan 09, 2023 at 04:47:01PM +0200, Yishai Hadas wrote:
> > > >    	if (sgt_append->prv) {
> > > > +		unsigned long paddr =
> > > > +			(page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
> > > 1. page_to_pfn() * PAGE_SIZE is spelled page_to_phys()
> > >
> > > 2. physical addresses have type phys_addr_t.  Oh, wait, paddr isn't a
> > > physical address, it's a pfn because you divide by PAGE_SIZE at the end.
> > > But you will get truncation on 32-bit, because page_to_pfn() has type
> > > unsigned long.  Anyway, this shouldn't be called paddr.  Maybe last_pfn
> > > or something?
> > >
> > > > +			 sgt_append->prv->offset + sgt_append->prv->length) /
> > > > +			PAGE_SIZE;
> > In this area of the patch I just reverted to what we had in 6.1 [1] where we
> > were good.
> >
> > I can send V1 and replace the 'paddr' to be called 'last_pfn' if it's really
> > needed.
> Please change it as Matthew describes so it is clearer
>
> Jason

OK

Once cleaning the code and renaming to 'last_pfn', could also see that 
we had to use below '- 1' instead of '- PAGE_SIZE'.

The expected diff compared to V0 can be seen below.

Will repeat some testing and send V1 once be ready.

+++ b/lib/scatterlist.c
@@ -470,18 +470,16 @@ int sg_alloc_append_table_from_pages(struct 
sg_append_table *sgt_append,
                return -EOPNOTSUPP;

        if (sgt_append->prv) {
-               unsigned long paddr =
-                       (page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
-                        sgt_append->prv->offset + 
sgt_append->prv->length) /
-                       PAGE_SIZE;
+               unsigned long last_pfn = 
(page_to_phys(sg_page(sgt_append->prv)) +
+                       sgt_append->prv->offset + 
sgt_append->prv->length) / PAGE_SIZE;

                if (WARN_ON(offset))
                        return -EINVAL;

                /* Merge contiguous pages into the last SG */
                prv_len = sgt_append->prv->length;
-               if (page_to_pfn(pages[0]) == paddr) {
-                       last_pg = pfn_to_page(paddr - PAGE_SIZE);
+               if (page_to_pfn(pages[0]) == last_pfn) {
+                       last_pg = pfn_to_page(last_pfn - 1);
                        while (n_pages && pages_are_mergeable(pages[0], 
last_pg)) {
                                if (sgt_append->prv->length + PAGE_SIZE 
> max_segment)
                                        break;

Yishai