Re: [Xen-devel] [PATCH v3 18/20] net/xen-netback: Make it running on 64KB page granularity

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Aug 10, 2015 at 10:57:48AM +0100, Julien Grall wrote:
[...]
> 
> >>+           info.page = page;
> >>+           gnttab_foreach_grant_in_range(page, offset, bytes,
> >>+                                         xenvif_gop_frag_copy_grant,
> >>+                                         &info);
> >
> >Looks like I need to at least wait until the API is settle before giving
> >my ack.
> >
> >>            size -= bytes;
> >>+           offset = 0;
> >
> >This looks wrong. Should be offset += bytes.
> 
> With the new implementation of the loop, each iteration will be on a
> different page.
> So only the first page has an offset different than zero.
> 
> >
> >>
> >>-           /* Next frame */
> >>-           if (offset == PAGE_SIZE && size) {
> >>+           /* Next page */
> >>+           if (size) {
> >>                    BUG_ON(!PageCompound(page));
> >>                    page++;
> >>-                   offset = 0;
> >
> >And this should not be deleted, I think.
> >
> >What is the reason for changing offset calculation? I think there is
> >still compound page when using 64K page.
> 
> The compound pages are still working ... gnttab_foreach_grant_in_range is
> called once per page. So the offset can be reset to 0 every time. No need to
> add code which would make the result less clear.
> 
> We only need to know if the size is not 0 to get the next page.
> 
> The patch may not be clear enough to see it's working so I've copied the
> result loop below:
> 
>         while (size > 0) {
>                 BUG_ON(offset >= PAGE_SIZE);
> 
>                 bytes = PAGE_SIZE - offset;
>                 if (bytes > size)
>                         bytes = size;
> 
>                 info.page = page;
>                 gnttab_foreach_grant_in_range(page, offset, bytes,
>                                              xenvif_gop_frag_copy_grant,
>                                               &info);
>                 size -= bytes;
>                 offset = 0;
> 
>                 /* Next page */
>                 if (size) {
>                         BUG_ON(!PageCompound(page));
>                         page++;
>                 }
>         }
> 

Right. That doesn't mean the original code was wrong or anything. But I
don't want to bikeshed about this.

Please add a comment saying that offset is always 0 starting from second
iteration because the gnttab_foreach_grant_in_range makes sure we handle
one page in one go.

Wei.


> Regards,
> 
> -- 
> Julien Grall

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