On 27.02.20 06:58, John Hubbard wrote:
> On 2/26/20 7:11 PM, Stephen Rothwell wrote:
>> Hi all,
>>
>> Today's linux-next merge of the akpm-current tree got a conflict in:
>>
>> mm/gup.c
>>
>> between commit:
>>
>> 732b80e677b8 ("mm/gup/writeback: add callbacks for inaccessible pages")
>>
>> from the kvms390 tree and commit:
>>
>> 9947ea2c1e60 ("mm/gup: track FOLL_PIN pages")
>>
>> from the akpm-current tree.
>>
>> I fixed it up (see below - maybe not optimally) and can carry the fix as
>> necessary. This is now fixed as far as linux-next is concerned, but any
>> non trivial conflicts should be mentioned to your upstream maintainer
>> when your tree is submitted for merging. You may also want to consider
>> cooperating with the maintainer of the conflicting tree to minimise any
>> particularly complex conflicts.
>>
>
> Yes. Changes to mm/gup.c really should normally go through linux-mm and
> Andrew's tree, if at all possible. This would have been caught, and figured out
> on linux-mm, had that been done--instead of leaving the linux-next maintainer
> trying to guess at how to resolve the conflict.
Yes. This patch should go via Andrew. Claudio is going to provide a fixed up
version that takes care of the new semantics.
This patch was posted several times on linux-mm (also before rc1) and I will
drop it from my tree due to the conflict.
>
> +Cc David Hildenbrand, who I see looked at the kvms390 proposed patch a bit.
> Maybe he has some opinions, especially about my questions below.
>
> The fix-up below may (or may not) need some changes:
>
>
> diff --cc mm/gup.c
> index 354bcfbd844b,f589299b0d4a..000000000000
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@@ -269,18 -470,11 +468,19 @@@ retry
> goto retry;
> }
>
> + /* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
> + if (unlikely(!try_grab_page(page, flags))) {
> + page = ERR_PTR(-ENOMEM);
> + goto out;
> + }
> + if (flags & FOLL_GET) {
>
>
> If I'm reading the diff correctly, I believe that line should *maybe* be changed to:
>
> if (flags & (FOLL_GET | FOLL_PIN)) {
>
> ...because each of those flags has a similar effect: pinned pages for DMA or RDMA
> use. So either flag will require a call to arch_make_page_accessible()...except that
> I'm not sure that's what you want. Would the absence of a call to
> arch_make_page_accessible() cause things like pin_user_pages() to not work correctly?
> Seems like it would, to me.
>
> (I'm pretty unhappy that we have to ask this at the linux-next level.)
>
> Also below...
>
>
> - if (unlikely(!try_get_page(page))) {
> - page = ERR_PTR(-ENOMEM);
> - goto out;
> - }
> + ret = arch_make_page_accessible(page);
> + if (ret) {
> + put_page(page);
>
>
> put_page() only works with FOLL_GET. So if we do allow to get here via either FOLL_GET or
> FOLL_PIN, the we need to do an unpin_user_page(), like this:
>
> if (flags & FOLL_PIN)
> unpin_user_page(page);
> else
> put_page(page);
>
>
>
> + page = ERR_PTR(ret);
> + goto out;
> + }
> + }
> if (flags & FOLL_TOUCH) {
> if ((flags & FOLL_WRITE) &&
> !pte_dirty(pte) && !PageDirty(page))
>
> thanks,
>
