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SubjectRe: linux-next: manual merge of the akpm-current tree with the kvms390 tree
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.

+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)

+ page = ERR_PTR(ret);
+ goto out;
+ }
+ }
if (flags & FOLL_TOUCH) {
if ((flags & FOLL_WRITE) &&
!pte_dirty(pte) && !PageDirty(page))

John Hubbard

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