Re: [PATCH v4 04/23] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

[Dan Williams]

On Wed, Nov 13, 2019 at 2:49 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> On 11/13/19 2:00 PM, Dan Williams wrote:
> ...
> >> Ugh, when did all this HMM specific manipulation sneak into the
> >> generic ZONE_DEVICE path? It used to be gated by pgmap type with its
> >> own put_zone_device_private_page(). For example it's certainly
> >> unnecessary and might be broken (would need to check) to call
> >> mem_cgroup_uncharge() on a DAX page. ZONE_DEVICE users are not a
> >> monolith and the HMM use case leaks pages into code paths that DAX
> >> explicitly avoids.
> >
> > It's been this way for a while and I did not react previously,
> > apologies for that. I think __ClearPageActive, __ClearPageWaiters, and
> > mem_cgroup_uncharge, belong behind a device-private conditional. The
> > history here is:
> >
> > Move some, but not all HMM specifics to hmm_devmem_free():
> >      2fa147bdbf67 mm, dev_pagemap: Do not clear ->mapping on final put
> >
> > Remove the clearing of mapping since no upstream consumers needed it:
> >      b7a523109fb5 mm: don't clear ->mapping in hmm_devmem_free
> >
> > Add it back in once an upstream consumer arrived:
> >      7ab0ad0e74f8 mm/hmm: fix ZONE_DEVICE anon page mapping reuse
> >
> > We're now almost entirely free of ->page_free callbacks except for
> > that weird nouveau case, can that FIXME in nouveau_dmem_page_free()
> > also result in killing the ->page_free() callback altogether? In the
> > meantime I'm proposing a cleanup like this:
>
>
> OK, assuming this is acceptable (no obvious problems jump out at me,
> and we can also test it with HMM), then how would you like to proceed, as
> far as patches go: add such a patch as part of this series here, or as a
> stand-alone patch either before or after this series? Or something else?
> And did you plan on sending it out as such?

I think it makes sense to include it in your series since you're
looking to refactor the implementation. I can send you one based on
current linux-next as a lead-in cleanup before the refactor. Does that
work for you?

>
> Also, the diffs didn't quite make it through intact to my "git apply", so
> I'm re-posting the diff in hopes that this time it survives:

Apologies for that. For quick "how about this" patch examples, I just
copy and paste into gmail and it sometimes clobbers it.

>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index f9f76f6ba07b..21db1ce8c0ae 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -338,13 +338,7 @@ static void pmem_release_disk(void *__pmem)
>         put_disk(pmem->disk);
>   }
>
> -static void pmem_pagemap_page_free(struct page *page)
> -{
> -       wake_up_var(&page->_refcount);
> -}
> -
>   static const struct dev_pagemap_ops fsdax_pagemap_ops = {
> -       .page_free              = pmem_pagemap_page_free,
>         .kill                   = pmem_pagemap_kill,
>         .cleanup                = pmem_pagemap_cleanup,
>   };
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 03ccbdfeb697..157edb8f7cf8 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -419,12 +419,6 @@ void __put_devmap_managed_page(struct page *page)
>          * holds a reference on the page.
>          */
>         if (count == 1) {
> -               /* Clear Active bit in case of parallel mark_page_accessed */
> -               __ClearPageActive(page);
> -               __ClearPageWaiters(page);
> -
> -               mem_cgroup_uncharge(page);
> -
>                 /*
>                  * When a device_private page is freed, the page->mapping field
>                  * may still contain a (stale) mapping value. For example, the
> @@ -446,10 +440,17 @@ void __put_devmap_managed_page(struct page *page)
>                  * handled differently or not done at all, so there is no need
>                  * to clear page->mapping.
>                  */
> -               if (is_device_private_page(page))
> -                       page->mapping = NULL;
> +               if (is_device_private_page(page)) {
> +                       /* Clear Active bit in case of parallel mark_page_accessed */
> +                       __ClearPageActive(page);
> +                       __ClearPageWaiters(page);
>
> -               page->pgmap->ops->page_free(page);
> +                       mem_cgroup_uncharge(page);
> +
> +                       page->mapping = NULL;
> +                       page->pgmap->ops->page_free(page);
> +               } else
> +                       wake_up_var(&page->_refcount);
>         } else if (!count)
>                 __put_page(page);
>   }
> --
> 2.24.0
>
>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> >
> > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> > index ad8e4df1282b..4eae441f86c9 100644
> > --- a/drivers/nvdimm/pmem.c
> > +++ b/drivers/nvdimm/pmem.c
> > @@ -337,13 +337,7 @@ static void pmem_release_disk(void *__pmem)
> >          put_disk(pmem->disk);
> >   }
> >
> > -static void pmem_pagemap_page_free(struct page *page)
> > -{
> > -       wake_up_var(&page->_refcount);
> > -}
> > -
> >   static const struct dev_pagemap_ops fsdax_pagemap_ops = {
> > -       .page_free              = pmem_pagemap_page_free,
> >          .kill                   = pmem_pagemap_kill,
> >          .cleanup                = pmem_pagemap_cleanup,
> >   };
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 03ccbdfeb697..157edb8f7cf8 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -419,12 +419,6 @@ void __put_devmap_managed_page(struct page *page)
> >           * holds a reference on the page.
> >           */
> >          if (count == 1) {
> > -               /* Clear Active bit in case of parallel mark_page_accessed */
> > -               __ClearPageActive(page);
> > -               __ClearPageWaiters(page);
> > -
> > -               mem_cgroup_uncharge(page);
> > -
> >                  /*
> >                   * When a device_private page is freed, the page->mapping field
> >                   * may still contain a (stale) mapping value. For example, the
> > @@ -446,10 +440,17 @@ void __put_devmap_managed_page(struct page *page)
> >                   * handled differently or not done at all, so there is no need
> >                   * to clear page->mapping.
> >                   */
> > -               if (is_device_private_page(page))
> > -                       page->mapping = NULL;
> > +               if (is_device_private_page(page)) {
> > +                       /* Clear Active bit in case of parallel
> > mark_page_accessed */
> > +                       __ClearPageActive(page);
> > +                       __ClearPageWaiters(page);
> >
> > -               page->pgmap->ops->page_free(page);
> > +                       mem_cgroup_uncharge(page);
> > +
> > +                       page->mapping = NULL;
> > +                       page->pgmap->ops->page_free(page);
> > +               } else
> > +                       wake_up_var(&page->_refcount);
> >          } else if (!count)
> >                  __put_page(page);
> >   }
> >