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SubjectRe: [PATCH v4 04/23] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages
On Wed, Nov 13, 2019 at 2:49 PM John Hubbard <jhubbard@nvidia.com> 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);
> > }
> >

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