Re: [PATCH 2/2] mm: fix dev_pagemap reference counting around get_dev_pagemap

Dan Williams
2017-12-06T02:43:36Z

On Mon, Dec 4, 2017 at 4:34 PM, Christoph Hellwig <hch@lst.de> wrote:
> Both callers of get_dev_pagemap that pass in a pgmap don't actually hold a
> reference to the pgmap they pass in, contrary to the comment in the function.
>
> Change the calling convention so that get_dev_pagemap always consumes the
> previous reference instead of doing this using an explicit earlier call to
> put_dev_pagemap in the callers.
>
> The callers will still need to put the final reference after finishing the
> loop over the pages.

I don't think we need this change, but perhaps the reasoning should be
added to the code as a comment... details below.

>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> kernel/memremap.c | 17 +++++++++--------
> mm/gup.c | 7 +++++--
> 2 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index f0b54eca85b0..502fa107a585 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -506,22 +506,23 @@ struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
> * @pfn: page frame number to lookup page_map
> * @pgmap: optional known pgmap that already has a reference
> *
> - * @pgmap allows the overhead of a lookup to be bypassed when @pfn lands in the
> - * same mapping.
> + * If @pgmap is non-NULL and covers @pfn it will be returned as-is. If @pgmap
> + * is non-NULL but does not cover @pfn the reference to it while be released.
> */
> struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> struct dev_pagemap *pgmap)
> {
> - const struct resource *res = pgmap ? pgmap->res : NULL;
> resource_size_t phys = PFN_PHYS(pfn);
>
> /*
> - * In the cached case we're already holding a live reference so
> - * we can simply do a blind increment
> + * In the cached case we're already holding a live reference.
> */
> - if (res && phys >= res->start && phys <= res->end) {
> - percpu_ref_get(pgmap->ref);
> - return pgmap;
> + if (pgmap) {
> + const struct resource *res = pgmap ? pgmap->res : NULL;
> +
> + if (res && phys >= res->start && phys <= res->end)
> + return pgmap;
> + put_dev_pagemap(pgmap);
> }
>
> /* fall back to slow path lookup */
> diff --git a/mm/gup.c b/mm/gup.c
> index d3fb60e5bfac..9d142eb9e2e9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1410,7 +1410,6 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>
> VM_BUG_ON_PAGE(compound_head(page) != head, page);
>
> - put_dev_pagemap(pgmap);
> SetPageReferenced(page);
> pages[*nr] = page;
> (*nr)++;
> @@ -1420,6 +1419,8 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> ret = 1;
>
> pte_unmap:
> + if (pgmap)
> + put_dev_pagemap(pgmap);
> pte_unmap(ptem);
> return ret;
> }
> @@ -1459,10 +1460,12 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
> SetPageReferenced(page);
> pages[*nr] = page;
> get_page(page);
> - put_dev_pagemap(pgmap);

It's safe to do the put_dev_pagemap() here because the pgmap cannot be
released until the corresponding put_page() for that get_page() we
just did occurs. So we're only holding the pgmap reference long enough
to take individual page references.

We used to take and put individual pgmap references inside get_page()
/ put_page(), but that got simplified in this commit to just take and
put page reference at devm_memremap_pages() setup / teardown time:

71389703839e mm, zone_device: Replace {get, put}_zone_device_page()
with a single reference to fix pmem crash

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