Re: [Linux-nvdimm] [PATCH v2 08/10] x86: support kmap_atomic_pfn_t() for persistent memory

[Dan Williams]

On Wed, May 6, 2015 at 1:05 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> It would be unfortunate if the kmap infrastructure escaped its current
> 32-bit/HIGHMEM bonds and leaked into 64-bit code.  Instead, if the user
> has enabled CONFIG_PMEM_IO we direct the kmap_atomic_pfn_t()
> implementation to scan a list of pre-mapped persistent memory address
> ranges inserted by the pmem driver.
>
> The __pfn_t to resource lookup is indeed inefficient walking of a linked list,
> but there are two mitigating factors:
>
> 1/ The number of persistent memory ranges is bounded by the number of
>    DIMMs which is on the order of 10s of DIMMs, not hundreds.
>
> 2/ The lookup yields the entire range, if it becomes inefficient to do a
>    kmap_atomic_pfn_t() a PAGE_SIZE at a time the caller can take
>    advantage of the fact that the lookup can be amortized for all kmap
>    operations it needs to perform in a given range.
>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  arch/Kconfig             |    3 +
>  arch/x86/Kconfig         |    2 +
>  arch/x86/kernel/Makefile |    1
>  arch/x86/kernel/kmap.c   |   95 ++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/block/pmem.c     |    6 +++
>  include/linux/highmem.h  |   23 +++++++++++
>  6 files changed, 130 insertions(+)
>  create mode 100644 arch/x86/kernel/kmap.c
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index f7f800860c00..69d3a3fa21af 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -206,6 +206,9 @@ config HAVE_DMA_CONTIGUOUS
>  config HAVE_DMA_PFN
>         bool
>
> +config HAVE_KMAP_PFN
> +       bool
> +
>  config GENERIC_SMP_IDLE_THREAD
>         bool
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 1fae5e842423..eddaea839500 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1434,7 +1434,9 @@ config X86_PMEM_LEGACY
>           Say Y if unsure.
>
>  config X86_PMEM_DMA
> +       depends on !HIGHMEM
>         def_bool PMEM_IO
> +       select HAVE_KMAP_PFN
>         select HAVE_DMA_PFN
>
>  config HIGHPTE
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 9bcd0b56ca17..44c323342996 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -96,6 +96,7 @@ obj-$(CONFIG_PARAVIRT)                += paravirt.o paravirt_patch_$(BITS).o
>  obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= paravirt-spinlocks.o
>  obj-$(CONFIG_PARAVIRT_CLOCK)   += pvclock.o
>  obj-$(CONFIG_X86_PMEM_LEGACY)  += pmem.o
> +obj-$(CONFIG_X86_PMEM_DMA)     += kmap.o
>
>  obj-$(CONFIG_PCSPKR_PLATFORM)  += pcspeaker.o
>
> diff --git a/arch/x86/kernel/kmap.c b/arch/x86/kernel/kmap.c
> new file mode 100644
> index 000000000000..d597c475377b
> --- /dev/null
> +++ b/arch/x86/kernel/kmap.c
> @@ -0,0 +1,95 @@
> +/*
> + * Copyright(c) 2015 Intel Corporation. All rights reserved.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of version 2 of the GNU General Public License as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * General Public License for more details.
> + */
> +#include <linux/rcupdate.h>
> +#include <linux/rculist.h>
> +#include <linux/highmem.h>
> +#include <linux/device.h>
> +#include <linux/slab.h>
> +#include <linux/mm.h>
> +
> +static LIST_HEAD(ranges);
> +
> +struct kmap {
> +       struct list_head list;
> +       struct resource *res;
> +       struct device *dev;
> +       void *base;
> +};
> +
> +static void teardown_kmap(void *data)
> +{
> +       struct kmap *kmap = data;
> +
> +       dev_dbg(kmap->dev, "kmap unregister %pr\n", kmap->res);
> +       list_del_rcu(&kmap->list);
> +       synchronize_rcu();
> +       kfree(kmap);
> +}
> +
> +int devm_register_kmap_pfn_range(struct device *dev, struct resource *res,
> +               void *base)
> +{
> +       struct kmap *kmap = kzalloc(sizeof(*kmap), GFP_KERNEL);
> +       int rc;
> +
> +       if (!kmap)
> +               return -ENOMEM;
> +
> +       INIT_LIST_HEAD(&kmap->list);
> +       kmap->res = res;
> +       kmap->base = base;
> +       kmap->dev = dev;
> +       rc = devm_add_action(dev, teardown_kmap, kmap);
> +       if (rc) {
> +               kfree(kmap);
> +               return rc;
> +       }
> +       dev_dbg(kmap->dev, "kmap register %pr\n", kmap->res);
> +       list_add_rcu(&kmap->list, &ranges);
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(devm_register_kmap_pfn_range);
> +
> +void *kmap_atomic_pfn_t(__pfn_t pfn)
> +{
> +       struct page *page = __pfn_t_to_page(pfn);
> +       resource_size_t addr;
> +       struct kmap *kmap;
> +
> +       if (page)
> +               return kmap_atomic(page);
> +       addr = __pfn_t_to_phys(pfn);
> +       rcu_read_lock();
> +       list_for_each_entry_rcu(kmap, &ranges, list)
> +               if (addr >= kmap->res->start && addr <= kmap->res->end)
> +                       return kmap->base + addr - kmap->res->start;
> +
> +       /* only unlock in the error case */
> +       rcu_read_unlock();
> +       return NULL;
> +}
> +EXPORT_SYMBOL(kmap_atomic_pfn_t);
> +
> +void kunmap_atomic_pfn_t(void *addr)
> +{
> +       rcu_read_unlock();
> +
> +       /*
> +        * If the original __pfn_t had an entry in the memmap then
> +        * 'addr' will be outside of vmalloc space i.e. it came from
> +        * page_address()
> +        */
> +       if (!is_vmalloc_addr(addr))
> +               kunmap_atomic(addr);

rcu_read_unlock() should move here.
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