linux-kernel - Re: [PATCH 0/3] resource: find_next_iomem

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Date:   Tue, 18 Jun 2019 11:30:23 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Borislav Petkov <bp@...e.de>,
        Toshi Kani <toshi.kani@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 0/3] resource: find_next_iomem_res() improvements

On Tue, Jun 18, 2019 at 10:42 AM Nadav Amit <namit@...are.com> wrote:
>
> > On Jun 17, 2019, at 11:44 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > On Wed, Jun 12, 2019 at 9:59 PM Nadav Amit <namit@...are.com> wrote:
> >> Running some microbenchmarks on dax keeps showing find_next_iomem_res()
> >> as a place in which significant amount of time is spent. It appears that
> >> in order to determine the cacheability that is required for the PTE,
> >> lookup_memtype() is called, and this one traverses the resources list in
> >> an inefficient manner. This patch-set tries to improve this situation.
> >
> > Let's just do this lookup once per device, cache that, and replay it
> > to modified vmf_insert_* routines that trust the caller to already
> > know the pgprot_values.
>
> IIUC, one device can have multiple regions with different characteristics,
> which require difference cachability.

Not for pmem. It will always be one common cacheability setting for
the entirety of persistent memory.

> Apparently, that is the reason there
> is a tree of resources. Please be more specific about where you want to
> cache it, please.

The reason for lookup_memtype() was to try to prevent mixed
cacheability settings of pages across different processes . The
mapping type for pmem/dax is established by one of:

drivers/nvdimm/pmem.c:413:              addr =
devm_memremap_pages(dev, &pmem->pgmap);
drivers/nvdimm/pmem.c:425:              addr =
devm_memremap_pages(dev, &pmem->pgmap);
drivers/nvdimm/pmem.c:432:              addr = devm_memremap(dev,
pmem->phys_addr,
drivers/nvdimm/pmem.c-433-                              pmem->size,
ARCH_MEMREMAP_PMEM);

...and is constant for the life of the device and all subsequent mappings.

> Perhaps you want to cache the cachability-mode in vma->vm_page_prot (which I
> see being done in quite a few cases), but I don’t know the code well enough
> to be certain that every vma should have a single protection and that it
> should not change afterwards.

No, I'm thinking this would naturally fit as a property hanging off a
'struct dax_device', and then create a version of vmf_insert_mixed()
and vmf_insert_pfn_pmd() that bypass track_pfn_insert() to insert that
saved value.