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

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Date:   Tue, 18 Jun 2019 21:56:43 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dan Williams <dan.j.williams@...el.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 Jun 18, 2019, at 11:30 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
> 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.

Thanks for the detailed explanation. I’ll give it a try (the moment I find
some free time). I still think that patch 2/3 is beneficial, but based on
your feedback, patch 3/3 should be dropped.