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

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Message-ID: <9387A285-B768-4B58-B91B-61B70D964E6E@vmware.com>
Date:   Tue, 18 Jun 2019 17:42:06 +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 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. Apparently, that is the reason there
is a tree of resources. Please be more specific about where you want to
cache it, please.

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.