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

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Message-ID: <D463DD43-C09F-4B6E-B1BC-7E1CA5C8A9C4@vmware.com>
Date:   Tue, 16 Jul 2019 22:13:09 +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 Jul 16, 2019, at 3:07 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
> On Tue, Jul 16, 2019 at 3:01 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>> On Tue, 18 Jun 2019 21:56:43 +0000 Nadav Amit <namit@...are.com> wrote:
>> 
>>>> ...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.
>> 
>> It has been a while.  What should we do with
>> 
>> resource-fix-locking-in-find_next_iomem_res.patch
> 
> This one looks obviously correct to me, you can add:
> 
> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> 
>> resource-avoid-unnecessary-lookups-in-find_next_iomem_res.patch
> 
> This one is a good bug report that we need to go fix pgprot lookups
> for dax, but I don't think we need to increase the trickiness of the
> core resource lookup code in the meantime.

I think that traversing big parts of the tree that are known to be
irrelevant is wasteful no matter what, and this code is used in other cases.

I don’t think the new code is so tricky - can you point to the part of the
code that you find tricky?