linux-kernel - Re: [PATCH] perf: map pages in advance

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Message-ID: <c8ecd378-c197-4e06-94ef-e03b5ee28616@redhat.com>
Date: Fri, 29 Nov 2024 14:12:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kan Liang <kan.liang@...ux.intel.com>, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] perf: map pages in advance

On 29.11.24 14:02, Lorenzo Stoakes wrote:
> On Fri, Nov 29, 2024 at 01:59:01PM +0100, David Hildenbrand wrote:
>> On 29.11.24 13:55, Lorenzo Stoakes wrote:
>>> On Fri, Nov 29, 2024 at 01:45:42PM +0100, David Hildenbrand wrote:
>>>> On 29.11.24 13:26, Peter Zijlstra wrote:
>>>>> On Fri, Nov 29, 2024 at 01:12:57PM +0100, David Hildenbrand wrote:
>>>>>
>>>>>> Well, I think we simply will want vm_insert_pages_prot() that stops treating
>>>>>> these things like folios :) . *likely*  we'd want a distinct memdesc/type.
>>>>>>
>>>>>> We could start that work right now by making some user (iouring,
>>>>>> ring_buffer) set a new page->_type, and checking that in
>>>>>> vm_insert_pages_prot() + vm_normal_page(). If set, don't touch the refcount
>>>>>> and the mapcount.
>>>>>>
>>>>>> Because then, we can just make all the relevant drivers set the type, refuse
>>>>>> in vm_insert_pages_prot() anything that doesn't have the type set, and
>>>>>> refuse in vm_normal_page() any pages with this memdesc.
>>>>>>
>>>>>> Maybe we'd have to teach CoW to copy from such pages, maybe not. GUP of
>>>>>> these things will stop working, I hope that is not a problem.
>>>>>
>>>>> Well... perf-tool likes to call write() upon these pages in order to
>>>>> write out the data from the mmap() into a file.
>>>
>>> I'm confused about what you mean, write() using the fd should work fine, how
>>> would they interact with the mmap? I mean be making a silly mistake here
>>
>> write() to file from the mmap()'ed address range to *some* file.
>>
> 
> Yeah sorry my brain melted down briefly, for some reason was thinking of read()
> writing into the buffer...
> 
>> This will GUP the pages you inserted.
>>
>> GUP does not work on PFNMAP.
> 
> Well it _does_ if struct page **pages is set to NULL :)

Hm? :)

check_vma_flags() unconditionally refuses VM_PFNMAP.

-- 
Cheers,

David / dhildenb