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

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Message-ID: <74be541b-22ad-42b5-b3c5-79b201e28f04@redhat.com>
Date: Fri, 29 Nov 2024 13:45:42 +0100
From: David Hildenbrand <david@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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 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 think a PFNMAP as used in this patch here will already stop making 
that work ....

-- 
Cheers,

David / dhildenb