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

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Message-ID: <e97dba32-b5a8-43b1-9bea-e42daf6078c1@lucifer.local>
Date: Fri, 29 Nov 2024 12:55:24 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.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 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

>
> I think a PFNMAP as used in this patch here will already stop making that
> work ....

Actually I think GUP should work fine, except if you explicitly use a version
that expects an array of struct page *'s, because obviously those can't be
provided?

>
> --
> Cheers,
>
> David / dhildenb
>