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

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Message-ID: <6cab3e8a-dff7-41d1-af22-f18b8f2820dc@lucifer.local>
Date: Fri, 29 Nov 2024 13:02:30 +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 01:59:01PM +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 :)

Anyway let me go try this and see what happens...

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