From: Guillaume Morin <guillaume@morinfr.org>
To: David Hildenbrand <david@redhat.com>
Cc: Guillaume Morin <guillaume@morinfr.org>,
oleg@redhat.com, linux-kernel@vger.kernel.org,
linux-trace-kernel@vger.kernel.org, muchun.song@linux.dev
Subject: Re: [RFC][PATCH] uprobe: support for private hugetlb mappings
Date: Mon, 22 Apr 2024 22:53:20 +0200 [thread overview]
Message-ID: <ZibOQI9kwzE98n12@bender.morinfr.org> (raw)
In-Reply-To: <22fcde31-16c4-42d0-ad99-568173ec4dd0@redhat.com>
On 22 Apr 20:59, David Hildenbrand wrote:
> > The benefit - to me - is very clear. People do use hugetlb mappings to
> > run code in production environments. The perf benefits are there for some
> > workloads. Intel has published a whitepaper about it etc.
> > Uprobes are a very good tool to do live tracing. If you can restart the
> > process and reproduce, you should be able to disable hugetlb remapping
> > but if you need to look at a live process, there are not many options.
> > Not being able to use uprobes is crippling.
>
> Please add all that as motivation to the patch description or cover letter.
>
> > > Yes, libhugetlbfs exists. But why do we have to support uprobes with it?
> > > Nobody cared until now, why care now?
> >
> > I think you could ask the same question for every new feature patch :)
>
> I have to, because it usually indicates a lack of motivation in the
> cover-letter/patch description :P
My cover letter was indeed lacking. I will make sure to add this kind of
details next time.
> > Since the removal a few releases ago of the __morecore() hook in glibc,
> > the main feature of libhugetlbfs is ELF segments remapping. I think
> > there are definitely a lot of users that simply deal with this
> > unnecessary limitation.
> >
> > I am certainly not shoving this patch through anyone's throat if there
> > is no interest. But we definitely find it a very useful feature ...
>
> Let me try to see if we can get this done cleaner.
>
> One ugly part (in general here) is the custom page replacement in the
> registration part.
>
> We are guaranteed to have a MAP_PRIVATE mapping. Instead of replacing pages
> ourselves (which we likely shouldn't do ...) ... maybe we could use
> FAULT_FLAG_UNSHARE faults such that we will get an anonymous folio
> populated. (like KSM does nowadays)
>
> Punching FOLL_PIN|FOLL_LONGTERM into GUP would achieve the same thing, but
> using FOLL_WRITE would not work on many file systems. So maybe we have to
> trigger an unsharing fault ourselves.
>
> That would do the page replacement for us and we "should" be able to lookup
> an anonymous folio that we can then just modify, like ptrace would.
>
> But then, there is also unregistration part, with weird conditional page
> replacement. Zapping the anon page if the content matches the content of the
> original page is one thing. But why are we placing an existing anonymous
> page by a new anonymous page when the content from the original page differs
> (but matches the one from the just copied page?)?
>
> I'll have to further think about that one. It's all a bit nasty.
Sounds good to me. I am willing to help with the code when you have a
plan or testing as you see fit. Let me know.
> One thing to note is that hugetlb folios don't grow on trees. Likely, Many
> setups *don't* reserve extra hugetlb folios and you might just easily be
> running out of free hugetlb folios that you can use to break COW here
> (replace a file hugetlb by a fresh anon hugetlb page). Likely it's easy to
> make register or unregister fail.
Agreed.
--
Guillaume Morin <guillaume@morinfr.org>
next prev parent reply other threads:[~2024-04-22 20:53 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-19 18:37 [RFC][PATCH] uprobe: support for private hugetlb mappings Guillaume Morin
2024-04-22 9:39 ` David Hildenbrand
2024-04-22 18:11 ` Guillaume Morin
2024-04-22 18:59 ` David Hildenbrand
2024-04-22 20:53 ` Guillaume Morin [this message]
2024-04-24 20:09 ` David Hildenbrand
2024-04-24 20:44 ` Guillaume Morin
2024-04-24 21:00 ` David Hildenbrand
2024-04-25 15:19 ` Guillaume Morin
2024-04-25 15:42 ` David Hildenbrand
2024-04-25 19:56 ` David Hildenbrand
2024-04-26 0:09 ` Guillaume Morin
2024-04-26 7:19 ` David Hildenbrand
2024-04-26 19:55 ` Guillaume Morin
2024-04-30 15:22 ` Guillaume Morin
2024-04-30 18:21 ` David Hildenbrand
2024-04-30 18:58 ` Guillaume Morin
2024-04-30 19:25 ` David Hildenbrand
2024-05-02 3:59 ` Guillaume Morin
2024-05-16 17:44 ` Guillaume Morin
2024-05-16 19:52 ` David Hildenbrand
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ZibOQI9kwzE98n12@bender.morinfr.org \
--to=guillaume@morinfr.org \
--cc=david@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-trace-kernel@vger.kernel.org \
--cc=muchun.song@linux.dev \
--cc=oleg@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).