Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

Guillaume Morin
2024-04-24T20:44:55Z

On 24 Apr 22:09, David Hildenbrand wrote:
> > > 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.
>
> ^ realizing that we already use FOLL_FORCE, so we can just use FOLL_WRITE to
> break COW.

It was never clear to me why uprobes was not doing FOLL_WRITE in the
first place, I must say.

One issue here is that FOLL_FORCE|FOLL_WRITE is not implemented for
hugetlb mappings. However this was also on my TODO and I have a draft
patch that implements it.

>
> > >
> > > 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.
>
> I'm hacking on a redesign that removes the manual COW breaking logic and
> *might* make it easier to integrate hugetlb. (very likely, but until I have
> the redesign running I cannot promise anything :) )
>
> I'll let you know once I have something ready so you could integrate the
> hugetlb portion.

Sounds good.