linux-kernel - Re: [PATCH v3 2/8] binder: concurrent page installation

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Message-ID: <ZzNxEsF2HjQzI0ak@google.com>
Date: Tue, 12 Nov 2024 15:15:30 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Christian Brauner <brauner@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	Barry Song <v-songbaohua@...o.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH v3 2/8] binder: concurrent page installation

On Tue, Nov 12, 2024 at 03:53:51PM +0100, David Hildenbrand wrote:
> On 12.11.24 15:43, Carlos Llamas wrote:
> > On Tue, Nov 12, 2024 at 12:10:20PM +0100, David Hildenbrand wrote:
> > > On 08.11.24 20:10, Carlos Llamas wrote:
> > > > +	ret = vm_insert_page(vma, addr, page);
> > > > +	switch (ret) {
> > > > +	case -EBUSY:
> > > > +		/*
> > > > +		 * EBUSY is ok. Someone installed the pte first but the
> > > > +		 * lru_page->page_ptr has not been updated yet. Discard
> > > > +		 * our page and look up the one already installed.
> > > > +		 */
> > > > +		ret = 0;
> > > > +		__free_page(page);
> > > > +		npages = get_user_pages_remote(alloc->mm, addr, 1, 0, &page, NULL);
> > > 
> > > This will trigger a page fault if we don't find what we expect (are races
> > > with e.g., MADV_DONTNEED possible?), is that really desired or not a
> > > problem?
> > 
> > This is fine. As of now, binder blocks its page faults:
> > 
> > 	static vm_fault_t binder_vm_fault(struct vm_fault *vmf)
> > 	{
> > 		return VM_FAULT_SIGBUS;
> > 	}
> > 
> > If we race with something like MADV_DONTNEED then we would just
> > propagate the -EFAULT error. I could add FOLL_NOFAULT to the gup remote
> > call to make it evident we don't care though.
> 
> Might make it clearer ... or at least adding a comment how this is supposed
> to work. :)

Sounds good. I'll add both FOLL_NOFAULT flag and a comment for v4.

Cheers,
Carlos Llamas