linux-kernel - Re: [PATCH v3 3/5] mm: madvise: implement lightweight guard page mechanism

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Message-ID: <7c7185ed-f997-484a-b1d1-91ae6c761266@suse.cz>
Date: Fri, 25 Oct 2024 23:56:52 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney"
 <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
 David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
 Richard Henderson <richard.henderson@...aro.org>,
 Matt Turner <mattst88@...il.com>,
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
 Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
 Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...nel.org>,
 linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
 Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
 linux-kselftest@...r.kernel.org, Sidhartha Kumar
 <sidhartha.kumar@...cle.com>, Jeff Xu <jeffxu@...omium.org>,
 Christoph Hellwig <hch@...radead.org>, linux-api@...r.kernel.org,
 John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v3 3/5] mm: madvise: implement lightweight guard page
 mechanism

On 10/25/24 19:12, Lorenzo Stoakes wrote:
> On Wed, Oct 23, 2024 at 05:24:40PM +0100, Lorenzo Stoakes wrote:
>> Implement a new lightweight guard page feature, that is regions of userland
>> virtual memory that, when accessed, cause a fatal signal to arise.
> 
> <snip>
> 
> Hi Andrew - Could you apply the below fix-patch? I realise we must handle
> fatal signals and conditional rescheduling in the vector_madvise() special
> case.
> 
> Thanks!
> 
> ----8<----
> From 546d7e1831c71599fc733d589e0d75f52e84826d Mon Sep 17 00:00:00 2001
> From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Date: Fri, 25 Oct 2024 18:05:48 +0100
> Subject: [PATCH] mm: yield on fatal signal/cond_sched() in vector_madvise()
> 
> While we have to treat -ERESTARTNOINTR specially here as we are looping
> through a vector of operations and can't simply restart the entire
> operation, we mustn't hold up fatal signals or RT kernels.

For plain madvise() syscall returning -ERESTARTNOINTR does the right thing
and checks fatal_signal_pending() before returning, right?

Uh actually can we be just returning -ERESTARTNOINTR or do we need to use
restart_syscall()?

> ---
>  mm/madvise.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 48eba25e25fe..127aa5d86656 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1713,8 +1713,14 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
>  		 * we have already rescinded locks, it should be no problem to
>  		 * simply try again.
>  		 */
> -		if (ret == -ERESTARTNOINTR)
> +		if (ret == -ERESTARTNOINTR) {
> +			if (fatal_signal_pending(current)) {
> +				ret = -EINTR;
> +				break;
> +			}
> +			cond_resched();

Should be unnecessary as we're calling an operation that takes a rwsem so
there are reschedule points already. And with lazy preempt hopefully
cond_resched()s will become history, so let's not add more only to delete later.

>  			continue;
> +		}
>  		if (ret < 0)
>  			break;
>  		iov_iter_advance(iter, iter_iov_len(iter));
> --
> 2.47.0