Re: [PATCH v3 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

From: Catalin Marinas <catalin.marinas@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Josef Bacik <josef@toxicpanda.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chris Mason <clm@fb.com>, David Sterba <dsterba@suse.com>,
	Will Deacon <will@kernel.org>,
	linux-fsdevel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults
Date: Wed, 6 Apr 2022 21:40:23 +0100	[thread overview]
Message-ID: <Yk36tyvswgl3VcM0@arm.com> (raw)
In-Reply-To: <20220406180922.1522433-4-catalin.marinas@arm.com>

On Wed, Apr 06, 2022 at 07:09:22PM +0100, Catalin Marinas wrote:
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 238cee5b5254..d49e8254f823 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -2556,8 +2556,13 @@ static noinline int search_ioctl(struct inode *inode,
>  	key.offset = sk->min_offset;
>  
>  	while (1) {
> +		size_t len = *buf_size - sk_offset;
>  		ret = -EFAULT;
> -		if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset))
> +		/*
> +		 * Ensure that the whole user buffer is faulted in at sub-page
> +		 * granularity, otherwise the loop may live-lock.
> +		 */
> +		if (fault_in_subpage_writeable(ubuf + sk_offset, len))
>  			break;

This doesn't need a new 'len' variable. It's a left-over from the v2
where fault_in_writeable() took the size and a min_size argument, both
being 'len'.

-- 
Catalin