linux-kernel - Re: [PATCH v3 3/3] btrfs: Avoid live-lock in search

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Date:   Wed, 6 Apr 2022 21:40:23 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Josef Bacik <josef@...icpanda.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chris Mason <clm@...com>, David Sterba <dsterba@...e.com>,
        Will Deacon <will@...nel.org>, linux-fsdevel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] btrfs: Avoid live-lock in search_ioctl() on
 hardware with sub-page faults

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