linux-kernel - Re: [PATCH] userfaultfd: wake on unregister for minor faults as well as missing

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Date:   Tue, 25 Oct 2022 14:34:17 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] userfaultfd: wake on unregister for minor faults as well
 as missing

On Tue, Oct 25, 2022 at 11:21:49AM -0700, Axel Rasmussen wrote:
> This was an overlooked edge case when minor faults were added. In
> general, minor faults have the same rough edge here as missing faults:
> if we unregister while there are waiting threads, they will just remain
> waiting forever, as there is no way for userspace to wake them after
> unregistration. To work around this, userspace needs to carefully wake
> everything before unregistering.
> 
> So, wake for minor faults just like we already do for missing faults as
> part of the unregistration process.
> 
> Cc: stable@...r.kernel.org
> Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode")
> Reported-by: Lokesh Gidra <lokeshgidra@...gle.com>
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
> ---
>  fs/userfaultfd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 07c81ab3fd4d..7daee4b9481c 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1606,7 +1606,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>  			start = vma->vm_start;
>  		vma_end = min(end, vma->vm_end);
>  
> -		if (userfaultfd_missing(vma)) {
> +		if (userfaultfd_missing(vma) || userfaultfd_minor(vma)) {
>  			/*
>  			 * Wake any concurrent pending userfault while
>  			 * we unregister, so they will not hang
> -- 
> 2.38.0.135.g90850a2211-goog

Thanks, Axel.  Is wr-protect mode also prone to this?  Would a test case
help too?

-- 
Peter Xu