linux-kernel - Re: [PATCH 3/9] userfaultfd/shmem: support minor fault registration for shmem

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Date:   Fri, 9 Apr 2021 12:50:24 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Colascione <dancol@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Shaohua Li <shli@...com>, Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Wang Qing <wangqing@...o.com>, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        Brian Geffon <bgeffon@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH 3/9] userfaultfd/shmem: support minor fault registration
 for shmem

On Thu, Apr 08, 2021 at 04:43:21PM -0700, Axel Rasmussen wrote:

[...]

> @@ -1820,16 +1821,27 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>  
>  	page = pagecache_get_page(mapping, index,
>  					FGP_ENTRY | FGP_HEAD | FGP_LOCK, 0);
> -	if (xa_is_value(page)) {
> +	swapped = xa_is_value(page);
> +	if (swapped) {
>  		error = shmem_swapin_page(inode, index, &page,
>  					  sgp, gfp, vma, fault_type);
>  		if (error == -EEXIST)
>  			goto repeat;
> -
>  		*pagep = page;
> -		return error;
> +		if (error)
> +			return error;
>  	}
>  
> +	if (page && vma && userfaultfd_minor(vma)) {
> +		unlock_page(page);
> +		put_page(page);
> +		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
> +		return 0;
> +	}

If we need to consider swapping for UFFDIO_CONTINUE later (as Hugh pointed out
previously, which looks the right thing to do), it's indeed a bit awkward to
swapin here.  Maybe move this chunk to right after pagecache_get_page()
returns?  Then no need to touch the rest.

> +
> +	if (swapped)
> +		return 0;
> +
>  	if (page)
>  		hindex = page->index;
>  	if (page && sgp == SGP_WRITE)
> -- 
> 2.31.1.295.g9ea45b61b8-goog
> 

-- 
Peter Xu