linux-ext4 - Re: [PATCH v5 1/5] mm: add vm_insert_mixed

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Date:   Tue, 25 Jul 2017 01:14:00 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()

On Mon, Jul 24, 2017 at 11:06:12AM -0600, Ross Zwisler wrote:
> @@ -1658,14 +1658,35 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  	if (!pte)
>  		goto out;
>  	retval = -EBUSY;
> -	if (!pte_none(*pte))
> -		goto out_unlock;
> +	if (!pte_none(*pte)) {
> +		if (mkwrite) {
> +			/*
> +			 * For read faults on private mappings the PFN passed
> +			 * in may not match the PFN we have mapped if the
> +			 * mapped PFN is a writeable COW page.  In the mkwrite
> +			 * case we are creating a writable PTE for a shared
> +			 * mapping and we expect the PFNs to match.
> +			 */

Can we?

I guess it's up to filesystem if it wants to reuse the same spot to write
data or not. I think your assumptions works for ext4 and xfs. I wouldn't
be that sure for btrfs or other filesystems with CoW support.

-- 
 Kirill A. Shutemov