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Date:   Tue, 22 Nov 2022 16:35:52 +0100
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Subject: Re: [PATCH mm-unstable v1 08/20] mm: extend FAULT_FLAG_UNSHARE
 support to anything in a COW mapping
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To:     David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Cc:     x86@kernel.org, linux-alpha@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
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        etnaviv@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-samsung-soc@vger.kernel.org, linux-rdma@vger.kernel.org,
        linux-media@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-perf-users@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        John Hubbard <jhubbard@nvidia.com>,
        Peter Xu <peterx@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>, Nadav Amit <namit@vmware.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Shuah Khan <shuah@kernel.org>,
        Lucas Stach <l.stach@pengutronix.de>,
        David Airlie <airlied@gmail.com>,
        Oded Gabbay <ogabbay@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Christoph Hellwig <hch@infradead.org>,
        Alex Williamson <alex.williamson@redhat.com>
References: <20221116102659.70287-1-david@redhat.com>
 <20221116102659.70287-9-david@redhat.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20221116102659.70287-9-david@redhat.com>
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On 11/16/22 11:26, David Hildenbrand wrote:
> Extend FAULT_FLAG_UNSHARE to break COW on anything mapped into a
> COW (i.e., private writable) mapping and adjust the documentation
> accordingly.
> 
> FAULT_FLAG_UNSHARE will now also break COW when encountering the shared
> zeropage, a pagecache page, a PFNMAP, ... inside a COW mapping, by
> properly replacing the mapped page/pfn by a private copy (an exclusive
> anonymous page).
> 
> Note that only do_wp_page() needs care: hugetlb_wp() already handles
> FAULT_FLAG_UNSHARE correctly. wp_huge_pmd()/wp_huge_pud() also handles it
> correctly, for example, splitting the huge zeropage on FAULT_FLAG_UNSHARE
> such that we can handle FAULT_FLAG_UNSHARE on the PTE level.
> 
> This change is a requirement for reliable long-term R/O pinning in
> COW mappings.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  include/linux/mm_types.h | 8 ++++----
>  mm/memory.c              | 4 ----
>  2 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5e7f4fac1e78..5e9aaad8c7b2 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1037,9 +1037,9 @@ typedef struct {
>   * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
>   * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
>   * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
> - * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to unshare (and mark
> - *                      exclusive) a possibly shared anonymous page that is
> - *                      mapped R/O.
> + * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to break COW in a
> + *                      COW mapping, making sure that an exclusive anon page is
> + *                      mapped after the fault.
>   * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
>   *                        We should only access orig_pte if this flag set.
>   *
> @@ -1064,7 +1064,7 @@ typedef struct {
>   *
>   * The combination FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE is illegal.
>   * FAULT_FLAG_UNSHARE is ignored and treated like an ordinary read fault when
> - * no existing R/O-mapped anonymous page is encountered.
> + * applied to mappings that are not COW mappings.
>   */
>  enum fault_flag {
>  	FAULT_FLAG_WRITE =		1 << 0,
> diff --git a/mm/memory.c b/mm/memory.c
> index d47ad33c6487..56b21ab1e4d2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3432,10 +3432,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  		}
>  		wp_page_reuse(vmf);
>  		return 0;
> -	} else if (unshare) {
> -		/* No anonymous page -> nothing to do. */
> -		pte_unmap_unlock(vmf->pte, vmf->ptl);
> -		return 0;
>  	}
>  copy:
>  	/*