linux-kernel - Re: [PATCH mm-unstable v1 08/20] mm: extend FAULT_FLAG_UNSHARE support to anything in a COW mapping

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Date:   Tue, 22 Nov 2022 16:35:52 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        sparclinux@...r.kernel.org, linux-um@...ts.infradead.org,
        etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-samsung-soc@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-perf-users@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Shuah Khan <shuah@...nel.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH mm-unstable v1 08/20] mm: extend FAULT_FLAG_UNSHARE
 support to anything in a COW mapping

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@...hat.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.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:
>  	/*