linux-ext4 - Re: [PATCHv3 12/41] thp: handle write-protection faults for file THP

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Date:   Tue, 11 Oct 2016 17:47:50 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCHv3 12/41] thp: handle write-protection faults for file THP

On Thu 15-09-16 14:54:54, Kirill A. Shutemov wrote:
> For filesystems that wants to be write-notified (has mkwrite), we will
> encount write-protection faults for huge PMDs in shared mappings.
> 
> The easiest way to handle them is to clear the PMD and let it refault as
> wriable.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  mm/memory.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 83be99d9d8a1..aad8d5c6311f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3451,8 +3451,17 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
>  		return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
>  				fe->flags);
>  
> +	if (fe->vma->vm_flags & VM_SHARED) {
> +		/* Clear PMD */
> +		zap_page_range_single(fe->vma, fe->address,
> +				HPAGE_PMD_SIZE, NULL);
> +		VM_BUG_ON(!pmd_none(*fe->pmd));
> +
> +		/* Refault to establish writable PMD */
> +		return 0;
> +	}
> +

Since we want to write-protect the page table entry on each page writeback
and write-enable then on the next write, this is relatively expensive.
Would it be that complicated to handle this fully in ->pmd_fault handler
like we do for DAX?

Maybe it doesn't have to be done now but longer term I guess it might make
sense.

Otherwise the patch looks good so feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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