Date:   Mon, 6 Feb 2023 14:25:46 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Stevens <stevensd@chromium.org>
Cc:     linux-mm@kvack.org, Peter Xu <peterx@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Yang Shi <shy828301@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm/khugepaged: skip shmem with userfaultfd
Message-ID: <Y+EN6oDi5k5L9651@casper.infradead.org>
References: <20230206112856.1802547-1-stevensd@google.com>
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On Mon, Feb 06, 2023 at 08:28:56PM +0900, David Stevens wrote:
> @@ -1747,6 +1748,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
>  	int nr_none = 0, result = SCAN_SUCCEED;
>  	bool is_shmem = shmem_file(file);
> +	bool i_mmap_locked = false;

you don't need this ...

> +	 * While iterating, we may drop the page cache lock multiple times. It
> +	 * is safe to replace pages in the page cache with hpage while doing so
> +	 * because nobody is able to map or otherwise access the content of
> +	 * hpage until we unlock it. However, we cannot insert hpage into empty
> +	 * indicies until we know we won't have to drop the page cache lock

"indices".

> @@ -1967,6 +1974,46 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  		put_page(page);
>  		goto xa_unlocked;
>  	}
> +
> +	if (nr_none) {
> +		struct vm_area_struct *vma;
> +		int nr_none_check = 0;
> +
> +		xas_unlock_irq(&xas);
> +		i_mmap_lock_read(mapping);
> +		i_mmap_locked = true;
> +		xas_lock_irq(&xas);
> +
> +		xas_set(&xas, start);
> +		for (index = start; index < end; index++) {
> +			if (!xas_next(&xas))
> +				nr_none_check++;
> +		}
> +
> +		if (nr_none != nr_none_check) {
> +			result = SCAN_PAGE_FILLED;

... you can unlock the i_mmap_lock here before the goto.


I think you need to add a case in madvise_collapse_errno().  It should
probably return -EBUSY, I would think?