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Date: Mon, 1 Jan 2024 00:18:48 +0000
From: Matthew Wilcox <willy@infradead.org>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 9/9] xfs: connect in-memory btrees to xfiles
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References: <170404829556.1748854.13886473250848576704.stgit@frogsfrogsfrogs>
 <170404829726.1748854.1262147267981918901.stgit@frogsfrogsfrogs>
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On Sun, Dec 31, 2023 at 12:15:54PM -0800, Darrick J. Wong wrote:
> +/* Ensure that there is storage backing the given range. */
> +int
> +xfile_prealloc(
> +	struct xfile		*xf,
> +	loff_t			pos,
> +	u64			count)
> +{
> +	struct inode		*inode = file_inode(xf->file);
> +	struct address_space	*mapping = inode->i_mapping;
> +	const struct address_space_operations *aops = mapping->a_ops;
> +	struct page		*page = NULL;
> +	unsigned int		pflags;
> +	int			error = 0;
> +
> +	if (count > MAX_RW_COUNT)
> +		return -E2BIG;
> +	if (inode->i_sb->s_maxbytes - pos < count)
> +		return -EFBIG;
> +
> +	trace_xfile_prealloc(xf, pos, count);
> +
> +	pflags = memalloc_nofs_save();
> +	while (count > 0) {
> +		void		*fsdata = NULL;
> +		unsigned int	len;
> +		int		ret;
> +
> +		len = min_t(ssize_t, count, PAGE_SIZE - offset_in_page(pos));
> +
> +		/*
> +		 * We call write_begin directly here to avoid all the freezer
> +		 * protection lock-taking that happens in the normal path.
> +		 * shmem doesn't support fs freeze, but lockdep doesn't know
> +		 * that and will trip over that.
> +		 */
> +		error = aops->write_begin(NULL, mapping, pos, len, &page,
> +				&fsdata);
> +		if (error)
> +			break;
> +
> +		/*
> +		 * xfile pages must never be mapped into userspace, so we skip
> +		 * the dcache flush.  If the page is not uptodate, zero it to
> +		 * ensure we never go lacking for space here.
> +		 */
> +		if (!PageUptodate(page)) {
> +			void	*kaddr = kmap_local_page(page);
> +
> +			memset(kaddr, 0, PAGE_SIZE);
> +			SetPageUptodate(page);
> +			kunmap_local(kaddr);
> +		}

Does the xfiles implementation prevent THPs from being created?
If not, this could lead to an entire THP being marked uptodate even
though we've only zeroed one page of it.