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From: Matthew Wilcox <willy@infradead.org>
To: David Howells <dhowells@redhat.com>
Cc: linux-fsdevel@vger.kernel.org,
	Dave Wysochanski <dwysocha@redhat.com>,
	Marc Dionne <marc.dionne@auristor.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@lst.de>,
	linux-mm@kvack.org, linux-cachefs@redhat.com,
	linux-afs@lists.infradead.org, linux-nfs@vger.kernel.org,
	linux-cifs@vger.kernel.org, ceph-devel@vger.kernel.org,
	v9fs-developer@lists.sourceforge.net,
	Trond Myklebust <trond.myklebust@hammerspace.com>,
	Anna Schumaker <anna.schumaker@netapp.com>,
	Steve French <sfrench@samba.org>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Jeff Layton <jlayton@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 01/31] iov_iter: Add ITER_XARRAY
Date: Fri, 23 Apr 2021 15:06:25 +0100	[thread overview]
Message-ID: <20210423140625.GC235567@casper.infradead.org> (raw)
In-Reply-To: <161918448151.3145707.11541538916600921083.stgit@warthog.procyon.org.uk>

On Fri, Apr 23, 2021 at 02:28:01PM +0100, David Howells wrote:
> +#define iterate_xarray(i, n, __v, skip, STEP) {		\
> +	struct page *head = NULL;				\
> +	size_t wanted = n, seg, offset;				\
> +	loff_t start = i->xarray_start + skip;			\
> +	pgoff_t index = start >> PAGE_SHIFT;			\
> +	int j;							\
> +								\
> +	XA_STATE(xas, i->xarray, index);			\
> +								\
> +	rcu_read_lock();						\
> +	xas_for_each(&xas, head, ULONG_MAX) {				\
> +		if (xas_retry(&xas, head))				\
> +			continue;					\
> +		if (WARN_ON(xa_is_value(head)))				\
> +			break;						\
> +		if (WARN_ON(PageHuge(head)))				\
> +			break;						\
> +		for (j = (head->index < index) ? index - head->index : 0; \
> +		     j < thp_nr_pages(head); j++) {			\

if head->index > index, something has gone disastrously wrong.

		for (j = index - head->index; j < thp_nr_pages(head); j++) { \

would be enough.

However ... the tree you were originally testing this against has the
page cache fixed to use only one entry per THP.  The tree you want to
apply this to inserts 2^n entries per THP.  They're all the head page,
but they're distinct entries as far as xas_for_each() is concerned.
So I think the loop you want looks like this:

+	rcu_read_lock();						\
+	xas_for_each(&xas, head, ULONG_MAX) {				\
+		if (xas_retry(&xas, head))				\
+			continue;					\
+		if (WARN_ON(xa_is_value(head)))				\
+			break;						\
+		if (WARN_ON(PageHuge(head)))				\
+			break;						\
+		__v.bv_page = head + index - head->index;		\
+		offset = offset_in_page(i->xarray_start + skip);	\
+		seg = PAGE_SIZE - offset;				\
+		__v.bv_offset = offset;					\
+		__v.bv_len = min(n, seg);				\
+		(void)(STEP);						\
+		n -= __v.bv_len;					\
+		skip += __v.bv_len;					\
+		if (n == 0)						\
+			break;						\
+	}								\
+	rcu_read_unlock();						\

Now, is this important?  There are no filesystems which do I/O to THPs
today.  So it's not possible to pick up the fact that it doesn't work,
and I hope to have the page cache fixed soon.  And fixing this now
will create more work later as part of fixing the page cache.  But I
wouldn't feel right not mentioning this problem ...

(also, iov_iter really needs to be fixed to handle bvecs which cross
page boundaries, but that's a fight for another day)

  reply	other threads:[~2021-04-23 14:07 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-23 13:27 [PATCH v7 00/31] Network fs helper library & fscache kiocb API David Howells
2021-04-23 13:28 ` [PATCH v7 01/31] iov_iter: Add ITER_XARRAY David Howells
2021-04-23 14:06   ` Matthew Wilcox [this message]
2021-04-23 14:33   ` David Howells
2021-04-25 13:14   ` Al Viro
2021-04-25 13:58   ` David Howells
2021-04-25 14:16     ` Al Viro
2021-04-26 18:54   ` Al Viro
2021-04-26 19:15     ` Jeff Layton
2021-04-26 19:23   ` David Howells
2021-04-26 19:52     ` Al Viro
2021-04-23 13:28 ` [PATCH v7 02/31] mm: Add set/end/wait functions for PG_private_2 David Howells
2021-04-23 13:28 ` [PATCH v7 03/31] mm/filemap: Pass the file_ra_state in the ractl David Howells
2021-04-23 13:28 ` [PATCH v7 04/31] fs: Document file_ra_state David Howells
2021-04-23 13:28 ` [PATCH v7 05/31] mm/readahead: Handle ractl nr_pages being modified David Howells
2021-04-23 13:29 ` [PATCH v7 06/31] mm: Implement readahead_control pageset expansion David Howells
2021-04-23 13:29 ` [PATCH v7 07/31] netfs: Make a netfs helper module David Howells
2021-04-29  8:04   ` Geert Uytterhoeven
2021-04-29  8:41   ` David Howells
2021-04-29  8:43     ` Dominique Martinet
2021-04-23 13:29 ` [PATCH v7 08/31] netfs: Documentation for helper library David Howells
2021-04-23 13:29 ` [PATCH v7 09/31] netfs, mm: Move PG_fscache helper funcs to linux/netfs.h David Howells
2021-04-23 13:29 ` [PATCH v7 10/31] netfs, mm: Add set/end/wait_on_page_fscache() aliases David Howells
2021-04-23 13:30 ` [PATCH v7 11/31] netfs: Provide readahead and readpage netfs helpers David Howells
2021-04-23 13:30 ` [PATCH v7 12/31] netfs: Add tracepoints David Howells
2021-04-23 13:30 ` [PATCH v7 13/31] netfs: Gather stats David Howells
2021-04-23 13:30 ` [PATCH v7 14/31] netfs: Add write_begin helper David Howells
2021-04-23 13:31 ` [PATCH v7 15/31] netfs: Define an interface to talk to a cache David Howells
2021-04-23 13:31 ` [PATCH v7 16/31] netfs: Add a tracepoint to log failures that would be otherwise unseen David Howells
2021-04-23 13:31 ` [PATCH v7 17/31] fscache, cachefiles: Add alternate API to use kiocb for read/write to cache David Howells
2021-04-23 13:32 ` [PATCH v7 18/31] afs: Disable use of the fscache I/O routines David Howells
2021-04-23 13:32 ` [PATCH v7 19/31] afs: Pass page into dirty region helpers to provide THP size David Howells
2021-04-23 13:32 ` [PATCH v7 20/31] afs: Print the operation debug_id when logging an unexpected data version David Howells
2021-04-23 13:32 ` [PATCH v7 21/31] afs: Move key to afs_read struct David Howells
2021-04-23 13:33 ` [PATCH v7 22/31] afs: Don't truncate iter during data fetch David Howells
2021-04-23 13:33 ` [PATCH v7 23/31] afs: Log remote unmarshalling errors David Howells
2021-04-23 13:33 ` [PATCH v7 24/31] afs: Set up the iov_iter before calling afs_extract_data() David Howells
2021-04-23 13:33 ` [PATCH v7 25/31] afs: Use ITER_XARRAY for writing David Howells
2021-04-23 13:33 ` [PATCH v7 26/31] afs: Wait on PG_fscache before modifying/releasing a page David Howells
2021-04-23 13:34 ` [PATCH v7 27/31] afs: Extract writeback extension into its own function David Howells
2021-04-23 13:34 ` [PATCH v7 28/31] afs: Prepare for use of THPs David Howells
2021-04-23 13:34 ` [PATCH v7 29/31] afs: Use the fs operation ops to handle FetchData completion David Howells
2021-04-23 13:34 ` [PATCH v7 30/31] afs: Use new netfs lib read helper API David Howells
2021-04-23 13:34 ` [PATCH v7 31/31] afs: Use the netfs_write_begin() helper David Howells
2021-04-25 23:14 ` [PATCH] iov_iter: Four fixes for ITER_XARRAY David Howells
2021-04-26 17:14   ` Al Viro
2021-04-26 19:02   ` Jeff Layton
2021-04-26 21:38   ` David Wysochanski
2021-04-26 21:06 ` [PATCH] netfs: Miscellaneous fixes David Howells
2021-04-26 21:09   ` Matthew Wilcox
2021-04-26 21:20   ` [PATCH v2] " David Howells
2021-04-26 22:06     ` Matthew Wilcox
2021-04-26 22:17     ` Jeff Layton

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