[PATCH 05/13] iomap: factor out a iomap_writepage_handle_eof helper

Christoph Hellwig
2023-11-26T12:47:46Z

Most of iomap_do_writepage is dedidcated to handling a folio crossing or
beyond i_size. Split this is into a separate helper and update the
commens to deal with folios instead of pages and make them more readable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
fs/iomap/buffered-io.c | 128 ++++++++++++++++++++---------------------
1 file changed, 62 insertions(+), 66 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 8148e4c9765dac..4a5a21809b0182 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1768,6 +1768,64 @@ iomap_add_to_ioend(struct inode *inode, loff_t pos, struct folio *folio,
wbc_account_cgroup_owner(wbc, &folio->page, len);
}

+/*
+ * Check interaction of the folio with the file end.
+ *
+ * If the folio is entirely beyond i_size, return false. If it straddles
+ * i_size, adjust end_pos and zero all data beyond i_size.
+ */
+static bool iomap_writepage_handle_eof(struct folio *folio, struct inode *inode,
+ u64 *end_pos)
+{
+ u64 isize = i_size_read(inode);
+
+ if (*end_pos > isize) {
+ size_t poff = offset_in_folio(folio, isize);
+ pgoff_t end_index = isize >> PAGE_SHIFT;
+
+ /*
+ * If the folio is entirely ouside of i_size, skip it.
+ *
+ * This can happen due to a truncate operation that is in
+ * progress and in that case truncate will finish it off once
+ * we've dropped the folio lock.
+ *
+ * Note that the pgoff_t used for end_index is an unsigned long.
+ * If the given offset is greater than 16TB on a 32-bit system,
+ * then if we checked if the folio is fully outside i_size with
+ * "if (folio->index >= end_index + 1)", "end_index + 1" would
+ * overflow and evaluate to 0. Hence this folio would be
+ * redirtied and written out repeatedly, which would result in
+ * an infinite loop; the user program performing this operation
+ * would hang. Instead, we can detect this situation by
+ * checking if the folio is totally beyond i_size or if its
+ * offset is just equal to the EOF.
+ */
+ if (folio->index > end_index ||
+ (folio->index == end_index && poff == 0))
+ return false;
+
+ /*
+ * The folio straddles i_size.
+ *
+ * It must be zeroed out on each and every writepage invocation
+ * because it may be mmapped:
+ *
+ * A file is mapped in multiples of the page size. For a
+ * file that is not a multiple of the page size, the
+ * remaining memory is zeroed when mapped, and writes to that
+ * region are not written out to the file.
+ *
+ * Also adjust the writeback range to skip all blocks entirely
+ * beyond i_size.
+ */
+ folio_zero_segment(folio, poff, folio_size(folio));
+ *end_pos = isize;
+ }
+
+ return true;
+}
+
/*
* We implement an immediate ioend submission policy here to avoid needing to
* chain multiple ioends and hence nest mempool allocations which can violate
@@ -1906,78 +1964,16 @@ static int iomap_do_writepage(struct folio *folio,
{
struct iomap_writepage_ctx *wpc = data;
struct inode *inode = folio->mapping->host;
- u64 end_pos, isize;
+ u64 end_pos = folio_pos(folio) + folio_size(folio);

trace_iomap_writepage(inode, folio_pos(folio), folio_size(folio));

- /*
- * Is this folio beyond the end of the file?
- *
- * The folio index is less than the end_index, adjust the end_pos
- * to the highest offset that this folio should represent.
- * -----------------------------------------------------
- * | file mapping | <EOF> |
- * -----------------------------------------------------
- * | Page ... | Page N-2 | Page N-1 | Page N | |
- * ^--------------------------------^----------|--------
- * | desired writeback range | see else |
- * ---------------------------------^------------------|
- */
- isize = i_size_read(inode);
- end_pos = folio_pos(folio) + folio_size(folio);
- if (end_pos > isize) {
- /*
- * Check whether the page to write out is beyond or straddles
- * i_size or not.
- * -------------------------------------------------------
- * | file mapping | <EOF> |
- * -------------------------------------------------------
- * | Page ... | Page N-2 | Page N-1 | Page N | Beyond |
- * ^--------------------------------^-----------|---------
- * | | Straddles |
- * ---------------------------------^-----------|--------|
- */
- size_t poff = offset_in_folio(folio, isize);
- pgoff_t end_index = isize >> PAGE_SHIFT;
-
- /*
- * Skip the page if it's fully outside i_size, e.g.
- * due to a truncate operation that's in progress. We've
- * cleaned this page and truncate will finish things off for
- * us.
- *
- * Note that the end_index is unsigned long. If the given
- * offset is greater than 16TB on a 32-bit system then if we
- * checked if the page is fully outside i_size with
- * "if (page->index >= end_index + 1)", "end_index + 1" would
- * overflow and evaluate to 0. Hence this page would be
- * redirtied and written out repeatedly, which would result in
- * an infinite loop; the user program performing this operation
- * would hang. Instead, we can detect this situation by
- * checking if the page is totally beyond i_size or if its
- * offset is just equal to the EOF.
- */
- if (folio->index > end_index ||
- (folio->index == end_index && poff == 0))
- goto unlock;
-
- /*
- * The page straddles i_size. It must be zeroed out on each
- * and every writepage invocation because it may be mmapped.
- * "A file is mapped in multiples of the page size. For a file
- * that is not a multiple of the page size, the remaining
- * memory is zeroed when mapped, and writes to that region are
- * not written out to the file."
- */
- folio_zero_segment(folio, poff, folio_size(folio));
- end_pos = isize;
+ if (!iomap_writepage_handle_eof(folio, inode, &end_pos)) {
+ folio_unlock(folio);
+ return 0;
}

return iomap_writepage_map(wpc, wbc, inode, folio, end_pos);
-
-unlock:
- folio_unlock(folio);
- return 0;
}

int
--
2.39.2