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From: Jens Axboe <axboe@kernel.dk>
To: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Cc: hch@infradead.org, akpm@linux-foundation.org,
	Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper
Date: Mon,  8 Feb 2021 15:18:27 -0700	[thread overview]
Message-ID: <20210208221829.17247-2-axboe@kernel.dk> (raw)
In-Reply-To: <20210208221829.17247-1-axboe@kernel.dk>

For O_DIRECT reads/writes, we check if we need to issue a call to
filemap_write_and_wait_range() to issue and/or wait for writeback for any
page in the given range. The existing mechanism just checks for a page in
the range, which is suboptimal for IOCB_NOWAIT as we'll fallback to the
slow path (and needing retry) if there's just a clean page cache page in
the range.

Provide filemap_range_needs_writeback() which tries a little harder to
check if we actually need to issue and/or wait for writeback in the
range.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/fs.h |  2 ++
 mm/filemap.c       | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd47deea7c17..def89222dfe9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2633,6 +2633,8 @@ static inline int filemap_fdatawait(struct address_space *mapping)
 
 extern bool filemap_range_has_page(struct address_space *, loff_t lstart,
 				  loff_t lend);
+extern bool filemap_range_needs_writeback(struct address_space *,
+					  loff_t lstart, loff_t lend);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);
 extern int __filemap_fdatawrite_range(struct address_space *mapping,
diff --git a/mm/filemap.c b/mm/filemap.c
index aa0e0fb04670..1ed7acac8a1b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -633,6 +633,52 @@ static bool mapping_needs_writeback(struct address_space *mapping)
 	return mapping->nrpages;
 }
 
+/**
+ * filemap_range_needs_writeback - check if range potentially needs writeback
+ * @mapping:           address space within which to check
+ * @start_byte:        offset in bytes where the range starts
+ * @end_byte:          offset in bytes where the range ends (inclusive)
+ *
+ * Find at least one page in the range supplied, usually used to check if
+ * direct writing in this range will trigger a writeback. Used by O_DIRECT
+ * read/write with IOCB_NOWAIT, to see if the caller needs to do
+ * filemap_write_and_wait_range() before proceeding.
+ *
+ * Return: %true if the caller should do filemap_write_and_wait_range() before
+ * doing O_DIRECT to a page in this range, %false otherwise.
+ */
+bool filemap_range_needs_writeback(struct address_space *mapping,
+				   loff_t start_byte, loff_t end_byte)
+{
+	struct page *page = NULL, *head;
+	XA_STATE(xas, &mapping->i_pages, start_byte >> PAGE_SHIFT);
+	pgoff_t max = end_byte >> PAGE_SHIFT;
+
+	if (!mapping_needs_writeback(mapping))
+		return false;
+	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) &&
+	    !mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
+		return false;
+	if (end_byte < start_byte)
+		return false;
+
+	rcu_read_lock();
+	xas_for_each(&xas, head, max) {
+		if (xas_retry(&xas, head))
+			continue;
+		if (xa_is_value(head))
+			continue;
+		page = find_subpage(head, xas.xa_index);
+		if (PageDirty(page) || PageLocked(page) || PageWriteback(page))
+			break;
+		page = NULL;
+	}
+	rcu_read_unlock();
+
+	return page != NULL;
+}
+EXPORT_SYMBOL_GPL(filemap_range_needs_writeback);
+
 /**
  * filemap_write_and_wait_range - write out & wait on a file range
  * @mapping:	the address_space for the pages
-- 
2.30.0


  reply	other threads:[~2021-02-08 22:19 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-08 22:18 [PATCHSET 0/3] Improve IOCB_NOWAIT O_DIRECT Jens Axboe
2021-02-08 22:18 ` Jens Axboe [this message]
2021-02-08 23:02   ` [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper Matthew Wilcox
2021-02-08 23:21     ` Jens Axboe
2021-02-08 22:18 ` [PATCH 2/3] mm: use filemap_range_needs_writeback() for O_DIRECT IO Jens Axboe
2021-02-08 22:18 ` [PATCH 3/3] iomap: " Jens Axboe
2021-02-08 23:28 ` [PATCHSET 0/3] Improve IOCB_NOWAIT O_DIRECT Dave Chinner
2021-02-08 23:37   ` Jens Axboe
2021-02-09  0:14     ` Dave Chinner
2021-02-09  2:15       ` Jens Axboe
2021-02-09  2:30 [PATCHSET v2 0/3] Improve IOCB_NOWAIT O_DIRECT reads Jens Axboe
2021-02-09  2:30 ` [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper Jens Axboe
2021-02-09  7:47   ` Christoph Hellwig
2021-02-09 14:30     ` Jens Axboe
2021-02-24 16:44 [PATCHSET v3 0/3] Improve IOCB_NOWAIT O_DIRECT reads Jens Axboe
2021-02-24 16:44 ` [PATCH 1/3] mm: provide filemap_range_needs_writeback() helper Jens Axboe
2021-02-24 16:50   ` Matthew Wilcox
2021-02-24 16:53     ` Jens Axboe
2021-02-24 17:22   ` Jan Kara

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