[PATCH 3/6] mm: make buffered writes work with RWF_UNCACHED

[Jens Axboe <axboe@xxxxxxxxx>]

If RWF_UNCACHED is set for io_uring (or pwritev2(2)), we'll drop the
cache instantiated for buffered writes. If new pages aren't
instantiated, we leave them alone. This provides similar semantics to
reads with RWF_UNCACHED set.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 include/linux/fs.h |  1 +
 mm/filemap.c       | 41 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index bf58db1bc032..5ea5fc167524 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -285,6 +285,7 @@ enum positive_aop_returns {
 #define AOP_FLAG_NOFS			0x0002 /* used by filesystem to direct
 						* helper code (eg buffer layer)
 						* to clear GFP_FS from alloc */
+#define AOP_FLAG_UNCACHED		0x0004
 
 /*
  * oh the beauties of C type declarations.
diff --git a/mm/filemap.c b/mm/filemap.c
index 522152ed86d8..0b5f29b30c34 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3277,10 +3277,12 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
 					pgoff_t index, unsigned flags)
 {
 	struct page *page;
-	int fgp_flags = FGP_LOCK|FGP_WRITE|FGP_CREAT;
+	int fgp_flags = FGP_LOCK|FGP_WRITE;
 
 	if (flags & AOP_FLAG_NOFS)
 		fgp_flags |= FGP_NOFS;
+	if (!(flags & AOP_FLAG_UNCACHED))
+		fgp_flags |= FGP_CREAT;
 
 	page = pagecache_get_page(mapping, index, fgp_flags,
 			mapping_gfp_mask(mapping));
@@ -3301,6 +3303,9 @@ ssize_t generic_perform_write(struct file *file,
 	ssize_t written = 0;
 	unsigned int flags = 0;
 
+	if (iocb->ki_flags & IOCB_UNCACHED)
+		flags |= AOP_FLAG_UNCACHED;
+
 	do {
 		struct page *page;
 		unsigned long offset;	/* Offset into pagecache page */
@@ -3333,10 +3338,16 @@ ssize_t generic_perform_write(struct file *file,
 			break;
 		}
 
+retry:
 		status = a_ops->write_begin(file, mapping, pos, bytes, flags,
 						&page, &fsdata);
-		if (unlikely(status < 0))
+		if (unlikely(status < 0)) {
+			if (status == -ENOMEM && (flags & AOP_FLAG_UNCACHED)) {
+				flags &= ~AOP_FLAG_UNCACHED;
+				goto retry;
+			}
 			break;
+		}
 
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_page(page);
@@ -3372,6 +3383,32 @@ ssize_t generic_perform_write(struct file *file,
 		balance_dirty_pages_ratelimited(mapping);
 	} while (iov_iter_count(i));
 
+	if (written && (iocb->ki_flags & IOCB_UNCACHED)) {
+		loff_t end;
+
+		pos = iocb->ki_pos;
+		end = pos + written;
+
+		status = filemap_write_and_wait_range(mapping, pos, end);
+		if (status)
+			goto out;
+
+		/*
+		 * No pages were created for this range, we're done
+		 */
+		if (flags & AOP_FLAG_UNCACHED)
+			 goto out;
+
+		/*
+		 * Try to invalidate cache pages for the range we just wrote.
+		 * We don't care if invalidation fails as the write has still
+		 * worked and leaving clean uptodate pages in the page cache
+		 * isn't a corruption vector for uncached IO.
+		 */
+		 invalidate_inode_pages2_range(mapping,
+					pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
+	}
+out:
 	return written ? written : status;
 }
 EXPORT_SYMBOL(generic_perform_write);
-- 
2.24.1