[PATCH 2/2] mm/filemap: Prevent waiting for memory for NOWAIT reads

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

Readahead memory allocations won't block for much today, as they're
already marked as NOFS and NORETRY, but they can still sleep, and
they shouldn't if the read is marked as IOCB_NOWAIT.  Clearing the
DIRECT_RECLAIM flag will prevent sleeping.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/filemap.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index d1458ecf2f51..2be27b686518 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2435,21 +2435,27 @@ static int filemap_create_page(struct file *file,
 
 static int filemap_readahead(struct kiocb *iocb, struct file *file,
 		struct address_space *mapping, struct page *page,
-		pgoff_t last_index)
+		pgoff_t index, pgoff_t last_index)
 {
+	DEFINE_READAHEAD(ractl, file, &file->f_ra, mapping, index);
+
 	if (iocb->ki_flags & IOCB_NOIO)
 		return -EAGAIN;
-	page_cache_async_readahead(mapping, &file->f_ra, file, page,
-			page->index, last_index - page->index);
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		ractl.gfp_flags &= ~__GFP_DIRECT_RECLAIM;
+
+	if (page)
+		page_cache_async_ra(&ractl, page, last_index - index);
+	else
+		page_cache_sync_ra(&ractl, last_index - index);
 	return 0;
 }
 
 static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 		struct pagevec *pvec)
 {
-	struct file *filp = iocb->ki_filp;
-	struct address_space *mapping = filp->f_mapping;
-	struct file_ra_state *ra = &filp->f_ra;
+	struct file *file = iocb->ki_filp;
+	struct address_space *mapping = file->f_mapping;
 	pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
 	pgoff_t last_index;
 	struct page *page;
@@ -2462,16 +2468,16 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 
 	filemap_get_read_batch(mapping, index, last_index, pvec);
 	if (!pagevec_count(pvec)) {
-		if (iocb->ki_flags & IOCB_NOIO)
-			return -EAGAIN;
-		page_cache_sync_readahead(mapping, ra, filp, index,
-				last_index - index);
+		err = filemap_readahead(iocb, file, mapping, NULL, index,
+					last_index);
+		if (err)
+			return err;
 		filemap_get_read_batch(mapping, index, last_index, pvec);
 	}
 	if (!pagevec_count(pvec)) {
 		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
 			return -EAGAIN;
-		err = filemap_create_page(filp, mapping,
+		err = filemap_create_page(file, mapping,
 				iocb->ki_pos >> PAGE_SHIFT, pvec);
 		if (err == AOP_TRUNCATED_PAGE)
 			goto retry;
@@ -2480,7 +2486,8 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 
 	page = pvec->pages[pagevec_count(pvec) - 1];
 	if (PageReadahead(page)) {
-		err = filemap_readahead(iocb, filp, mapping, page, last_index);
+		err = filemap_readahead(iocb, file, mapping, page, page->index,
+					last_index);
 		if (err)
 			goto err;
 	}
-- 
2.30.2