mm: use limited read-ahead to satisfy read

For the case where read-ahead is disabled on the file, or if the cgroup
is congested, ensure that we can at least do 1 page of read-ahead to
make progress on the read in an async fashion. This could potentially be
larger, but it's not needed in terms of functionality, so let's error on
the side of caution as larger counts of pages may run into reclaim
issues (particularly if we're congested).

This makes sure we're not hitting the potentially sync ->readpage() path
for IO that is marked IOCB_WAITQ, which could cause us to block. It also
means we'll use the same path for IO, regardless of whether or not
read-ahead happens to be disabled on the lower level device.
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Reported-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: NHao_Xu <haoxu@linux.alibaba.com>
[axboe: updated for new ractl API]
Signed-off-by: NJens Axboe <axboe@kernel.dk>

mm/readahead.c

@@ -552,15 +552,23 @@ static void ondemand_readahead(struct readahead_control *ractl,
 void page_cache_sync_ra(struct readahead_control *ractl,
 		struct file_ra_state *ra, unsigned long req_count)
 {
-	/* no read-ahead */
-	if (!ra->ra_pages)
-		return;
+	bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
 
-	if (blk_cgroup_congested())
-		return;
+	/*
+	 * Even if read-ahead is disabled, issue this request as read-ahead
+	 * as we'll need it to satisfy the requested range. The forced
+	 * read-ahead will do the right thing and limit the read to just the
+	 * requested range, which we'll set to 1 page for this case.
+	 */
+	if (!ra->ra_pages || blk_cgroup_congested()) {
+		if (!ractl->file)
+			return;
+		req_count = 1;
+		do_forced_ra = true;
+	}
 
 	/* be dumb */
-	if (ractl->file && (ractl->file->f_mode & FMODE_RANDOM)) {
+	if (do_forced_ra) {
 		force_page_cache_ra(ractl, ra, req_count);
 		return;
 	}