Willy reports that there's a case where async buffered reads will be
blocking, and that's due to not using read-ahead to generate the reads
when read-ahead is disabled. io_uring relies on read-ahead triggering
the reads, if not, it needs to fallback to threaded helpers.
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).
Reported-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
mm/readahead.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)
diff --git a/mm/readahead.c b/mm/readahead.c
index 3c9a8dd7c56c..b014d122e656 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -568,15 +568,23 @@ void page_cache_sync_readahead(struct address_space *mapping,
struct file_ra_state *ra, struct file *filp,
pgoff_t index, unsigned long req_count)
{
- /* no read-ahead */
- if (!ra->ra_pages)
- return;
+ bool do_forced_ra = filp && (filp->f_mode & FMODE_RANDOM);
- if (blk_cgroup_congested())
- return;
+ /*
+ * Even if read-ahead is disabled, start this request as read-ahead.
+ * This makes regular read-ahead disabled use the same path as normal
+ * reads, instead of having to punt to ->readpage() manually. We limit
+ * ourselves to 1 page for this case, to avoid causing problems if
+ * we're congested or tight on memory.
+ */
+ if (!ra->ra_pages || blk_cgroup_congested()) {
+ if (!filp)
+ return;
+ req_count = 1;
+ do_forced_ra = true;
+ }
- /* be dumb */
- if (filp && (filp->f_mode & FMODE_RANDOM)) {
+ if (do_forced_ra) {
force_page_cache_readahead(mapping, filp, index, req_count);
return;
}
--
2.28.0
