When the user increased the read-ahead size through sysfs this value
currently get lost if the device is reprobe, including on a resume
from suspend.
As there is no hardware limitation for the read-ahead size there is
no real need to reset it or track a separate hardware limitation
like for max_sectors.
This restores the pre-atomic queue limit behavior in the sd driver as
sd did not use blk_queue_io_opt and thus never updated the read ahead
size to the value based of the optimal I/O, but changes behavior for
all other drivers. As the new behavior seems useful and sd is the
driver for which the readahead size tweaks are most useful that seems
like a worthwhile trade off.
Fixes: 804e498e0496 ("sd: convert to the atomic queue limits API")
Reported-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Tested-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
---
block/blk-settings.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 6b2dbe645d23..4817e7ca03f8 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -61,8 +61,14 @@ void blk_apply_bdi_limits(struct backing_dev_info *bdi,
/*
* For read-ahead of large files to be effective, we need to read ahead
* at least twice the optimal I/O size.
+ *
+ * There is no hardware limitation for the read-ahead size and the user
+ * might have increased the read-ahead size through sysfs, so don't ever
+ * decrease it.
*/
- bdi->ra_pages = max(lim->io_opt * 2 / PAGE_SIZE, VM_READAHEAD_PAGES);
+ bdi->ra_pages = max3(bdi->ra_pages,
+ lim->io_opt * 2 / PAGE_SIZE,
+ VM_READAHEAD_PAGES);
bdi->io_pages = lim->max_sectors >> PAGE_SECTORS_SHIFT;
}
--
2.47.2
