mm: add bdi_get_max_bytes() function

This adds a function to return the specified value for max_bytes. It
converts the stored max_ratio of the bdi to the corresponding bytes
value. It introduces the bdi_get_bytes helper function to do the
conversion. This is an approximation as it is based on the value that is
returned by global_dirty_limits(), which can change. The helper function
will also be used by the min_bytes bdi knob.

Link: https://lkml.kernel.org/r/20221119005215.3052436-6-shr@devkernel.io
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Cc: Chris Mason <clm@meta.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 1b50c028e5add88fc8b8c79225a623798696199d..473686c32775e845190e2e379fe9ad17a23c75ff 100644 (file)
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -105,6 +105,7 @@ static inline unsigned long wb_stat_error(void)
 /* BDI ratio is expressed as part per 1000000 for finer granularity. */
 #define BDI_RATIO_SCALE 10000
 
+u64 bdi_get_max_bytes(struct backing_dev_info *bdi);
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio);
 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
 int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit);

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index dd98b26543027ac51cfe7a2a174c57d4035a3e80..719404e0d03d8e1b6fec31edf69fcda8f3be15fb 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -650,6 +650,18 @@ void wb_domain_exit(struct wb_domain *dom)
  */
 static unsigned int bdi_min_ratio;
 
+static u64 bdi_get_bytes(unsigned int ratio)
+{
+       unsigned long background_thresh;
+       unsigned long dirty_thresh;
+       u64 bytes;
+
+       global_dirty_limits(&background_thresh, &dirty_thresh);
+       bytes = (dirty_thresh * PAGE_SIZE * ratio) / BDI_RATIO_SCALE / 100;
+
+       return bytes;
+}
+
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
        unsigned int delta;
@@ -701,6 +713,11 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 }
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
+u64 bdi_get_max_bytes(struct backing_dev_info *bdi)
+{
+       return bdi_get_bytes(bdi->max_ratio);
+}
+
 int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit)
 {
        if (strict_limit > 1)