From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
The readahead flag is set on a folio based on the lookahead_size and
nr_to_read. For example, when the readahead happens from index to index
+ nr_to_read, then the readahead `mark` offset from index is set at
nr_to_read - lookahead_size.
There are some scenarios where the lookahead_size > nr_to_read. If this
happens, readahead flag is not set on any folio on the current
readahead window.
There are two problems at the moment in the way `mark` is calculated
when lookahead_size > nr_to_read:
- unsigned long `mark` will be assigned a negative value which can lead
to unexpected results in extreme cases due to wrap around.
- The current calculation for `mark` with mapping_min_order > 0 gives
incorrect results when lookahead_size > nr_to_read due to rounding
up operation.
Explicitly initialize `mark` to be ULONG_MAX and only calculate it
when lookahead_size is within the readahead window.
Fixes: 26cfdb395eef ("readahead: allocate folios with mapping_min_order in readahead")
Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
---
mm/readahead.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/mm/readahead.c b/mm/readahead.c
index 3dc6c7a128dd..475d2940a1ed 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -206,9 +206,9 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
unsigned long nr_to_read, unsigned long lookahead_size)
{
struct address_space *mapping = ractl->mapping;
- unsigned long ra_folio_index, index = readahead_index(ractl);
+ unsigned long index = readahead_index(ractl);
gfp_t gfp_mask = readahead_gfp_mask(mapping);
- unsigned long mark, i = 0;
+ unsigned long mark = ULONG_MAX, i = 0;
unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
/*
@@ -232,9 +232,14 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
* index that only has lookahead or "async_region" to set the
* readahead flag.
*/
- ra_folio_index = round_up(readahead_index(ractl) + nr_to_read - lookahead_size,
- min_nrpages);
- mark = ra_folio_index - index;
+ if (lookahead_size <= nr_to_read) {
+ unsigned long ra_folio_index;
+
+ ra_folio_index = round_up(readahead_index(ractl) +
+ nr_to_read - lookahead_size,
+ min_nrpages);
+ mark = ra_folio_index - index;
+ }
nr_to_read += readahead_index(ractl) - index;
ractl->_index = index;
base-commit: d61a00525464bfc5fe92c6ad713350988e492b88
--
2.44.1
