The commit
9425c591e06a ("page cache: fix page_cache_next/prev_miss off by one")
updated the page_cache_next_miss() to return the index beyond
range.
But it breaks the start/size of ra in ondemand_readahead() because
the offset by one is accumulated to readahead_index. As a consequence,
not best readahead order is picked.
Tracing of the order parameter of filemap_alloc_folio() showed:
page order : count distribution
0 : 892073 | |
1 : 0 | |
2 : 65120457 |****************************************|
3 : 32914005 |******************** |
4 : 33020991 |******************** |
with 9425c591e06a9.
With parent commit:
page order : count distribution
0 : 3417288 |**** |
1 : 0 | |
2 : 877012 |* |
3 : 288 | |
4 : 5607522 |******* |
5 : 29974228 |****************************************|
Fix the issue by removing the offset by one when page_cache_next_miss()
returns no gaps in the range.
After the fix:
page order : count distribution
0 : 2598561 |*** |
1 : 0 | |
2 : 687739 | |
3 : 288 | |
4 : 207210 | |
5 : 32628260 |****************************************|
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202306211346.1e9ff03e-oliver.sang@xxxxxxxxx
Fixes: 9425c591e06a ("page cache: fix page_cache_next/prev_miss off by one")
Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
---
Changes from v1:
- only removing offset by one when there is no gaps found by
page_cache_next_miss()
- Update commit message to include the histogram of page order
after fix
mm/readahead.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/mm/readahead.c b/mm/readahead.c
index 47afbca1d122..a93af773686f 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -614,9 +614,17 @@ static void ondemand_readahead(struct readahead_control *ractl,
max_pages);
rcu_read_unlock();
- if (!start || start - index > max_pages)
+ if (!start || start - index - 1 > max_pages)
return;
+ /*
+ * If no gaps in the range, page_cache_next_miss() returns
+ * index beyond range. Adjust it back to make sure
+ * ractl->_index is updated correctly later.
+ */
+ if ((start - index - 1) == max_pages)
+ start--;
+
ra->start = start;
ra->size = start - index; /* old async_size */
ra->size += req_size;
--
2.39.2
