[PATCH v2] readahead: Correct the start and size in ondemand_readahead()

[Yin Fengwei]

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