The patch titled
Subject: readahead: correct the start and size in ondemand_readahead()
has been added to the -mm mm-unstable branch. Its filename is
readahead-correct-the-start-and-size-in-ondemand_readahead.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/readahead-correct-the-start-and-size-in-ondemand_readahead.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Subject: readahead: correct the start and size in ondemand_readahead()
Date: Wed, 28 Jun 2023 12:43:03 +0800
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 |****************************************|
Link: https://lkml.kernel.org/r/20230628044303.1412624-1-fengwei.yin@xxxxxxxxx
Fixes: 9425c591e06a ("page cache: fix page_cache_next/prev_miss off by one")
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202306211346.1e9ff03e-oliver.sang@xxxxxxxxx
Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Cc: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/readahead.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
--- a/mm/readahead.c~readahead-correct-the-start-and-size-in-ondemand_readahead
+++ a/mm/readahead.c
@@ -613,9 +613,17 @@ static void ondemand_readahead(struct re
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;
_
Patches currently in -mm which might be from fengwei.yin@xxxxxxxxx are
readahead-correct-the-start-and-size-in-ondemand_readahead.patch
