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Subject[PATCH 3/3] mm: make PageReadahead more strict
Date
PG_readahead flag is shared with PG_reclaim but PG_reclaim is only
used in write context while PG_readahead is used for read context.

To make it clear, let's introduce PageReadahead wrapper with
!PageWriteback so it could make code clear and we could drop
PageWriteback check in page_cache_async_readahead, which removes
pointless dropping mmap_sem.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
include/linux/page-flags.h | 28 ++++++++++++++++++++++++++--
mm/readahead.c | 6 ------
2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 1bf83c8fcaa7..f91a9b2a49bd 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -363,8 +363,32 @@ PAGEFLAG(MappedToDisk, mappedtodisk, PF_NO_TAIL)
/* PG_readahead is only used for reads; PG_reclaim is only for writes */
PAGEFLAG(Reclaim, reclaim, PF_NO_TAIL)
TESTCLEARFLAG(Reclaim, reclaim, PF_NO_TAIL)
-PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
- TESTCLEARFLAG(Readahead, reclaim, PF_NO_COMPOUND)
+
+SETPAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
+CLEARPAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
+
+/*
+ * Since PG_readahead is shared with PG_reclaim of the page flags,
+ * PageReadahead should double check whether it's readahead marker
+ * or PG_reclaim. It could be done by PageWriteback check because
+ * PG_reclaim is always with PG_writeback.
+ */
+static inline int PageReadahead(struct page *page)
+{
+ VM_BUG_ON_PGFLAGS(PageCompound(page), page);
+
+ return (page->flags & (1UL << PG_reclaim | 1UL << PG_writeback)) ==
+ (1UL << PG_reclaim);
+}
+
+/* Clear PG_readahead only if it's PG_readahead, not PG_reclaim */
+static inline int TestClearPageReadahead(struct page *page)
+{
+ VM_BUG_ON_PGFLAGS(PageCompound(page), page);
+
+ return !PageWriteback(page) ||
+ test_and_clear_bit(PG_reclaim, &page->flags);
+}

#ifdef CONFIG_HIGHMEM
/*
diff --git a/mm/readahead.c b/mm/readahead.c
index 2fe72cd29b47..85b15e5a1d7b 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -553,12 +553,6 @@ page_cache_async_readahead(struct address_space *mapping,
if (!ra->ra_pages)
return;

- /*
- * Same bit is used for PG_readahead and PG_reclaim.
- */
- if (PageWriteback(page))
- return;
-
ClearPageReadahead(page);

/*
--
2.25.0.225.g125e21ebc7-goog
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