[patch 100/181] mm: move PageDoubleMap bit

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From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Subject: mm: move PageDoubleMap bit

Patch series "Fix PageDoubleMap".

This is a purely theoretical problem for now as none of the filesystems
which use PG_private_2 (ie PG_fscache) are being converted at this time,
but it's confusing to leave it like this.


This patch (of 2):

PG_private_2 is defined as being PF_ANY (applicable to tail pages as well
as regular & head pages).  That means that the first tail page of a
double-map page will appear to have Private2 set.  Use the Workingset bit
instead which is defined as PF_HEAD so any attempt to access the
Workingset bit on a tail page will redirect to the head page's Workingset
bit.

Link: https://lkml.kernel.org/r/20200629151933.15671-1-willy@xxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20200629151933.15671-2-willy@xxxxxxxxxxxxx
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 include/linux/page-flags.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/page-flags.h~mm-move-pagedoublemap-bit
+++ a/include/linux/page-flags.h
@@ -167,7 +167,7 @@ enum pageflags {
 	PG_slob_free = PG_private,
 
 	/* Compound pages. Stored in first tail page's flags */
-	PG_double_map = PG_private_2,
+	PG_double_map = PG_workingset,
 
 	/* non-lru isolated movable page */
 	PG_isolated = PG_reclaim,
_




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