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mm: add comment on storage key dirty bit semantics

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Add comments that dirty bit in storage key gets set whenever page content
is changed.  Hopefully if someone will use this function, he'll have a
look at one of the two places where we comment on this.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jan Kara 2012-12-11 16:01:09 -08:00 committed by Linus Torvalds
parent 712cd386fd
commit e749eb9553
2 changed files with 8 additions and 3 deletions
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3
arch/s390/include/asm/page.h
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@ -158,6 +158,9 @@ static inline int page_reset_referenced(unsigned long addr)
* race against modification of the referenced bit. This function
* should therefore only be called if it is not mapped in any
* address space.
*
* Note that the bit gets set whenever page content is changed. That means
* also when the page is modified by DMA or from inside the kernel.
*/
#define __HAVE_ARCH_PAGE_TEST_AND_CLEAR_DIRTY
static inline int page_test_and_clear_dirty(unsigned long pfn, int mapped)

8
mm/rmap.c
View file

@ -1151,9 +1151,11 @@ void page_remove_rmap(struct page *page)
* containing the swap entry, but page not yet written to swap.
*
* And we can skip it on file pages, so long as the filesystem
* participates in dirty tracking; but need to catch shm and tmpfs
* and ramfs pages which have been modified since creation by read
* fault.
* participates in dirty tracking (note that this is not only an
* optimization but also solves problems caused by dirty flag in
* storage key getting set by a write from inside kernel); but need to
* catch shm and tmpfs and ramfs pages which have been modified since
* creation by read fault.
*
* Note that mapping must be decided above, before decrementing
* mapcount (which luckily provides a barrier): once page is unmapped,