Commit e749eb95 authored by Jan Kara's avatar Jan Kara Committed by Linus Torvalds

mm: add comment on storage key dirty bit semantics

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: default avatarJan 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 712cd386
...@@ -158,6 +158,9 @@ static inline int page_reset_referenced(unsigned long addr) ...@@ -158,6 +158,9 @@ static inline int page_reset_referenced(unsigned long addr)
* race against modification of the referenced bit. This function * race against modification of the referenced bit. This function
* should therefore only be called if it is not mapped in any * should therefore only be called if it is not mapped in any
* address space. * 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 #define __HAVE_ARCH_PAGE_TEST_AND_CLEAR_DIRTY
static inline int page_test_and_clear_dirty(unsigned long pfn, int mapped) static inline int page_test_and_clear_dirty(unsigned long pfn, int mapped)
......
...@@ -1151,9 +1151,11 @@ void page_remove_rmap(struct page *page) ...@@ -1151,9 +1151,11 @@ void page_remove_rmap(struct page *page)
* containing the swap entry, but page not yet written to swap. * containing the swap entry, but page not yet written to swap.
* *
* And we can skip it on file pages, so long as the filesystem * And we can skip it on file pages, so long as the filesystem
* participates in dirty tracking; but need to catch shm and tmpfs * participates in dirty tracking (note that this is not only an
* and ramfs pages which have been modified since creation by read * optimization but also solves problems caused by dirty flag in
* fault. * 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 * Note that mapping must be decided above, before decrementing
* mapcount (which luckily provides a barrier): once page is unmapped, * mapcount (which luckily provides a barrier): once page is unmapped,
......
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