On Fri 30-10-15 16:01:43, Minchan Kim wrote:
> Basically, MADV_FREE relies on dirty bit in page table entry to decide
> whether VM allows to discard the page or not. IOW, if page table entry
> includes marked dirty bit, VM shouldn't discard the page.
>
> However, as a example, if swap-in by read fault happens, page table entry
> doesn't have dirty bit so MADV_FREE could discard the page wrongly.
>
> For avoiding the problem, MADV_FREE did more checks with PageDirty
> and PageSwapCache. It worked out because swapped-in page lives on
> swap cache and since it is evicted from the swap cache, the page has
> PG_dirty flag. So both page flags check effectively prevent
> wrong discarding by MADV_FREE.
>
> However, a problem in above logic is that swapped-in page has
> PG_dirty still after they are removed from swap cache so VM cannot
> consider the page as freeable any more even if madvise_free is
> called in future.
>
> Look at below example for detail.
>
> ptr = malloc();
> memset(ptr);
> ..
> ..
> .. heavy memory pressure so all of pages are swapped out
> ..
> ..
> var = *ptr; -> a page swapped-in and could be removed from
> swapcache. Then, page table doesn't mark
> dirty bit and page descriptor includes PG_dirty
> ..
> ..
> madvise_free(ptr); -> It doesn't clear PG_dirty of the page.
> ..
> ..
> ..
> .. heavy memory pressure again.
> .. In this time, VM cannot discard the page because the page
> .. has *PG_dirty*
>
> To solve the problem, this patch clears PG_dirty if only the page is owned
> exclusively by current process when madvise is called because PG_dirty
> represents ptes's dirtiness in several processes so we could clear it only
> if we own it exclusively.
>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/madvise.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 9ee9df8c768d..fc24104d6b3a 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -303,11 +303,19 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> if (!page)
> continue;
>
> - if (PageSwapCache(page)) {
> + if (PageSwapCache(page) || PageDirty(page)) {
> if (!trylock_page(page))
> continue;
> + /*
> + * If page is shared with others, we couldn't clear
> + * PG_dirty of the page.
> + */
> + if (page_count(page) != 1 + !!PageSwapCache(page)) {
> + unlock_page(page);
> + continue;
> + }
>
> - if (!try_to_free_swap(page)) {
> + if (PageSwapCache(page) && !try_to_free_swap(page)) {
> unlock_page(page);
> continue;
> }
> --
> 1.9.1
--
Michal Hocko
SUSE Labs
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