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    SubjectRe: [PATCH V3 3/7] mm: reclaim MADV_FREE pages
    On Fri, Feb 17, 2017 at 02:41:08PM +0900, Minchan Kim wrote:
    > Hi Johannes,
    >
    > On Thu, Feb 16, 2017 at 01:40:18PM -0500, Johannes Weiner wrote:
    > > On Tue, Feb 14, 2017 at 11:36:09AM -0800, Shaohua Li wrote:
    > > > @@ -911,7 +911,7 @@ static void page_check_dirty_writeback(struct page *page,
    > > > * Anonymous pages are not handled by flushers and must be written
    > > > * from reclaim context. Do not stall reclaim based on them
    > > > */
    > > > - if (!page_is_file_cache(page)) {
    > > > + if (!page_is_file_cache(page) || page_is_lazyfree(page)) {
    > >
    > > Do we need this? MADV_FREE clears the dirty bit off the page; we could
    > > just let them go through with the function without any special-casing.
    >
    > I thought some driver potentially can do GUP with FOLL_TOUCH so that the
    > lazyfree page can have PG_dirty with !PG_swapbacked. In this case,
    > throttling logic of shrink_page_list can be confused?

    Yep, agreed. We should filter these pages here.

    > > > @@ -1142,7 +1144,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
    > > > * The page is mapped into the page tables of one or more
    > > > * processes. Try to unmap it here.
    > > > */
    > > > - if (page_mapped(page) && mapping) {
    > > > + if (page_mapped(page) && (mapping || lazyfree)) {
    > >
    > > Do we actually need to filter for mapping || lazyfree? If we fail to
    > > allocate swap, we don't reach here. If the page is a truncated file
    > > page, ttu returns pretty much instantly with SWAP_AGAIN. We should be
    > > able to just check for page_mapped() alone, no?
    >
    > try_to_unmap_one assumes every anonymous pages reached will have swp_entry
    > so it should be changed to check PageSwapCache if we go to the way.

    Yep, I think it should check page_mapping(). To me that would make the
    most sense, see other email: "Don't unmap a ram page with valid data
    when there is no secondary storage mapping to maintain integrity."

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