lkml.org 
[lkml]   [2020]   [Nov]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock
    On Mon, Nov 02, 2020 at 02:49:27PM +0000, Matthew Wilcox wrote:
    > On Mon, Nov 02, 2020 at 09:41:10AM -0500, Johannes Weiner wrote:
    > > On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote:
    > > > From: Hugh Dickins <hughd@google.com>
    > > >
    > > > It is necessary for page_idle_get_page() to recheck PageLRU() after
    > > > get_page_unless_zero(), but holding lru_lock around that serves no
    > > > useful purpose, and adds to lru_lock contention: delete it.
    > > >
    > > > See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
    > > > discussion that led to lru_lock there; but __page_set_anon_rmap() now
    > > > uses WRITE_ONCE(),
    > >
    > > That doesn't seem to be the case in Linus's or Andrew's tree. Am I
    > > missing a dependent patch series?
    > >
    > > > and I see no other risk in page_idle_clear_pte_refs() using
    > > > rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly but
    > > > not entirely prevented by page_count() check in ksm.c's
    > > > write_protect_page(): that risk being shared with page_referenced()
    > > > and not helped by lru_lock).
    > >
    > > Isn't it possible, as per Minchan's description, for page->mapping to
    > > point to a struct anon_vma without PAGE_MAPPING_ANON set, and rmap
    > > thinking it's looking at a struct address_space?
    >
    > I don't think it can point to an anon_vma without the ANON bit set.
    > Minchan's concern in that email was that it might still be NULL.

    Hm, no, the thread is a lengthy discussion about whether the store
    could be split such that page->mapping is actually pointing to
    something invalid (anon_vma without the PageAnon bit).

    From his email:

    CPU 0 CPU 1

    do_anonymous_page
    __page_set_anon_rmap
    /* out of order happened so SetPageLRU is done ahead */
    SetPageLRU(page)
    /* Compilr changed store operation like below */
    page->mapping = (struct address_space *) anon_vma;
    /* Big stall happens */
    /* idletacking judged it as LRU page so pass the page
    in page_reference */
    page_refernced
    page_rmapping return true because
    page->mapping has some vaule but not complete
    so it calls rmap_walk_file.
    it's okay to pass non-completed anon page in rmap_walk_file?

    \
     
     \ /
      Last update: 2020-11-02 21:22    [W:4.203 / U:0.292 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site