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    SubjectRe: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
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    On 1/29/20 3:25 AM, Kirill A. Shutemov wrote:
    > On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
    >> When debugging a problem that involves compound pages, it is extremely
    >> helpful if dump_page() reports not only the page->_refcount, but also
    >> the refcount of the head page of the compound page. That's because the
    >> head page collects refcounts for the entire compound page.
    >>
    >> Therefore, enhance dump_page() so as to print out the refcount of the
    >> head page of a compound page.
    >>
    >> This approach (printing information about a struct page that is not the
    >> struct page that was passed into dump_page()) has a precedent:
    >> compound_mapcount is already being printed.
    >
    > refcount on a tail must always be 0. I think we should only print it when
    > it is non-zero, emphasizing this fact with a standalone message.
    >

    Hi Kirill,

    Yes, good point, that sounds like just the right balance. And it avoids adding
    a new item to print in the common case, which is very nice. Here's what I've
    changed it to for the next version (I'll also rewrite the commit description
    accordingly):


    diff --git a/mm/debug.c b/mm/debug.c
    index a90da5337c14..3a45e2b77de0 100644
    --- a/mm/debug.c
    +++ b/mm/debug.c
    @@ -75,12 +75,17 @@ void __dump_page(struct page *page, const char *reason)
    */
    mapcount = PageSlab(page) ? 0 : page_mapcount(page);

    - if (PageCompound(page))
    - pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
    - "index:%#lx compound_mapcount: %d\n",
    - page, page_ref_count(page), mapcount,
    + if (PageCompound(page)) {
    + pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
    + "index:%#lx compound_mapcount:%d\n",
    + page, page_ref_count(compound_head(page)), mapcount,
    page->mapping, page_to_pgoff(page),
    compound_mapcount(page));
    +
    + if (page != compound_head(page) && page_ref_count(page) != 0)
    + pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
    + "this tail page\n", page, page_ref_count(page));
    + }
    else
    pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
    page, page_ref_count(page), mapcount,

    thanks,
    --
    John Hubbard
    NVIDIA

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