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    SubjectRe: [PATCH] mm: Warn once when a page is freed with PG_mlocked set V2
    On Thu, 23 Jul 2009 11:29:39 +0100 Mel Gorman <mel@csn.ul.ie> wrote:

    > On Wed, Jul 22, 2009 at 04:06:49PM -0700, Andrew Morton wrote:
    > > On Wed, 15 Jul 2009 10:31:54 -0400 (EDT)
    > > Christoph Lameter <cl@linux-foundation.org> wrote:
    > >
    > > > On Wed, 15 Jul 2009, Mel Gorman wrote:
    > > >
    > > > > -static inline int free_pages_check(struct page *page)
    > > > > +static inline int free_pages_check(struct page *page, int wasMlocked)
    > > > > {
    > > > > + WARN_ONCE(wasMlocked, KERN_WARNING
    > > > > + "Page flag mlocked set for process %s at pfn:%05lx\n"
    > > > > + "page:%p flags:0x%lX\n",
    > > > > + current->comm, page_to_pfn(page),
    > > > > + page, page->flags|__PG_MLOCKED);
    > > > > +
    > > > > if (unlikely(page_mapcount(page) |
    > > >
    > > > There is already a free_page_mlocked() that is only called if the mlock
    > > > bit is set. Move it into there to avoid having to run two checks in the
    > > > hot codee path?
    > >
    > > Agreed.
    > >
    > > This patch gratuitously adds hotpath overhead. Moving the change to be
    > > inside those preexisting wasMlocked tests will reduce its overhead a lot.
    > >
    >
    > It adds code duplication then, one of which is in a fast path.
    >
    > > As it stands, I'm really doubting that the patch's utility is worth its
    > > cost.
    > >
    >
    > I'm happy to let this one drop. It seemed like it would be nice for debugging
    > while there are still corner cases where mlocked pages are getting freed
    > instead of torn down but we already account for that situation occuring. While
    > I think it'll be tricky to spot, it's probably preferable to having warnings
    > spew out onto dmesg.

    If we do in it the way which Christoph recommends, the additional
    overhead is miniscule?




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