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    SubjectRe: [RFC] vmalloc: add warning in __vmalloc
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    On 27 April 2012 20:36, David Rientjes <rientjes@google.com> wrote:
    > On Fri, 27 Apr 2012, Minchan Kim wrote:
    >
    >> Now there are several places to use __vmalloc with GFP_ATOMIC,
    >> GFP_NOIO, GFP_NOFS but unfortunately __vmalloc calls map_vm_area
    >> which calls alloc_pages with GFP_KERNEL to allocate page tables.
    >> It means it's possible to happen deadlock.
    >> I don't know why it doesn't have reported until now.
    >>
    >> Firstly, I tried passing gfp_t to lower functions to support __vmalloc
    >> with such flags but other mm guys don't want and decided that
    >> all of caller should be fixed.
    >>
    >> http://marc.info/?l=linux-kernel&m=133517143616544&w=2
    >>
    >> To begin with, let's listen other's opinion whether they can fix it
    >> by other approach without calling __vmalloc with such flags.
    >>
    >> So this patch adds warning to detect and to be fixed hopely.
    >> I Cced related maintainers.
    >> If I miss someone, please Cced them.
    >>
    >> side-note:
    >>   I added WARN_ON instead of WARN_ONCE to detect all of callers
    >>   and each WARN_ON for each flag to detect to use any flag easily.
    >>   After we fix all of caller or reduce such caller, we can merge
    >>   a warning with WARN_ONCE.
    >>
    >
    > I disagree with this approach since it's going to violently spam an
    > innocent kernel user's log with no ratelimiting and for a situation that
    > actually may not be problematic.

    With WARN_ON_ONCE, it should be good.

    >
    > Passing any of these bits (the difference between GFP_KERNEL and
    > GFP_ATOMIC) only means anything when we're going to do reclaim.  And I'm
    > suspecting we would have seen problems with this already since
    > pte_alloc_kernel() does __GFP_REPEAT on most architectures meaning that it
    > will loop infinitely in the page allocator until at least one page is
    > freed (since its an order-0 allocation) which would hardly ever happen if
    > __GFP_FS or __GFP_IO actually meant something in this context.
    >
    > In other words, we would already have seen these deadlocks and it would
    > have been diagnosed as a vmalloc(GFP_ATOMIC) problem.  Where are those bug
    > reports?

    That's not sound logic to disprove a bug.

    I think simply most callers are permissive and don't mask out flags.
    But for example a filesystem holding an fs lock and then doing
    vmalloc(GFP_NOFS) can certainly deadlock.
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