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    SubjectRe: introducing __GFP_PANIC
    [Pekka Enberg - Mon, May 04, 2009 at 11:32:21AM +0300]
    | Hi Cyrill,
    |
    | On Mon, 2009-05-04 at 12:14 +0400, Cyrill Gorcunov wrote:
    | > mm: introduce __GFP_PANIC
    | >
    | > Sometime we need that memory obtained via kmalloc
    | > should always be granted. If there is no enough
    | > memory we just can't go further.
    | >
    | > For such a case we introduce __GFP_PANIC panic
    | > modificator. If memory can't be granted -- we just
    | > panic.
    | >
    | > Note that __GFP_PANIC implicitly turn off failslab
    | > facility on such kind calls.
    | >
    | > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    | > ---
    | > include/linux/gfp.h | 13 ++++++++++---
    | > include/linux/slab_def.h | 1 +
    | > mm/failslab.c | 3 +++
    | > mm/page_alloc.c | 17 +++++++++++++++--
    | > 4 files changed, 29 insertions(+), 5 deletions(-)
    | >
    | > Index: linux-2.6.git/include/linux/gfp.h
    | > =====================================================================
    | > --- linux-2.6.git.orig/include/linux/gfp.h
    | > +++ linux-2.6.git/include/linux/gfp.h
    | > @@ -7,6 +7,7 @@
    | > #include <linux/topology.h>
    | >
    | > struct vm_area_struct;
    | > +void oom_panic(gfp_t gfp_mask, unsigned int order);
    | >
    | > /*
    | > * GFP bitmasks..
    | > @@ -58,7 +59,9 @@ struct vm_area_struct;
    | > #define __GFP_NOTRACK ((__force gfp_t)0)
    | > #endif
    | >
    | > -#define __GFP_BITS_SHIFT 22 /* Room for 22 __GFP_FOO bits */
    | > +#define __GFP_PANIC ((__force gfp_t)0x400000u) /* Panic on page alloction failure */
    | > +
    | > +#define __GFP_BITS_SHIFT 23 /* Room for 23 __GFP_FOO bits */
    | > #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
    | >
    | > /* This equals 0, but use constants in case they ever change */
    | > @@ -196,8 +199,10 @@ __alloc_pages_nodemask(gfp_t gfp_mask, u
    | > static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
    | > unsigned int order)
    | > {
    | > - if (unlikely(order >= MAX_ORDER))
    | > + if (unlikely(order >= MAX_ORDER)) {
    | > + oom_panic(gfp_mask, order);
    |
    | This...
    |
    | > return NULL;
    | > + }
    | >
    | > /* Unknown node is current node */
    | > if (nid < 0)
    | > @@ -212,8 +217,10 @@ extern struct page *alloc_pages_current(
    | > static inline struct page *
    | > alloc_pages(gfp_t gfp_mask, unsigned int order)
    | > {
    | > - if (unlikely(order >= MAX_ORDER))
    | > + if (unlikely(order >= MAX_ORDER)) {
    | > + oom_panic(gfp_mask, order);
    |
    | ...this...
    |
    | > return NULL;
    | > + }
    | >
    | > return alloc_pages_current(gfp_mask, order);
    | > }
    | > Index: linux-2.6.git/include/linux/slab_def.h
    | > =====================================================================
    | > --- linux-2.6.git.orig/include/linux/slab_def.h
    | > +++ linux-2.6.git/include/linux/slab_def.h
    | > @@ -143,6 +143,7 @@ static __always_inline void *kmalloc(siz
    | > i++;
    | > #include <linux/kmalloc_sizes.h>
    | > #undef CACHE
    | > + oom_panic(flags, get_order(size));
    |
    | ...and this look fishy. They're static inlines that get expanded
    | everywhere and they're known to be performance sensitive paths. I don't
    | see much point in checking for >= MAX_ORDER at all because we will get a
    | nice oops anyway for that.

    Yep, I noticed that it is really ugly cases to put oom_panic
    here. But shouldn't we cover all the sites? If I'm not missing
    something, for slab with builtin constant greater then KMALLOC_MAX_SIZE
    the kmalloc caller could get NULL even if __GFP_PANIC specified.
    Which in turn means __GFP_PANIC just doesn't work properly here.

    Anyway, I'll update the patch with fixes you proposed should be done.
    Wait a bit :)

    |
    | __GFP_PANIC is an annotation saying that it's okay for a particular
    | call-site not to check for NULL because we never expect to run out of
    | memory at that point. But we don't really need to panic() for all the
    | possible *errors*, just for the out-of-memory case.
    |
    | Pekka
    |

    Which means, __GFP_PANIC will not grant the guarantee to a caller
    that he will not get NULL deref even with __GFP_PANIC specified?

    Perhaps I missing something...

    -- Cyrill


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