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    Subject[RFC][PATCH 2/6] in_emergency Trigger
    Create the 'in_emergency' trigger, to allow userspace to turn access to the
    critical pool on and off. The rationale behind this is to ensure that the
    critical pool stays full for *actual* emergency situations, and isn't used
    for transient, low-mem situations.

    -Matt
    Create a userspace trigger: /proc/sys/vm/in_emergency that notifies the kernel
    that the system is in an emergency state, and allows the kernel to delve into
    the 'critical pool' to satisfy __GFP_CRITICAL allocations.

    Signed-off-by: Matthew Dobson <colpatch@us.ibm.com>

    Index: linux-2.6.15-rc5+critical_pool/Documentation/sysctl/vm.txt
    ===================================================================
    --- linux-2.6.15-rc5+critical_pool.orig/Documentation/sysctl/vm.txt 2005-12-13 16:01:57.783326968 -0800
    +++ linux-2.6.15-rc5+critical_pool/Documentation/sysctl/vm.txt 2005-12-13 16:02:40.935766800 -0800
    @@ -27,6 +27,7 @@ Currently, these files are in /proc/sys/
    - laptop_mode
    - block_dump
    - critical_pages
    +- in_emergency

    ==============================================================

    @@ -112,3 +113,12 @@ This is used to force the Linux VM to re
    emergency (__GFP_CRITICAL) allocations. Allocations with this flag
    MUST succeed.
    The number written into this file is the number of pages to reserve.
    +
    +==============================================================
    +
    +in_emergency:
    +
    +This is used to let the Linux VM know that userspace thinks that the system is
    +in an emergency situation.
    +Writing a non-zero value into this file tells the VM we *are* in an emergency
    +situation & writing zero tells the VM we *are not* in an emergency situation.
    Index: linux-2.6.15-rc5+critical_pool/include/linux/sysctl.h
    ===================================================================
    --- linux-2.6.15-rc5+critical_pool.orig/include/linux/sysctl.h 2005-12-13 16:02:13.757898464 -0800
    +++ linux-2.6.15-rc5+critical_pool/include/linux/sysctl.h 2005-12-13 16:02:40.937766496 -0800
    @@ -181,6 +181,7 @@ enum
    VM_LEGACY_VA_LAYOUT=27, /* legacy/compatibility virtual address space layout */
    VM_SWAP_TOKEN_TIMEOUT=28, /* default time for token time out */
    VM_CRITICAL_PAGES=29, /* # of pages to reserve for __GFP_CRITICAL allocs */
    + VM_IN_EMERGENCY=30, /* tell the VM if we are/aren't in an emergency */
    };


    Index: linux-2.6.15-rc5+critical_pool/kernel/sysctl.c
    ===================================================================
    --- linux-2.6.15-rc5+critical_pool.orig/kernel/sysctl.c 2005-12-13 16:01:57.784326816 -0800
    +++ linux-2.6.15-rc5+critical_pool/kernel/sysctl.c 2005-12-13 16:02:40.942765736 -0800
    @@ -859,6 +859,16 @@ static ctl_table vm_table[] = {
    .strategy = &sysctl_intvec,
    .extra1 = &zero,
    },
    + {
    + .ctl_name = VM_IN_EMERGENCY,
    + .procname = "in_emergency",
    + .data = &system_in_emergency,
    + .maxlen = sizeof(system_in_emergency),
    + .mode = 0644,
    + .proc_handler = &proc_dointvec,
    + .strategy = &sysctl_intvec,
    + .extra1 = &zero,
    + },
    { .ctl_name = 0 }
    };

    Index: linux-2.6.15-rc5+critical_pool/mm/page_alloc.c
    ===================================================================
    --- linux-2.6.15-rc5+critical_pool.orig/mm/page_alloc.c 2005-12-13 16:01:57.810322864 -0800
    +++ linux-2.6.15-rc5+critical_pool/mm/page_alloc.c 2005-12-13 16:02:40.946765128 -0800
    @@ -53,6 +53,10 @@ unsigned long totalram_pages __read_most
    unsigned long totalhigh_pages __read_mostly;
    long nr_swap_pages;

    +/* Is the sytem in an emergency situation? */
    +int system_in_emergency = 0;
    +EXPORT_SYMBOL(system_in_emergency);
    +
    /* The number of pages to maintain in the critical page pool */
    int critical_pages = 0;

    @@ -927,7 +931,7 @@ struct page * fastcall
    __alloc_pages(gfp_t gfp_mask, unsigned int order,
    struct zonelist *zonelist)
    {
    - const gfp_t wait = gfp_mask & __GFP_WAIT;
    + gfp_t wait = gfp_mask & __GFP_WAIT;
    struct zone **z;
    struct page *page;
    struct reclaim_state reclaim_state;
    @@ -936,6 +940,16 @@ __alloc_pages(gfp_t gfp_mask, unsigned i
    int alloc_flags;
    int did_some_progress;

    + if (is_emergency_alloc(gfp_mask)) {
    + /*
    + * If the system is 'in emergency' and this is a critical
    + * allocation, then make sure we don't sleep
    + */
    + gfp_mask &= ~__GFP_WAIT;
    + gfp_mask |= __GFP_HIGH;
    + wait = 0;
    + }
    +
    might_sleep_if(wait);

    restart:
    @@ -1070,7 +1084,7 @@ nopage:
    * Rather than fail one of these allocations, take a page,
    * if there are any, from the critical pool.
    */
    - if ((gfp_mask & __GFP_CRITICAL) && !order) {
    + if (is_emergency_alloc(gfp_mask) && !order) {
    page = get_critical_page(gfp_mask);
    if (page)
    goto got_pg;
    Index: linux-2.6.15-rc5+critical_pool/include/linux/mm.h
    ===================================================================
    --- linux-2.6.15-rc5+critical_pool.orig/include/linux/mm.h 2005-12-13 16:01:57.783326968 -0800
    +++ linux-2.6.15-rc5+critical_pool/include/linux/mm.h 2005-12-13 16:02:40.950764520 -0800
    @@ -33,6 +33,12 @@ extern int sysctl_legacy_va_layout;
    #endif

    extern int critical_pages;
    +extern int system_in_emergency;
    +
    +static inline int is_emergency_alloc(gfp_t gfpmask)
    +{
    + return system_in_emergency && (gfpmask & __GFP_CRITICAL);
    +}

    #include <asm/page.h>
    #include <asm/pgtable.h>
    \
     
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