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    Subject[PATCH v6 22/22] dax: Add reporting of major faults
    Date
    If we have to call get_block with the create argument set to 1, then
    the filesystem almost certainly had to zero the block. which is an I/O,
    which should be reported as a major fault.

    Note that major faults on DAX files happen for different reasons than
    major faults on non-DAX files. DAX files behave as if everything except
    file holes is already cached. That's all the more reason to report
    major faults when we do have to do I/O; it may be a valuable resource
    for sysadmins trying to diagnose performance problems.

    Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
    ---
    fs/dax.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    diff --git a/fs/dax.c b/fs/dax.c
    index cdc8012..79a67c5 100644
    --- a/fs/dax.c
    +++ b/fs/dax.c
    @@ -20,10 +20,12 @@
    #include <linux/fs.h>
    #include <linux/genhd.h>
    #include <linux/highmem.h>
    +#include <linux/memcontrol.h>
    #include <linux/mm.h>
    #include <linux/mutex.h>
    #include <linux/sched.h>
    #include <linux/uio.h>
    +#include <linux/vmstat.h>

    int dax_clear_blocks(struct inode *inode, sector_t block, long size)
    {
    @@ -286,6 +288,7 @@ static int do_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
    pgoff_t size;
    unsigned long pfn;
    int error;
    + int major = 0;

    size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
    if (vmf->pgoff >= size)
    @@ -301,6 +304,9 @@ static int do_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
    if (!buffer_written(&bh) && !vmf->cow_page) {
    if (vmf->flags & FAULT_FLAG_WRITE) {
    error = get_block(inode, block, &bh, 1);
    + count_vm_event(PGMAJFAULT);
    + mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
    + major = VM_FAULT_MAJOR;
    if (error || bh.b_size < PAGE_SIZE)
    return VM_FAULT_SIGBUS;
    } else {
    @@ -332,7 +338,7 @@ static int do_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
    /* -EBUSY is fine, somebody else faulted on the same PTE */
    if (error != -EBUSY)
    BUG_ON(error);
    - return VM_FAULT_NOPAGE;
    + return VM_FAULT_NOPAGE | major;
    }

    /**
    --
    1.8.5.3


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