linux-kernel - Re: [PATCH v5 3/5] vmcore: Introduce remap_oldmem_pfn

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Date:	Mon, 10 Jun 2013 17:37:39 +0200
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	HATAYAMA Daisuke <d.hatayama@...il.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Jan Willeke <willeke@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH v5 3/5] vmcore: Introduce remap_oldmem_pfn_range()

On Mon, 10 Jun 2013 22:40:24 +0900
HATAYAMA Daisuke <d.hatayama@...il.com> wrote:

> 2013/6/8 Michael Holzheu <holzheu@...ux.vnet.ibm.com>:
> <cut>
> > @@ -225,6 +251,56 @@ static ssize_t read_vmcore(struct file *file, char __user *buffer,
> >         return acc;
> >  }
> >
> > +static ssize_t read_vmcore(struct file *file, char __user *buffer,
> > +                          size_t buflen, loff_t *fpos)
> > +{
> > +       return __read_vmcore(buffer, buflen, fpos, 1);
> > +}
> > +
> > +/*
> > + * The vmcore fault handler uses the page cache and fills data using the
> > + * standard __vmcore_read() function.
> > + */
> > +static int mmap_vmcore_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> > +{
> > +       struct address_space *mapping = vma->vm_private_data;
> > +       pgoff_t index = vmf->pgoff;
> > +       struct page *page;
> > +       loff_t src;
> > +       char *buf;
> > +       int rc;
> > +
> > +find_page:
> > +       page = find_lock_page(mapping, index);
> > +       if (page) {
> > +               unlock_page(page);
> > +               rc = VM_FAULT_MINOR;
> > +       } else {
> > +               page = page_cache_alloc_cold(mapping);
> > +               if (!page)
> > +                       return VM_FAULT_OOM;
> > +               rc = add_to_page_cache_lru(page, mapping, index, GFP_KERNEL);
> > +               if (rc) {
> > +                       page_cache_release(page);
> > +                       if (rc == -EEXIST)
> > +                               goto find_page;
> > +                       /* Probably ENOMEM for radix tree node */
> > +                       return VM_FAULT_OOM;
> > +               }
> > +               buf = (void *) (page_to_pfn(page) << PAGE_SHIFT);
> > +               src = index << PAGE_CACHE_SHIFT;
> > +               __read_vmcore(buf, PAGE_SIZE, &src, 0);
> > +               unlock_page(page);
> > +               rc = VM_FAULT_MAJOR;
> > +       }
> > +       vmf->page = page;
> > +       return rc;
> > +}
> 
> How about reusing find_or_create_page()?

The function would then look like the following:

static int mmap_vmcore_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct address_space *mapping = vma->vm_private_data;
        pgoff_t index = vmf->pgoff;
        struct page *page;
        loff_t src;
        char *buf;

        page = find_or_create_page(mapping, index, GFP_KERNEL);
        if (!page)
                return VM_FAULT_OOM;
        src = index << PAGE_CACHE_SHIFT;
        buf = (void *) (page_to_pfn(page) << PAGE_SHIFT);
        __read_vmcore(buf, PAGE_SIZE, &src, 0);
        unlock_page(page);
        vmf->page = page;
        return 0;
}

I agree that this makes the function simpler but we have to copy
the page also if it has already been filled, correct?

But since normally only one process uses /proc/vmcore this might be
acceptable.

BTW: I also removed the VM_FAULT_MAJOR/MINOR because I think the fault
handler should return 0 if a page has been found.

Michael

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