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

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Message-ID: <CABOkKT05VR3V9rEz1LWnEJSEvkevhBFfQVVX5_DS0Z4rBG9EyA@mail.gmail.com>
Date:	Tue, 11 Jun 2013 21:42:15 +0900
From:	HATAYAMA Daisuke <d.hatayama@...il.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.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()

2013/6/11 Michael Holzheu <holzheu@...ux.vnet.ibm.com>:
> 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?
>

You can use for the purpose PG_uptodate flag.

> 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
>

Thanks.
HATAYAMA, Daisuke
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