Re: [RFC][PATCH 2/9] pagemap: remove file header

[Matt Mackall <mpm@selenic.com>]

On Tue, Aug 21, 2007 at 01:42:48PM -0700, Dave Hansen wrote:
> 
> The /proc/<pid>/pagemap file has a header containing:
>  * first byte:   0 for big endian, 1 for little
>  * second byte:  page shift (eg 12 for 4096 byte pages)
>  * third byte:   entry size in bytes (currently either 4 or 8)
>  * fourth byte:  header size
> 
> The endianness is only useful when examining a raw dump of
> pagemap from a different machine when you don't know the
> source of the file.  This is pretty rare, and the programs
> or scripts doing the copying off-machine can certainly be
> made to hold this information.
> 
> The page size is available in userspace at least with libc's
> getpagesize().  This will also never vary across processes,
> so putting it in a per-process file doesn't make any difference.
> If we need a "kernel's page size" exported to userspace,
> perhaps we can put it in /proc/meminfo.
> 
> The entry size is the really tricky one.  This can't just
> be sizeof(unsigned long) from userspace because we can have
> 32-bit processes on 64-bit kernels.  But, userspace can
> certainly derive this value if it lseek()s to the end of
> the file, and divides the file position by the size of its
> virtual address space.
> 
> In any case, I believe this information is redundant, and
> can be removed.
> 
> Signed-off-by: Dave Hansen <haveblue@us.ibm.com>

Yeah, looks like various folks thought this ought to go. So:

Acked-by: Matt Mackall <mpm@selenic.com>

> ---
> 
>  lxc-dave/fs/proc/task_mmu.c |   14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff -puN fs/proc/task_mmu.c~pagemap-no-header fs/proc/task_mmu.c
> --- lxc/fs/proc/task_mmu.c~pagemap-no-header	2007-08-21 13:30:50.000000000 -0700
> +++ lxc-dave/fs/proc/task_mmu.c	2007-08-21 13:30:50.000000000 -0700
> @@ -601,12 +601,12 @@ static ssize_t pagemap_read(struct file 
>  		goto out;
>  
>  	ret = -EIO;
> -	svpfn = src / sizeof(unsigned long) - 1;
> +	svpfn = src / sizeof(unsigned long);
>  	addr = PAGE_SIZE * svpfn;
> -	if ((svpfn + 1) * sizeof(unsigned long) != src)
> +	if (svpfn * sizeof(unsigned long) != src)
>  		goto out;
>  	evpfn = min((src + count) / sizeof(unsigned long) - 1,
> -		    ((~0UL) >> PAGE_SHIFT) + 1) - 1;
> +		    ((~0UL) >> PAGE_SHIFT) + 1);
>  	count = (evpfn - svpfn) * sizeof(unsigned long);
>  	end = PAGE_SIZE * evpfn;
>  	//printk("src %ld svpfn %d evpfn %d count %d\n", src, svpfn, evpfn, count);
> @@ -638,14 +638,6 @@ static ssize_t pagemap_read(struct file 
>  	pm.count = count;
>  	pm.out = (unsigned long __user *)buf;
>  
> -	if (svpfn == -1) {
> -		put_user((char)(ntohl(1) != 1), buf);
> -		put_user((char)PAGE_SHIFT, buf + 1);
> -		put_user((char)sizeof(unsigned long), buf + 2);
> -		put_user((char)sizeof(unsigned long), buf + 3);
> -		add_to_pagemap(pm.next, page[0], &pm);
> -	}
> -
>  	down_read(&mm->mmap_sem);
>  	vma = find_vma(mm, pm.next);
>  	while (pm.count > 0 && vma) {
> _

-- 
Mathematics is the supreme nostalgia of our time.

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