Re: [PATCH 9/10] kdump: read previous kernel's memory

[Eric W. Biederman]

Andrew Morton <akpm@xxxxxxxx> writes:

> Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>>
>> +ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
>> + size_t csize, unsigned long offset, int userbuf)
>> +{
>> +	void  *vaddr;
>> +
>> +	if (!csize)
>> +		return 0;
>> +
>> +	vaddr = kmap_atomic_pfn(pfn, KM_PTE0);
>> +
>> +	if (userbuf) {
>> +		if (copy_to_user(buf, (vaddr + offset), csize)) {
>> +			kunmap_atomic(vaddr, KM_PTE0);
>> +			return -EFAULT;
>
> The copy_*_user() inside kmap_atomic() is problematic.
>
> On some configs (eg, x86, highmem) the process is running atomically, hence
> the copy_*_user() will *refuse* to fault in the user's page if it's not
> present.  Because pagefaulting involves doing things which sleep.
>
> So
>
> a) This code will generate might_sleep() warnings at runtime and
>
> b) It'll return -EFAULT for user pages which haven't been faulted in yet.
>
>
> We do all sorts of gruesome tricks in mm/filemap.c to get around all this. 
> I don't think your code is as performance-sensitive, so a suitable fix
> might be to double-copy the data.  Make sure that the same physical page is
> used as a bounce page for each copy (ie: get the caller to pass it in) and
> that page will be cache-hot and the performance should be acceptable.
>
> If it really is performance-sensitive then you'll need to play filemap.c
> games.  It'd be better to use a sleeping kmap instead, if poss.  That's
> kmap().
>
> Please send an incremental patch when it's sorted.  

I'm going send my standard grumble that what is really needed
is to track why we can't use /dev/mem.  

We could solve this with a normal kmap except that we
quite possibly don't have a struct page for this page of memory.

/dev/mem does not allow reads in this situation because of how
problematic getting I/O reads correct is.  But not having a good
interface for mapping it into user space is similar.

Could we simply make this into a file that you can mmap
but you can't read?  I think that would be cleaner, and simpler.

Eric

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