Re: [PATCH v2 3/3] x86: Support local_flush_tlb_kernel_range

On 06/15/2012 12:39 PM, Dan Magenheimer wrote:

quoted

From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]

quoted

The compression code already compresses to a per-cpu page-pair
already and then that "zpage" is copied into the space allocated
for it by zsmalloc.  For that final copy, if the copy code knows
the target may cross a page boundary, has both target pages
kmap'ed, and is smart about doing the copy, the "pair mapping"
can be avoided for compression.

The problem is that by "smart" you mean "has access to zsmalloc
internals".  zcache, or any user, would need the know the kmapped
address of the first page, the offset to start at within that page, and
the kmapped address of the second page in order to do the smart copy
you're talking about.  Then the complexity to do the smart copy that
would have to be implemented in each user.

Or simply add a zsmalloc_copy in zsmalloc and require that
it be used by the caller (instead of a memcpy).

quoted

quoted

The decompression path calls lzo1x directly and it would be
a huge pain to make lzo1x smart about page boundaries.  BUT
since we know that the decompressed result will always fit
into a page (actually exactly a page), you COULD do an extra
copy to the end of the target page (using the same smart-
about-page-boundaries copying code from above) and then do
in-place decompression, knowing that the decompression will
not cross a page boundary.  So, with the extra copy, the "pair
mapping" can be avoided for decompression as well.

This is an interesting thought.

But this does result in a copy in the decompression (i.e. page fault)
path, where right now, it is copy free.  The compressed data is
decompressed directly from its zsmalloc allocation to the page allocated
in the fault path.

The page fault occurs as soon as the lzo1x compression code starts anyway,
a

s do all the cache faults... both just occur earlier, so the only

additional cost is the actual cpu instructions to move the sequence of
(compressed) bytes from the zsmalloc-allocated area to the end
of the target page.

TLB operations can be very expensive, not to mention (as the
subject of this thread attests) non-portable.
 

Even if you go for copying chunks followed by decompression, it still
requires two kmaps and kunmaps. Each of these require one local TLB
invlpg. So, a total of 2 local maps + unmaps even with this approach.

The only additional requirement of zsmalloc is that it requires two
mappings which are virtually contiguous. The cost is the same in both
approaches but the current zsmalloc approach presents a much cleaner
interface.

Thanks,
Nitin

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