Xen devel - Jul 2007 - [PATCH] [Linux, blktap] fix page reference count/file rss count leak

fix page reference count/file rss count leak when auto translated mode is
enabled.

Tapdisk process rss size becomes too large with auto translation enabled.
The example is as follows where dom0 has only several hundred megabytes.
This patch fixes it.
>  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
> 6758 root      15   0 39824 1.7t 1.7t S    0 188932.0  14:10.28 tapdisk
This is because page reference count/file rss size are incremented
when io request is accepted, but aren''t decremented when the request is
done.
This can be fixed by using vm_insert_page() in blktap_mmap() instead of
remap_pfn_range().


Details:
The tapdisk daemon mmaps blktap device and the blktap driver maps
page from the front end into the mmapped area and unmaps it when I/O
request is done.

When io request is accepted, dispatch_rw_block_io() is called.
With auto translated mode disabled, it directly manipulates the page table
without incrementing rss size.
With auto translated mode enabled, it calls vm_insert_page() which
increments page reference count/file rss.
When io request is done, fast_flush_area() is called.
With auto translated mode disabled, it directly manipulates the page table
without decrementing rss size.
With auto translated mode enabled, it calls zap_page_range() which
should decrements page reference count/file rss.
However (vma->vm_flags & VM_PFNMAP) is true, it doesn''t
decrement them
so that page reference count and file rss are leaked.
blktap driver allocates pages and never free them so that page reference
count leak doesn''t cause an issue (probably until overflow).


Without auto translation, it makes sense for blktap_mmap() to set
VM_PFNMAP with remap_pfn_range() because the blktap driver directly
manipulates page tables.
On the other hand with auto translation the VM_PFNMAP bit shouldn''t
set.
This can be achieved by using vm_insert_page() in blktap_mmap()
instead of remap_pfn_range()

-- 
yamahata


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