On 08/25/2011 07:53 PM, Yongqiang Yang wrote:
Hi Allison,
Currently, punch hole flushes all pages to disk and releases pages in
page cache, and then calls ext4_ext_map_blocks.
Assume that if a new page in the punching's range is mapped after
releasing pages and before down_write i_data_sem,
then ext4_ext_map_blocks will release map info of the page in extent
tree. However, up layers does not know this, and they think the page
is mapped.
I can not find how punch hole handle the situation above. Could you
shed a light on it?
Hi Yongqiang
This is a really good question and at the moment Im still looking into
it. :) The calling sequence in punch hole was modeled after truncate,
which also only locks i_data_sem when modifying the extent tree.
ext4_ext_map_blocks when called with the punch hole flag, only releases
blocks in the extent tree, using the same routines truncate does, but it
does not modify the state of the pages. Though that still does not
prevent the race condition you describe, so I am still investigating it.
I've found that I can catch a lot of race conditions by simply running
the stress test over night, and so far I havnt had anything like this
come up, but that certainly doesnt mean its not there. I will let you
know what I find. Thx!
Allison Henderson
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