Re: attempt to fix race in bigalloc code path
From: Aditya Kali <hidden>
Date: 2011-08-22 17:30:23
On Mon, Aug 22, 2011 at 6:54 AM, Yongqiang Yang [off-list ref] wrote:
Hi, I can not find the patch - attempt to fix race in bigalloc code path - in the mailing list. I don't think this patch can work. set_buffers_da_mapped() is called under i_data_sem, so it can not lock page. If the page is not locked, calling page_has_buffers() is not secure as Jan pointed out previously. If a page is partially mapped(block size < page size), setting BH_Da_Mapped without locking page is not safe too.
The BH_Da_Mapped is set only during page writeback when the page is locked (in write_cache_pages_da). So, the setting of BH_Da_Mapped should be safe, no ?
BTW: find_delalloc_range() does not lock page as ext4_fiemap_cb(). As Jan said, it can crash system. BUT we can not lock page in both scenarios, because i_data_sem is hold. So delayed extent list is necessary. I have finished the code implementing delayed extent list, which needs some further testing. What are your opinions?
I agree that having a delayed extent list will greatly simply this solution in this case.
-- Best Wishes Yongqiang Yang
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