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由 Nick Piggin 提交于
A bug was introduced into write_cache_pages cyclic writeout by commit 31a12666 ("mm: write_cache_pages cyclic fix"). The intention (and comments) is that we should cycle back and look for more dirty pages at the beginning of the file if there is no more work to be done. But the !done condition was dropped from the test. This means that any time the page writeout loop breaks (eg. due to nr_to_write == 0), we will set index to 0, then goto again. This will set done_index to index, then find done is set, so will proceed to the end of the function. When updating mapping->writeback_index for cyclic writeout, we now use done_index == 0, so we're always cycling back to 0. This seemed to be causing random mmap writes (slapadd and iozone) to start writing more pages from the LRU and writeout would slowdown, and caused bugzilla entry http://bugzilla.kernel.org/show_bug.cgi?id=12604 about Berkeley DB slowing down dramatically. With this patch, iozone random write performance is increased nearly 5x on my system (iozone -B -r 4k -s 64k -s 512m -s 1200m on ext2). Signed-off-by: NNick Piggin <npiggin@suse.de> Reported-and-tested-by: NJan Kara <jack@suse.cz> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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