[Ocfs2-devel] Buffer read will get starvation in case reading/writing the same file from different nodes concurrently
Gang He
ghe at suse.com
Mon Dec 7 19:21:49 PST 2015
Hello Guys,
There is a issue from the customer, who is complaining that buffer reading sometimes lasts too much time ( 1 - 10 seconds) in case reading/writing the same file from different nodes concurrently.
According to the demo code from the customer, we also can reproduce this issue at home (run the test program under SLES11SP4 OCFS2 cluster), actually this issue can be reproduced in openSuSe 13.2 (more newer code), but in direct-io mode, this issue will disappear.
Base on my investigation, the root cause is the buffer-io using cluster-lock is different from direct-io, I do not know why buffer-io use cluster-lock like this way.
the code details are as below,
in aops.c file,
281 static int ocfs2_readpage(struct file *file, struct page *page)
282 {
283 struct inode *inode = page->mapping->host;
284 struct ocfs2_inode_info *oi = OCFS2_I(inode);
285 loff_t start = (loff_t)page->index << PAGE_CACHE_SHIFT;
286 int ret, unlock = 1;
287
288 trace_ocfs2_readpage((unsigned long long)oi->ip_blkno,
289 (page ? page->index : 0));
290
291 ret = ocfs2_inode_lock_with_page(inode, NULL, 0, page); <<== this line
292 if (ret != 0) {
293 if (ret == AOP_TRUNCATED_PAGE)
294 unlock = 0;
295 mlog_errno(ret);
296 goto out;
297 }
in dlmglue.c file,
2 int ocfs2_inode_lock_with_page(struct inode *inode,
2443 struct buffer_head **ret_bh,
2444 int ex,
2445 struct page *page)
2446 {
2447 int ret;
2448
2449 ret = ocfs2_inode_lock_full(inode, ret_bh, ex, OCFS2_LOCK_NONBLOCK); <<== there, why using NONBLOCK mode to get the cluster lock? this way will let reading IO get starvation.
2450 if (ret == -EAGAIN) {
2451 unlock_page(page);
2452 if (ocfs2_inode_lock(inode, ret_bh, ex) == 0)
2453 ocfs2_inode_unlock(inode, ex);
2454 ret = AOP_TRUNCATED_PAGE;
2455 }
2456
2457 return ret;
2458 }
If you know the background behind the code, please tell us, why not use block way to get the lock in reading a page, then reading IO will get the page fairly when there is a concurrent writing IO from the other node.
Second, I tried to modify that line from OCFS2_LOCK_NONBLOCK to 0 (switch to blocking way), the reading IO will not be blocked too much time (can erase the customer's complaining), but a new problem arises, sometimes the reading IO and writing IO get a dead lock (why dead lock? I am looking at).
Thanks
Gang
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