gfs2: Rework how rgrp buffer_heads are managed
Before this patch, the rgrp code had a serious problem related to how it managed buffer_heads for resource groups. The problem caused file system corruption, especially in cases of journal replay. When an rgrp glock was demoted to transfer ownership to a different cluster node, do_xmote() first calls rgrp_go_sync and then rgrp_go_inval, as expected. When it calls rgrp_go_sync, that called gfs2_rgrp_brelse() that dropped the buffer_head reference count. In most cases, the reference count went to zero, which is right. However, there were other places where the buffers are handled differently. After rgrp_go_sync, do_xmote called rgrp_go_inval which called gfs2_rgrp_brelse a second time, then rgrp_go_inval's call to truncate_inode_pages_range would get rid of the pages in memory, but only if the reference count drops to 0. Unfortunately, gfs2_rgrp_brelse was setting bi->bi_bh = NULL. So when rgrp_go_sync called gfs2_rgrp_brelse, it lost the pointer to the buffer_heads in cases where the reference count was still 1. Therefore, when rgrp_go_inval called gfs2_rgrp_brelse a second time, it failed the check for "if (bi->bi_bh)" and thus failed to call brelse a second time. Because of that, the reference count on those buffers sometimes failed to drop from 1 to 0. And that caused function truncate_inode_pages_range to keep the pages in page cache rather than freeing them. The next time the rgrp glock was acquired, the metadata read of the rgrp buffers re-used the pages in memory, which were now wrong because they were likely modified by the other node who acquired the glock in EX (which is why we demoted the glock). This re-use of the page cache caused corruption because changes made by the other nodes were never seen, so the bitmaps were inaccurate. For some reason, the problem became most apparent when journal replay forced the replay of rgrps in memory, which caused newer rgrp data to be overwritten by the older in-core pages. A big part of the problem was that the rgrp buffer were released in multiple places: The go_unlock function would release them when the glock was released rather than when the glock is demoted, which is clearly wrong because our intent was to cache them until the glock is demoted from SH or EX. This patch attempts to clean up the mess and make one consistent and centralized mechanism for managing the rgrp buffer_heads by implementing several changes: 1. It eliminates the call to gfs2_rgrp_brelse() from rgrp_go_sync. We don't want to release the buffers or zero the pointers when syncing for the reasons stated above. It only makes sense to release them when the glock is actually invalidated (go_inval). And when we do, then we set the bh pointers to NULL. 2. The go_unlock function (which was only used for rgrps) is eliminated, as we've talked about doing many times before. The go_unlock function was called too early in the glock dq process, and should not happen until the glock is invalidated. 3. It also eliminates the call to rgrp_brelse in gfs2_clear_rgrpd. That will now happen automatically when the rgrp glocks are demoted, and shouldn't happen any sooner or later than that. Instead, function gfs2_clear_rgrpd has been modified to demote the rgrp glocks, and therefore, free those pages, before the remaining glocks are culled by gfs2_gl_hash_clear. This prevents the gl_object from hanging around when the glocks are culled. Signed-off-by:Bob Peterson <rpeterso@redhat.com> Reviewed-by:
Andreas Gruenbacher <agruenba@redhat.com>
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