Messages in this thread Patch in this message | | | From | NeilBrown <> | Date | Wed, 16 Apr 2014 14:03:36 +1000 | Subject | [PATCH 08/19] Set PF_FSTRANS while write_cache_pages calls ->writepage |
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It is normally safe for direct reclaim to enter filesystems even when a page is locked - as can happen if ->writepage allocates memory with GFP_KERNEL (which xfs does).
However if a localhost NFS mount is present, then a flush-* thread might hold a page locked and then in direct reclaim, ask nfs to commit an inode (nfs_release_page). When nfsd performs the fsync it might try to lock the same page, which leads to a deadlock.
A ->writepage should not allocate much memory, or do so very often, so it is safe to set PF_FSTRANS, and this removes the possible deadlock.
This was not detected by lockdep as it doesn't monitor the page lock. It was found as a real deadlock in testing.
Signed-off-by: NeilBrown <neilb@suse.de> --- mm/page-writeback.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 7106cb1aca8e..572e70b9a3f7 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -1909,6 +1909,7 @@ retry: for (i = 0; i < nr_pages; i++) { struct page *page = pvec.pages[i]; + unsigned int pflags; /* * At this point, the page may be truncated or @@ -1960,8 +1961,10 @@ continue_unlock: if (!clear_page_dirty_for_io(page)) goto continue_unlock; + current_set_flags_nested(&pflags, PF_FSTRANS); trace_wbc_writepage(wbc, mapping->backing_dev_info); ret = (*writepage)(page, wbc, data); + current_restore_flags_nested(&pflags, PF_FSTRANS); if (unlikely(ret)) { if (ret == AOP_WRITEPAGE_ACTIVATE) { unlock_page(page);
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