[PATCH 08/19] Set PF_FSTRANS while write_cache_pages calls ->writepage

[NeilBrown]

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@xxxxxxx>
---
 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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