Comment 0 for bug 1777029

[Impact]

Under heavy NFS + FSCache load, a user sometimes observes a hang in __fscache_wait_on_page_write+0x5f/0xa0.

Example traces:
[<ffffffffc009f1bf>] __fscache_wait_on_page_write+0x5f/0xa0 [fscache]
[<ffffffffc009f6da>] __fscache_uncache_all_inode_pages+0xba/0x120 [fscache]
[<ffffffffc0419ece>] nfs_fscache_open_file+0x4e/0xc0 [nfs]

[<ffffffffc034f1bf>] __fscache_wait_on_page_write+0x5f/0xa0 [fscache]
[<ffffffffc04104dc>] __nfs_fscache_invalidate_page+0x2c/0x80 [nfs]
[<ffffffffc03fb543>] nfs_invalidate_page+0x63/0x90 [nfs]
[<ffffffff811925c0>] truncate_inode_page+0x80/0x90

[Fix]
Cherry-pick 2c98425720233ae3e135add0c7e869b32913502f from upstream, which is a patch from the FSCache maintainer.

[Testcase]
The user has run a NFS stress-test with a similar home-grown patch, and will run a stress test on the proposed kernel.

[Regression Potential]
Patch is limited to FSCache, so regression potential is limited.