On Tue, Mar 27, 2018 at 07:52:48AM -0500, Goldwyn Rodrigues wrote: > I am not sure if I missed a condition in the code, but here is one of > the call lineup: > > writepages() -> writepage() -> kmalloc() -> __alloc_pages() -> > __alloc_pages_nodemask -> __alloc_pages_slowpath -> > __alloc_pages_direct_reclaim() -> try_to_free_pages() -> > do_try_to_free_pages() -> shrink_zones() -> shrink_node() -> > shrink_slab() -> do_shrink_slab() -> shrinker.scan_objects() -> > super_cache_scan() -> prune_icache_sb() -> fs/inode.c:dispose_list() -> > evict(inode) -> evict_inode() for ext4 -> filemap_write_and_wait() -> > filemap_fdatawrite(mapping) -> __filemap_fdatawrite_range() -> > do_writepages -> writepages() > > Please note, most filesystems currently have a safeguard in writepage() > which will return if the PF_MEMALLOC is set. The other safeguard is > __GFP_FS which we are trying to eliminate. But is that harmful? ext4_writepage() (for example) says that it will not deadlock in that circumstance: * We can get recursively called as show below. * * ext4_writepage() -> kmalloc() -> __alloc_pages() -> page_launder() -> * ext4_writepage() * * But since we don't do any block allocation we should not deadlock. * Page also have the dirty flag cleared so we don't get recurive page_lock. One might well argue that it's not *useful*; if we've gone into writepage already, there's no point in re-entering writepage. And the last thing we want to do is But I could see filesystems behaving differently when entered for writepage-for-regularly-scheduled-writeback versus writepage-for-shrinking, so maybe they can make progress. Maybe no real filesystem behaves that way. We need feedback from filesystem people.
