On 03/27/2018 09:21 AM, Matthew Wilcox wrote: > 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: No, it is not harmful. > > * 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. Yes, and it avoids this by checking for PF_MEMALLOC flag. > > 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. > do_writepages() is the same for both, and hence the memalloc_* API patch. > Maybe no real filesystem behaves that way. We need feedback from > filesystem people. The idea is to: * Keep a central location for check, rather than individual filesystem writepage(). It should reduce code as well. * Filesystem developers call memory allocations without thinking twice about which GFP flag to use: GFP_KERNEL or GFP_NOFS. In essence eliminate GFP_NOFS. -- Goldwyn
