On Fri, Jul 19, 2024 at 07:58:40PM +0930, Qu Wenruo wrote: > [BACKGROUND] > The function filemap_add_folio() charges the memory cgroup, > as we assume all page caches are accessible by user space progresses > thus needs the cgroup accounting. > > However btrfs is a special case, it has a very large metadata thanks to > its support of data csum (by default it's 4 bytes per 4K data, and can > be as large as 32 bytes per 4K data). > This means btrfs has to go page cache for its metadata pages, to take > advantage of both cache and reclaim ability of filemap. > > This has a tiny problem, that all btrfs metadata pages have to go through > the memcgroup charge, even all those metadata pages are not > accessible by the user space, and doing the charging can introduce some > latency if there is a memory limits set. > > Btrfs currently uses __GFP_NOFAIL flag as a workaround for this cgroup > charge situation so that metadata pages won't really be limited by > memcgroup. > > [ENHANCEMENT] > Instead of relying on __GFP_NOFAIL to avoid charge failure, use root > memory cgroup to attach metadata pages. > > With root memory cgroup, we directly skip the charging part, and only > rely on __GFP_NOFAIL for the real memory allocation part. > > Suggested-by: Michal Hocko <mhocko@xxxxxxxx> > Suggested-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx> > Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> > --- > fs/btrfs/extent_io.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c > index aa7f8148cd0d..cfeed7673009 100644 > --- a/fs/btrfs/extent_io.c > +++ b/fs/btrfs/extent_io.c > @@ -2971,6 +2971,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, > > struct btrfs_fs_info *fs_info = eb->fs_info; > struct address_space *mapping = fs_info->btree_inode->i_mapping; > + struct mem_cgroup *old_memcg; > const unsigned long index = eb->start >> PAGE_SHIFT; > struct folio *existing_folio = NULL; > int ret; > @@ -2981,8 +2982,17 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, > ASSERT(eb->folios[i]); > > retry: > + /* > + * Btree inode is a btrfs internal inode, and not exposed to any > + * user. > + * Furthermore we do not want any cgroup limits on this inode. > + * So we always use root_mem_cgroup as our active memcg when attaching > + * the folios. > + */ > + old_memcg = set_active_memcg(root_mem_cgroup); > ret = filemap_add_folio(mapping, eb->folios[i], index + i, > GFP_NOFS | __GFP_NOFAIL); > + set_active_memcg(old_memcg); It looks correct. But it's going through all dance to set up current->active_memcg, then have the charge path look that up, css_get(), call try_charge() only to bail immediately, css_put(), then update current->active_memcg again. All those branches are necessary when we want to charge to a "real" other cgroup. But in this case, we always know we're not charging, so it seems uncalled for. Wouldn't it be a lot simpler (and cheaper) to have a filemap_add_folio_nocharge()?
