On Mon, Dec 17, 2018 at 03:10:44PM +0100, Michal Hocko wrote: > On Mon 17-12-18 04:25:46, Matthew Wilcox wrote: > > It's worth noticing that squashfs _is_ in fact holding a page locked in > > squashfs_copy_cache() when it calls grab_cache_page_nowait(). I'm not > > sure if this will lead to trouble or not because I'm insufficiently > > familiar with the reclaim path. > > Hmm, this is more interesting then. If there is any memcg accounted > allocation down that path _and_ the squashfs writeout can lock more > pages and mark them writeback before they are really sent to the storage > then we have a problem. See [1] > > [1] http://lkml.kernel.org/r/20181213092221.27270-1-mhocko@xxxxxxxxxx Squashfs is read only, so it'll never have dirty pages and never do writeout. But ... maybe the GFP flags being used for grab_cache_page_nowait() are wrong. It does, after all, say "nowait". Perhaps it shouldn't be trying direct reclaim at all, but rather fail earlier. Like this: +++ b/mm/filemap.c @@ -1550,6 +1550,8 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset, gfp_mask |= __GFP_WRITE; if (fgp_flags & FGP_NOFS) gfp_mask &= ~__GFP_FS; + if (fgp_flags & FGP_NOWAIT) + gfp_mask &= ~__GFP_DIRECT_RECLAIM; page = __page_cache_alloc(gfp_mask); if (!page)
