Re: [PATCH] squashfs: enable __GFP_FS in ->readpage to prevent hang in mem alloc

[Matthew Wilcox]

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)