Re: [patch] mm: pagecache gfp flags fix

[Nick Piggin <npiggin@suse.de>]

On Mon, Dec 15, 2008 at 03:21:44PM -0800, Andrew Morton wrote:
> On Fri, 12 Dec 2008 05:41:20 +0100
> Nick Piggin <npiggin@suse.de> wrote:
> 
> > This patch doesn't actually fix a regression, but a longer standing bug.
> > --
> > 
> > Frustratingly, gfp_t is really divided into two classes of flags. One are the
> > context dependent ones (can we sleep? can we enter filesystem? block subsystem?
> > should we use some extra reserves, etc.). The other ones are the type of memory
> > required and depend on how the algorithm is implemented rather than the point
> > at which the memory is allocated (highmem? dma memory? etc).
> > 
> > Some of functions which allocate a page and add it to page cache take a gfp_t,
> > but sometimes those functions or their callers aren't really doing the right
> > thing: when allocating pagecache page, the memory type should be
> > mapping_gfp_mask(mapping). When allocating radix tree nodes, the memory type
> > should be kernel mapped (not highmem) memory. The gfp_t argument should only
> > really be needed for context dependent options.
> > 
> > This patch doesn't really solve that tangle in a nice way, but it does attempt
> > to fix a couple of bugs.
> > 
> > - find_or_create_page changes its radix-tree allocation to only include the
> >   main context dependent flags in order so the pagecache page may be allocated
> >   from arbitrary types of memory without affecting the radix-tree. In practice,
> >   slab allocations don't come from highmem anyway, and radix-tree only uses
> >   slab allocations. So there isn't a practical change (unless some fs uses
> >   GFP_DMA for pages).
> > 
> > - grab_cache_page_nowait() is changed to allocate radix-tree nodes with
> >   GFP_NOFS, because it is not supposed to reenter the filesystem. This bug
> >   could cause lock recursion if a filesystem is not expecting the function
> >   to reenter the fs (as-per documentation).
> > 
> > Filesystems should be careful about exactly what semantics they want and what
> > they get when fiddling with gfp_t masks to allocate pagecache. One should be
> > as liberal as possible with the type of memory that can be used, and same
> > for the the context specific flags.
> 
> ug.  So at present page_symlink() can call write_begin() which will do
> a GFP_KERNEL/GFP_USER allocation even though we hold fs locks?

Yes.

 
> In which calling context does this happen?

ext3/4 do it AFAIKS. I think some filesystems set GFP_NOFS in mapping_gfp_mask
too, but I don't think that's a very good idea because we don't always mask
allocations with mapping_gfp_mask (but still, they would be helped by this
patch too, if they were relying on it).

 
> This is a pretty big ugly patch.  I'm thinking that we merge into
> 2.6.29 and backport into 2.6.28.x.

Sounds good.

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