Re: [patch] mm: pagecache gfp flags fix

[Andrew Morton <akpm@linux-foundation.org>]

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?

In which calling context does this happen?

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