Re: [patch 1/2] mm: pagecache allocation gfp fixes

["Pekka Enberg" <penberg@cs.helsinki.fi>]

Hi Nick,

On Thu, Nov 27, 2008 at 11:34 AM, Nick Piggin <npiggin@suse.de> wrote:
>
> 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. Then grab_cache_page_nowait() is changed to allocate radix-tree
> nodes with GFP_NOFS, because it is not supposed to reenter the filesystem.
>
> 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.
>
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> ---
> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c
> +++ linux-2.6/mm/filemap.c
> @@ -741,7 +741,8 @@ repeat:
>                page = __page_cache_alloc(gfp_mask);
>                if (!page)
>                        return NULL;
> -               err = add_to_page_cache_lru(page, mapping, index, gfp_mask);
> +               err = add_to_page_cache_lru(page, mapping, index,
> +                       (gfp_mask & (__GFP_FS|__GFP_IO|__GFP_WAIT|__GFP_HIGH)));

Can we use GFP_RECLAIM_MASK here? I mean, surely we need to pass
__GFP_NOFAIL, for example, down to radix_tree_preload() et al?

>                if (unlikely(err)) {
>                        page_cache_release(page);
>                        page = NULL;
> @@ -950,7 +951,7 @@ grab_cache_page_nowait(struct address_sp
>                return NULL;
>        }
>        page = __page_cache_alloc(mapping_gfp_mask(mapping) & ~__GFP_FS);
> -       if (page && add_to_page_cache_lru(page, mapping, index, GFP_KERNEL)) {
> +       if (page && add_to_page_cache_lru(page, mapping, index, GFP_NOFS)) {
>                page_cache_release(page);
>                page = NULL;
>        }
>
> --
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