Messages in this thread | | | Date | Mon, 27 Nov 2006 16:32:20 +0000 (GMT) | From | Mel Gorman <> | Subject | Re: [PATCH 1/11] Add __GFP_MOVABLE flag and update callers |
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On Sun, 26 Nov 2006, Hugh Dickins wrote:
> On Sat, 25 Nov 2006, Linus Torvalds wrote: >> On Fri, 24 Nov 2006, Hugh Dickins wrote: >>> >>> You need to add in something like the patch below (mutatis mutandis >>> for whichever approach you end up taking): tmpfs uses highmem pages >>> for its swap vector blocks, noting where on swap the data pages are, >>> and allocates them with mapping_gfp_mask(inode->i_mapping); but we >>> don't have any mechanism in place for reclaiming or migrating those. >> >> I think this really just points out that you should _not_ put MOVABLE into >> the "mapping_gfp_mask()" at all. >> >> The mapping_gfp_mask() should really just contain the "constraints" on >> the allocation, not the "how the allocation is used". So things like "I >> need all my pages to be in the 32bit DMA'able region" is a constraint on >> the allocator, as is something like "I need the allocation to be atomic". >> >> But MOVABLE is really not a constraint on the allocator, it's a guarantee >> by the code _calling_ the allocator that it will then make sure that it >> _uses_ the allocation in a way that means that it is movable. >>
Later, MOVABLE might be a constraint. For example, hotpluggable nodes may only allow MOVABLE allocations to be allocated.
>> So it shouldn't be a property of the mapping itself, it should always be a >> property of the code that actually does the allocation. >> >> Hmm? > > Not anything I feel strongly about, but I don't see it that way. > > mapping_gfp_mask() seems to me nothing more than a pragmatic way > of getting the appropriate gfp_mask down to page_cache_alloc(). >
And that is important for any filesystem that uses generic_file_read(). As page_cache_alloc() has no knowledge of the filesystem, it depends on the mapping_gfp_mask() to determine if the pages are movable or not.
> alloc_inode() initializes it to whatever suits most filesystems > (currently GFP_HIGHUSER), and those who differ adjust it (e.g. > block_dev has good reason to avoid highmem so sets it to GFP_USER > instead). It used to be the case that several filesystems lacked > kmap() where needed, and those too would set GFP_USER: what you call > a constraint seems to me equally a property of the surrounding code. > > If __GFP_MOVABLE is coming in, and most fs's are indeed allocating > movable pages, then I don't see why MOVABLE shouldn't be in the > mapping_gfp_mask. Specifying MOVABLE constrains both the caller's > use of the pages, and the way they are allocated; as does HIGHMEM. >
From what I've seen, the majority of filesystems are suitable for using __GFP_MOVABLE and it would be clearer to have GFP_HIGH_MOVABLE as the default and setting GFP_HIGHUSER in filesystems like ramfs.
> And we shouldn't be guided by the way tmpfs (ab?)uses that gfp_mask > for its metadata allocations as well as its page_cache_alloc()s: > that's just a special case. Though the ramfs case is more telling > (its pagecache pages being not at present movable). > > Hugh >
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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