Re: [PATCH 1/4] compcache: xvmalloc memory allocator

From: Nitin Gupta
Date: Mon Aug 24 2009 - 15:37:00 EST

Hi Pekka,

On 08/24/2009 11:03 PM, Pekka Enberg wrote:


On Mon, Aug 24, 2009 at 7:37 AM, Nitin Gupta<ngupta@xxxxxxxxxx> wrote:
+ * xv_malloc - Allocate block of given size from pool.
+ * @pool: pool to allocate from
+ * @size: size of block to allocate
+ * @pagenum: page no. that holds the object
+ * @offset: location of object within pagenum
+ *
+ * On success,<pagenum, offset> identifies block allocated
+ * and 0 is returned. On failure,<pagenum, offset> is set to
+ * 0 and -ENOMEM is returned.
+ *
+ * Allocation requests with size> XV_MAX_ALLOC_SIZE will fail.
+ */
+int xv_malloc(struct xv_pool *pool, u32 size, u32 *pagenum, u32 *offset,
+ gfp_t flags)


What's the purpose of passing PFNs around? There's quite a lot of PFN
to struct page conversion going on because of it. Wouldn't it make
more sense to return (and pass) a pointer to struct page instead?

PFNs are 32-bit on all archs while for 'struct page *', we require 32-bit or
64-bit depending on arch. ramzswap allocates a table entry <pagenum, offset>
corresponding to every swap slot. So, the size of table will unnecessarily
increase on 64-bit archs. Same is the argument for xvmalloc free list sizes.

Also, xvmalloc and ramzswap itself does PFN -> 'struct page *' conversion
only when freeing the page or to get a deferencable pointer.


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