On Thu, 2006-08-10 at 12:23 -0700, Andrew Morton wrote: > On Thu, 10 Aug 2006 11:51:34 -0700 > Badari Pulavarty <pbadari@xxxxxxxxxx> wrote: > > > Andrew Morton wrote: > > > Also, JBD is presently feeding into submit_bh() buffer_heads which span two > > > machine pages, and some device drivers spit the dummy. It'd be better to > > > fix that once, rather than twice.. > > > > > Andrew, > > > > I looked at this few days ago. I am not sure how we end up having > > multiple pages (especially, > > why we end up having buffers with bh_size > pagesize) ? Do you know why ? > > > > It's one or both of the jbd_kmalloc(bh->b_size) calls in > fs/jbd/transaction.c. Here we're allocating data to attach to a bh which > later gets fed into submit_bh(). > > Problem is, with CONFIG_DEBUG_SLAB=y, the data which kmalloc() returns can > be offset by 4 bytes due to redzoning. > > Example: if the fs is using a 1k blocksize and we have a 4k pagesize, the > data coming back from kmalloc may have an address of 0xnnnnxc04, so the > data which we later feed into submit_bh() will span two pages. > > A simple fix would be to replace kmalloc() with a call to alloc_page(). > We'd need to work out how much memory that will worst-case-waste. If "not > much" then OK. > > If "quite a lot in the worst case" then we'd need something more elaborate. Would some like this be okay: #ifdef CONFIG_DEBUG_SLAB return alloc_page(... #else return kmalloc(... #endif This keeps it simple, and should be still be efficient in the non-DEBUG_SLAB case. > I'd suggest that ext3 implement ext3-private slab caches of size 1024, > 2048, 4096 and perhaps 8192. Those caches should be kmem_cache_create()d > on-demand at mount-time. They should be created with appropriate slab > options to defeat the redzoning. The transaction.c code should use the > appropriate slab (based on b_size) rather than using kmalloc(). The > up-to-four private slab caches should be destroyed on ext3 rmmod. -- David Kleikamp IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
