On Thu, 2011-01-06 at 18:05 +0000, Russell King - ARM Linux wrote: > On Thu, Jan 06, 2011 at 11:40:13AM -0600, James Bottomley wrote: > > On Wed, 2011-01-05 at 23:28 +0000, James Bottomley wrote: > > > Can you explain how the code works? it looks to me like you read the xdr > > > stuff through the vmap region then write it out directly to the pages? > > > > OK, I think I see how this is supposed to work: It's a sequential loop > > of reading in via the pages (i.e. through the kernel mapping) and then > > updating those pages via the vmap. In which case, I think this patch is > > what you need. > > > > The theory of operation is that the readdir on pages actually uses the > > network DMA operations to perform, so when it's finished, the underlying > > What network DMA operations - what if your NIC doesn't do DMA because > it's an SMSC device? So this is the danger area ... we might be caught by our own flushing tricks. I can't test this on parisc since all my network drivers use DMA (which automatically coheres the kernel mapping by flush/invalidate). What should happen is that the kernel mapping pages go through the ->readdir() path. Any return from this has to be ready to map the pages back to user space, so the kernel alias has to be flushed to make the underlying page up to date. The exception is pages we haven't yet mapped to userspace. Here we set the PG_dcache_dirty bit (sparc trick) but don't flush the page, since we expect the addition of a userspace mapping will detect this case and do the flush and clear the bit before the mapping goes live. I assume you're thinking that because this page is allocated and freed internally to NFS, it never gets a userspace mapping and therefore, we can return from ->readdir() with a dirty kernel cache (and the corresponding flag set)? I think that is a possible hypothesis in certain cases. James -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
