On Thu, Jan 15, 2009 at 10:57:30PM -0700, Matthew Wilcox wrote: > On Fri, Jan 16, 2009 at 02:51:19PM +0900, MinChan Kim wrote: > > On Thu, Jan 15, 2009 at 10:33:38PM -0700, Matthew Wilcox wrote: > > > Sorry, no. You have to call fluch_dcache_page() in two situations -- > > > when the kernel is going to read some data that userspace wrote, *and* > > > when userspace is going to read some data that the kernel wrote. From a > > > quick look at the patch, this seems to be the second case. The kernel > > > wrote data to a pagecache page, and userspace should be able to read it. > > > > > > To understand why this is necessary, consider a processor which is > > > virtually indexed and has a writeback cache. The kernel writes to a > > > page, then a user process reads from the same page through a different > > > address. The cache doesn't find the data the kernel wrote because it > > > has a different virtual index, so userspace reads stale data. > > > > I see. :) > > > > Thanks for quick reponse and good explaination. > > Hmm,.. one more question. > > > > I can't find flush_dcache_page call in mpage_readpage which is > > generic read function. In case of ext fs, it use mpage_readpage > > with readpage. > > > > who and where call flush_dcache_page in mpage_readpage call path? > > Most I/O devices will do DMA to the page in question and thus the kernel > hasn't written to it and the CPU won't have the data in cache. For the > few devices which can't do DMA, it's the responsibility of the device > driver to call flush_dcache_page() (or some other flushing primitive). Hmm.. Now I am confusing. If devicer driver or with DMA makes sure cache consistency, Why filesystem code have to handle it ? > See the gdth driver for an example: > > address = kmap_atomic(sg_page(sl), KM_BIO_SRC_IRQ) + sl->offset; > memcpy(address, buffer, cpnow); > flush_dcache_page(sg_page(sl)); > kunmap_atomic(address, KM_BIO_SRC_IRQ); > > -- > Matthew Wilcox Intel Open Source Technology Centre > "Bill, look, we understand that you're interested in selling us this > operating system, but compare it to ours. We can't possibly take such > a retrograde step." -- Kinds regards, MinChan Kim -- 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
