On Aug 7, 2013, at 5:00 PM, Andrew Morton wrote: > On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin <ecas...@coraid.com> wrote: > >> As discussed previously, the fact that some users of the block >> layer provide bios that point to pages with a zero _count means >> that it is not OK for the network layer to do a put_page on the >> skb frags during an skb_linearize, so the aoe driver gets a >> reference to pages in bios and puts the reference before ending >> the bio. And because it cannot use get_page on a page with a >> zero _count, it manipulates the value directly. >> >> It is not OK to increment the _count of a compound page tail, >> though, since the VM layer will VM_BUG_ON a non-zero _count. >> Block users that do direct I/O can result in the aoe driver >> seeing compound page tails in bios. In that case, the same logic >> works as long as the head of the compound page is used instead of >> the tails. This patch handles compound pages and does not BUG. >> >> It relies on the block layer user leaving the relationship >> between the page tail and its head alone for the duration between >> the submission of the bio and its completion, whether successful >> or not. > > What are the end-user-visible effects of the problem which is being > fixed? Please always include this info when fixing bugs so that others > can work out what kernel version(s) need the patch.
Sorry. I tried to cover that but maybe should have separated it more clearly from the running text. The patch changes the current behavior encountered with direct I/O and aoe: aoe can BUG when direct I/O used ... to a new behavior: aoe does not BUG when direct I/O is used And the user will never know that instead aoe is manipulating the _count of the head page, even though that's what's happening with this patch applied. (The user-visible effects that motivated the older patch that added the BUG to aoe when page tails are used for block I/O was a VM_BUG_ON that occurred when the aoe driver manipulated the _count of a compound page tail.) -- Ed Cashin ecas...@coraid.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
