From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Dan Williams" Subject: Re: kmap_atomic() oopses in current mainline Date: Thu, 19 Jul 2007 08:23:02 -0700 Message-ID: References: <20070719013304.3c060c99.akpm@linux-foundation.org> <20070719092856.GA15839@2ka.mipt.ru> <20070719023831.400f7905.akpm@linux-foundation.org> <20070719100132.GC15839@2ka.mipt.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "Andrew Morton" , netdev@vger.kernel.org, linux-kernel-announce@vger.kernel.org To: "Evgeniy Polyakov" Return-path: Received: from an-out-0708.google.com ([209.85.132.242]:26495 "EHLO an-out-0708.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964979AbXGSPXF (ORCPT ); Thu, 19 Jul 2007 11:23:05 -0400 Received: by an-out-0708.google.com with SMTP id d31so120347and for ; Thu, 19 Jul 2007 08:23:03 -0700 (PDT) In-Reply-To: <20070719100132.GC15839@2ka.mipt.ru> Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 7/19/07, Evgeniy Polyakov wrote: > On Thu, Jul 19, 2007 at 02:38:31AM -0700, Andrew Morton (akpm@linux-foundation.org) wrote: > > > > is very wrong if both ASYNC_TX_KMAP_DST and ASYNC_TX_KMAP_SRC can ever be > > > > set. We'll end up using the same kmap slot for both src add dest and we > > > > get either corrupted data or a BUG. > > > > > > So far it can not since the only user is raid code, which only allows to > > > perform either reading from bio or writing into one, which requires only > > > one mapping. > > > > hm, so we got lucky? > > I would say it was intentionally, current code can perform only one > operation in a time. Of course changing KM_USER from 0 to 1 in second > kmap_atomic will not force oceans to run out of coasts. > > Kind of: > > diff --git a/crypto/async_tx/async_memcpy.c b/crypto/async_tx/async_memcpy.c > index a973f4e..a48c7f3 100644 > --- a/crypto/async_tx/async_memcpy.c > +++ b/crypto/async_tx/async_memcpy.c > @@ -94,7 +94,7 @@ async_memcpy(struct page *dest, struct page *src, unsigned int dest_offset, > dest_buf = page_address(dest) + dest_offset; > > if (flags & ASYNC_TX_KMAP_SRC) > - src_buf = kmap_atomic(src, KM_USER0) + src_offset; > + src_buf = kmap_atomic(src, KM_USER1) + src_offset; > else > src_buf = page_address(src) + src_offset; > > @@ -104,7 +104,7 @@ async_memcpy(struct page *dest, struct page *src, unsigned int dest_offset, > kunmap_atomic(dest_buf, KM_USER0); > > if (flags & ASYNC_TX_KMAP_SRC) > - kunmap_atomic(src_buf, KM_USER0); > + kunmap_atomic(src_buf, KM_USER1); > > async_tx_sync_epilog(flags, depend_tx, cb_fn, cb_param); > } > > > > Btw, shouldn't it always be kmap_atomic() even if flag is not set. > > > That pages are usual one returned by alloc_page(). > > > > The code would work OK if the kmap_atomic()s were unconditional, but it > > would be a bit more expensive if the page is in highmem and we don't > > actually intend to access it with the CPU. > > > > kmap_atomic() against a non-highmem page is basically free: just an > > additional test_bit(). > Always kmap'ing the page is the way to go, since in this path the page is always accessed with the CPU. This also allows these ASYNC_TX_ flags to be killed off as they are not necessary. I'll cook up a patch, and be more careful about my kmap usage going forward. > As far as I recall there was an intention to do async memory copy to > userspace, so likely kmapping is a good idea. > > -- > Evgeniy Polyakov Thanks, Dan