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Message-ID: <f32338c8-db1a-ba0c-9254-922d96f2e601@intel.com> Date: Fri, 23 Sep 2022 11:50:36 -0700 From: Anirudh Venkataramanan <anirudh.venkataramanan@...el.com> To: Alexander Duyck <alexander.duyck@...il.com> CC: "Fabio M. De Francesco" <fmdefrancesco@...il.com>, Jakub Kicinski <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Netdev <netdev@...r.kernel.org>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, intel-wired-lan <intel-wired-lan@...ts.osuosl.org>, Alexander Duyck <alexanderduyck@...com>, John Fastabend <john.fastabend@...il.com>, Jesse Brandeburg <jesse.brandeburg@...el.com>, Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>, Ira Weiny <ira.weiny@...el.com>, LKML <linux-kernel@...r.kernel.org>, Tony Nguyen <anthony.l.nguyen@...el.com> Subject: Re: [Intel-wired-lan] [PATCH] ixgbe: Use kmap_local_page in ixgbe_check_lbtest_frame() On 9/23/2022 8:31 AM, Alexander Duyck wrote: > On Thu, Sep 22, 2022 at 3:38 PM Anirudh Venkataramanan > <anirudh.venkataramanan@...el.com> wrote: >> >> On 9/22/2022 1:58 PM, Alexander Duyck wrote: >>> On Thu, Sep 22, 2022 at 1:07 PM Anirudh Venkataramanan >>> <anirudh.venkataramanan@...el.com> wrote: >>>> >>>> >>>> Following Fabio's patches, I made similar changes for e1000/e1000e and >>>> submitted them to IWL [1]. >>>> >>>> Yesterday, Ira Weiny pointed me to some feedback from Dave Hansen on the >>>> use of page_address() [2]. My understanding of this feedback is that >>>> it's safer to use kmap_local_page() instead of page_address(), because >>>> you don't always know how the underlying page was allocated. >>>> >>>> This approach (of using kmap_local_page() instead of page_address()) >>>> makes sense to me. Any reason not to go this way? >>>> >>>> [1] >>>> >>>> https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-1-anirudh.venkataramanan@intel.com/ >>>> >>>> https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-2-anirudh.venkataramanan@intel.com/ >>>> >>>> [2] >>>> https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@intel.com/ >>>> >>>> Ani >>> >>> For the two patches you referenced the driver is the one allocating >>> the pages. So in such a case the page_address should be acceptable. >>> Specifically we are falling into alloc_page(GFP_ATOMIC) which should >>> fall into the first case that Dave Hansen called out. >> >> Right. However, I did run into a case in the chelsio inline crypto >> driver where it seems like the pages are allocated outside the driver. >> In such cases, kmap_local_page() would be the right approach, as the >> driver can't make assumptions on how the page was allocated. > > Right, but that is comparing apples and oranges. As I said for Tx it > would make sense, but since we are doing the allocations for Rx that > isn't the case so we don't need it. > >> ... and this makes me wonder why not just use kmap_local_page() even in >> cases where the page allocation was done in the driver. IMO, this is >> simpler because >> >> a) you don't have to care how a page was allocated. kmap_local_page() >> will create a temporary mapping if required, if not it just becomes a >> wrapper to page_address(). >> >> b) should a future patch change the allocation to be from highmem, you >> don't have to change a bunch of page_address() calls to be >> kmap_local_page(). >> >> Is using page_address() directly beneficial in some way? > > By that argument why don't we just leave the code alone and keep using > kmap? I am pretty certain that is the logic that had us using kmap in > the first place since it also dumps us with page_address in most cases > and we didn't care much about the other architectures. Well, my understanding is that kmap_local_page() doesn't have the overheads kmap() has, and that alone is reason enough to replace kmap() and kmap_atomic() with kmap_local_page() where possible. > If you look at > the kmap_local_page() it just adds an extra step or two to calling > page_address(). In this case it is adding extra complication to > something that isn't needed which is the reason why we are going > through this in the first place. If we are going to pull the bandage I > suggest we might as well just go all the way and not take a half-step > since we don't actually need kmap or its related calls for this. I don't really see this as "pulling the kmap() bandage", but a "use a more appropriate kmap function if you can" type situation. FWIW, I am not against using page_address(). Just wanted to hash this out and get to a conclusion before I made new changes. Ani
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