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Message-ID: <01fa5b0f-d209-6eb5-9911-b7522eb491d6@intel.com> Date: Fri, 23 Sep 2022 10:59:35 -0700 From: Anirudh Venkataramanan <anirudh.venkataramanan@...el.com> To: "Fabio M. De Francesco" <fmdefrancesco@...il.com>, Ira Weiny <ira.weiny@...el.com> CC: Alexander Duyck <alexander.duyck@...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>, 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 8:05 AM, Fabio M. De Francesco wrote: > Hi Anirudh, > > On Friday, September 23, 2022 12:38:02 AM CEST Anirudh Venkataramanan 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]. > > I saw your patches and they look good to me. I might comment and probably > review them, however I prefer to wait for Ira to do that. Furthermore, looking > again at your patches made me recall that I need to talk with him about > something that is only indirectly related with you work. > > Please don't rely on older patches of mine as models for your next patches. In > the last months I changed many things in the way I handle the removal of > kmap() in favour of a plain page_address() or decide to convert to > kmap_local_page(). Obviously I'm talking about pages which cannot come from > ZONE_HIGHMEM. > >>>> 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. > > Your understanding of Dave's message is absolutely correct. > >>>> 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@...el.com/ >>>> >>>> https://patchwork.ozlabs.org/project/intel-wired-lan/patch/ > 20220919180949.388785-2-anirudh.venkataramanan@...el.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. > > The mere fact that we are still discussing this particular topic is my only > fault. I mean that the guidelines about what to do with ZONE_NORMAL or lower > pages is not enough clear. I'll have to improve that paragraph. > > For now let me tell you what I'm doing whenever I have to decide between a > conversion from kmap{,_atomic}() to kmap_local_page() or a kmap() removal in > favour of page_address() use. > >> ... 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(). > > "a" and "b" are good arguments with sound logic. However there are a couple of > cases that you are not yet considering. > > As my main rule I prefer the use of kmap_local_page() whenever tracking if > pages can't come from Highmem is complex, especially when allocation is > performed in other translation units of the same driver or, worse, pages come > from different subsystems. > > Instead, I don't like to use kmap_local_page() when the allocation is in the > same function and you see immediately that it cannot come from ZONE_HIGHMEM. > > Sometimes it's so clear that using kmap_local_page() looks silly to me :-) > For example... > > void *silly_alloc_and_map() { > struct *page; > > page = alloc_page(GFP_KERNEL); > return kmap_local_page(page); > } > > In this case you know without any effort that the page cannot come from > ZONE_HIGHMEM. Therefore, why bother with mapping and unmapping (and perhaps > write a function for unmapping). That's fair. When I suggested using kmap_local_page() even for driver-local page allocations, I was thinking of situations where the page allocation and mapping/access happen in different functions in the same driver. Not that these are impossible to trace, just takes some more time and effort. > > While working on the removals or the conversions of kmap(), I noticed that > people tend to forget to call kunmap(). We have a limited amount of kmap() > slots. If the mapping space is fully utilized we'll have the next slot > available only after reboot or unloading and reloading a module. > > If I recall correctly, with kmap_local_page() we can map a maximum of 16 pages > per task_struct. Therefore, limits are everywhere and people tend to leak > resources. > > To summarize: whenever allocation is easily trackable, and pages cannot come > from ZONE_HIGHMEM, I prefer page_address(). How would you define "easily track-able"? Does it make more sense to instead say "if page allocation is module-local and can't come from highmem, then use page_address()". > > Honestly, if code is well designed I don't care whether or not within 5 days > or 10 years decide to change the allocation. I think it's like to refrain from > deleting unreachable code, variables, partially implemented functions, and so > on just because one day someone may think to make something useful from those > things. (a) is the primary reason to use kmap_local_page(). (b) is a co-traveler. > > Greg K-H taught me that I must see the code as is now and don't speculate > about possible future scenarios. I agree with him in full :-) > > Very different case where I _need_ page_address() are due to the strict rules > of nesting mapping and unmapping-mapping. I recall that I spent days on a > function in Btrfs because I could not map and unmap with the usual Last In - > First Out (LIFO) rule. Right, so maybe instead of me saying "use kmap_local_page() everywhere" I should have said "kmap_local_page() should be preferred where possible". To summarize, how about this for a guideline? - For module-local page allocations that can't come from highmem, using page_address() is acceptable. - For pages that are allocated outside the module but passed to the module, use the appropriate kmap() function. Ani
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